Geologic, hydrologic, and geochemical identification of flow paths in the Edwards Aquifer, northeastern Bexar and southern Comal Counties, Texas


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Geologic, hydrologic, and geochemical identification of flow paths in the Edwards Aquifer, northeastern Bexar and southern Comal Counties, Texas

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Geologic, hydrologic, and geochemical identification of flow paths in the Edwards Aquifer, northeastern Bexar and southern Comal Counties, Texas
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Scientific Investigations Report
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Otero, Cassi L.
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The U.S. Geological Survey, in cooperation with the San Antonio Water System, conducted a 4-year study during 2002?06 to identify major flow paths in the Edwards aquifer in northeastern Bexar and southern Comal Counties (study area). In the study area, faulting directs ground water into three hypothesized flow paths that move water, generally, from the southwest to the northeast. These flow paths are identified as the southern Comal flow path, the central Comal flow path, and the northern Comal flow path. Statistical correlations between water levels for six observation wells and between the water levels and discharges from Comal Springs and Hueco Springs yielded evidence for the hypothesized flow paths. Strong linear correlations were evident between the datasets from wells and springs within the same flow path and the datasets from wells in areas where flow between flow paths was suspected. Geochemical data (major ions, stable isotopes, sulfur hexafluoride, and tritium and helium) were used in graphical analyses to obtain evidence of the flow path from which wells or springs derive water. Major-ion geochemistry in samples from selected wells and springs showed relatively little variation. Samples from the southern Comal flow path were characterized by relatively high sulfate and chloride concentrations, possibly indicating that the water in the flow path was mixing with small amounts of saline water from the freshwater/saline-water transition zone. Samples from the central Comal flow path yielded the most varied major-ion geochemistry of the three hypothesized flow paths. Central Comal flow path samples were characterized, in general, by high calcium concentrations and low magnesium concentrations. Samples from the northern Comal flow path were characterized by relatively low sulfate and chloride concentrations and high magnesium concentrations. The high magnesium concentrations characteristic of northern Comal flow path samples from the recharge zone in Comal County might indicate that water from the Trinity a
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Scientific Investigations Report, Vol. 2007-5285 (2007).

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I n c o o p e r a t i o n w i t h t h e S a n A n t o n i o W a t e r S y s t e m G e o l o g i c , H y d r o l o g i c , a n d G e o c h e m i c a l I d e n t i f i c a t i o n o f F l o w P a t h s i n t h e E d w a r d s A q u i f e r , N o r t h e a s t e r n B e x a r a n d S o u t h e r n C o m a l C o u n t i e s , T e x a s S c i e n t i f i c I n v e s t i g a t i o n s R e p o r t 2 0 0 7 – 5 2 8 5 U . S . D e p a r t m e n t o f t h e I n t e r i o r U . S . G e o l o g i c a l S u r v e y

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Cover: U.S. Geological Survey hydrologic technicians preparing to collect samples on Landa Lake, New Braunfels, Texas.

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Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer, Northeastern Bexar and Southern Comal Counties, Texas By Cassi L. Otero In cooperation with the San Antonio Water System Scientific Investigations Report 2007 U.S. Department of the Interior U.S. Geological Survey

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U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director U.S. Geological Survey, Reston, Virginia: 2007 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Otero, C.L., 2007, Geologic, hydrologic, and geochemical identification of flow paths in the Edwards aquifer, north eastern Bexar and southern Comal Counties, Texas: U.S. Geological Survey Scientific Investigations Report 2007– 5285, 48 p.

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iii Contents Abstract .......................................................................................................................................................... 1 Introduction .................................................................................................................................................... 1 Purpose and Scope ............................................................................................................................. 3 Description of the Edwards Aquifer and Study Area ..................................................................... 3 Acknowledgments ............................................................................................................................... 6 Methods of Investigation ............................................................................................................................. 6 Data Collection and Sample Analysis .............................................................................................. 6 Quality Control and Quality Assurance of Geochemical Samples ............................................ 1 2 Methods of Data Analysis ......................................................................................................................... 1 2 Hydrologic Data .................................................................................................................................. 1 2 Geochemical Data ............................................................................................................................. 1 2 Geologic and Hydrologic Identification of Flow Paths ......................................................................... 1 3 Southern Comal Flow Path ............................................................................................................... 1 3 Central Comal Flow Path .................................................................................................................. 1 7 Northern Comal Flow Path ............................................................................................................... 1 7 Correlation of Water Levels ............................................................................................................. 1 8 Analysis of Hydrograph Recession Curves ................................................................................... 1 8 Geochemical Identification of Flow Paths .............................................................................................. 2 1 Major-Ion Chemistry .......................................................................................................................... 2 1 Stable Isotopes ................................................................................................................................... 2 1 Ground-Water Ages .......................................................................................................................... 2 5 Summary ....................................................................................................................................................... 2 8 References ................................................................................................................................................... 2 9 Appendix 1—Water-Level (2004) and Chemical Data (2003) ................................................... 3 1 1.1. Daily mean depth to water at well DX, Comal County, Texas, 2004 ................................................................................................................................... 3 3 1.2. Daily mean depth to water at well DX, Comal County, Texas, 2004 ..... 3 6 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003 ..................................................................................................... 3 9 Figures 1. Maps showing: 1. San Antonio segment of the Edwards aquifer, location of study area, and relation between Edwards and Trinity aquifers, south-central Texas ....................... 2 2. Study area and relation between recharge zone and confined zone of the Edwards aquifer and the Trinity aquifer, northeastern Bexar and southern Comal Counties, Texas ....................................................................................................... 4 3. Locations of major springs in the Comal Springs complex, southern Comal County, Texas ....................................................................................................................... 5

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4. Idealized block diagram of the Edwards aquifer between Comal Springs fault and Hueco Springs fault, southern Comal County, Texas ............................................................. 7 5. Maps showing: 5. Locations of wells and spring orifices from which water-quality samples were collected and locations of continuous water-level observation wells, northeastern Bexar and southern Comal Counties, Texas .......................................... 8 6. Locations of hypothesized ground-water flow paths, northeastern Bexar and southern Comal Counties, Texas ............................................................................ 14 7. Potentiometric surface of Edwards aquifer in northeastern Bexar and southern Comal Counties, Texas, fall 2000 ................................................................... 15 8. Hydrographs of water levels from six wells and discharge for Comal Springs and Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 2004–September 2006 ............................................................................................................... 16 9. Graph showing correlations between water levels for six wells and discharge for Comal Springs and Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 2004–September 2006 .................................................................... 19 10. Hydrographs of discharge at Comal Springs and Hueco Springs, southern Comal County, Texas, March 2004–September 2006, showing recession coefficients for recession curves ....................................................................................................................... 20 11. Trilinear diagrams showing composition of ground water in the Edwards aquifer in the southern, central, and northern Comal flow paths, northeastern Bexar and southern Comal Counties, Texas ............................................................................................. 22 12. Graphs showing: 12. Relation of calcium concentration to magnesium/calcium ratio in samples collected from wells and springs in the Edwards aquifer and wells in the Trinity aquifer, northeastern Bexar and southern Comal Counties, Texas. .............. 23 13. Relation between delta oxygen-18 and delta deuterium for selected wells and springs, northeastern Bexar and southern Comal Counties, Texas, 1996, with insert showing same plot with axes extended to range of rainfall samples ................................................................................................................. 2 4 14. Apparent ages of ground water in 2003 samples from selected wells and springs in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas, based on sulfur hexafluoride concentration .................................. 2 6 15. Maps showing apparent age of water in selected wells in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas, based on tritium/helium-3 ratio ................................................................................................................. 2 7 Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data were collected and compiled for this report .................................................................. 9 iv

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v Datums and Abbreviations Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29) or the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum. Specic conductance is given in microsiemens per centimeter at 25 degrees Celsius (S/cm). Concentrations of chemical constituents in water are given in either milligrams per liter (mg/L) or micrograms per liter (g/L).

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Abstract The U.S. Geological Survey, in cooperation with the San Antonio Water System, conducted a 4-year study dur ing 2002 to identify major flow paths in the Edwards aquifer in northeastern Bexar and southern Comal Counties (study area). In the study area, faulting directs ground water into three hypothesized flow paths that move water, gener ally, from the southwest to the northeast. These flow paths are identified as the southern Comal flow path, the central Comal flow path, and the northern Comal flow path. Statisti cal correlations between water levels for six observation wells and between the water levels and discharges from Comal Springs and Hueco Springs yielded evidence for the hypoth esized flow paths. Strong linear correlations were evident between the datasets from wells and springs within the same flow path and the datasets from wells in areas where flow between flow paths was suspected. Geochemical data (major ions, stable isotopes, sulfur hexafluoride, and tritium and helium) were used in graphical analyses to obtain evidence of the flow path from which wells or springs derive water. Major-ion geochemistry in samples from selected wells and springs showed relatively little variation. Samples from the southern Comal flow path were characterized by relatively high sulfate and chloride concentrations, possibly indicating that the water in the flow path was mixing with small amounts of saline water from the freshwater/saline-water transition zone. Samples from the central Comal flow path yielded the most varied major-ion geochemistry of the three hypothesized flow paths. Central Comal flow path samples were charac terized, in general, by high calcium concentrations and low magnesium concentrations. Samples from the northern Comal flow path were characterized by relatively low sulfate and chloride concentrations and high magnesium concentrations. The high magnesium concentrations characteristic of northern Comal flow path samples from the recharge zone in Comal County might indicate that water from the Trinity aquifer is entering the Edwards aquifer in the subsurface. A graph of the relation between the stable isotopes deuterium and delta-18 oxygen showed that, except for samples collected following an unusually intense rain storm, there was not much variation in stable isotope values among the flow paths. In the study area deuterium ranged from -36.00 to -20.89 per mil and delta-18 oxygen ranged from -6.03 to -3.70 per mil. Excluding samples collected following the intense rain storm, the deuterium range in the study area was -33.00 to -20.89 per mil and the delta-18 oxygen range was -4.60 to -3.70 per mil. Two ground-water age-dating techniques, sulfur hexafluoride concentrations and tritium/helium-3 isotope ratios, were used to compute apparent ages (time since recharge occurred) of water samples collected in the study area. In general, the apparent ages computed by the two methods do not seem to indicate direction of flow. Apparent ages computed for water samples in northeastern Bexar and southern Comal Counties do not vary greatly except for some very young water in the recharge zone in central Comal County. Introduction The Edwards aquifer is the main source of public water supply for the city of San Antonio, Texas, and the surround ing area and provides nearly all of the water for industrial, military, and irrigation use in the region (fig.1). Withdrawals from the aquifer to meet San Antonio’s increasing water-sup ply needs might be a threat to minimum mandated sustained flows at Comal Springs (Edwards Aquifer Authority, 2007), the largest spring in the Southwest. The springs supply water to downstream users, sustain federally-listed endangered spe cies, and support local economies through tourism. Increased knowledge of the complex hydrologic processes that control water availability in the Edwards aquifer in the vicinity of Comal Springs is imperative for optimal resource manage ment. The U.S. Geological Survey (USGS), in cooperation with the San Antonio Water System (SAWS), conducted a 4-year study during 2002 in northeastern Bexar County and southern Comal County to identify flow paths in the Edwards aquifer. The study area (fig. 1) includes small parts Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer, Northeastern Bexar and Southern Comal Counties, Texas By Cassi L. Otero

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Bracken ga p ! Alamo Heights horst ! GULF CO AST AL PLAI N FRIO COUNTY ZA VA LA COUNT Y AT ASCOSA COUNTY WILSON COUNTY UV ALDE COUNT Y MEDI NA COUNTY BEXAR COUNTY GU AD ALUP E COUNTY BA NDER A COUNT Y COMAL COUNTY CALD WELL COUNTY REAL COUNT Y KEND AL L COUNTY KERR COUNT Y HA YS COUNTY GILLESPIE COUNTY BLANC O COUNTY TRA VI S COUNTY KINNEY COUNT Y Boerne San Antoni o Braunfel s Bulverd e New 100' 99' 99' 98' 97' 29' 30' EXPLAN A TION T rinity aquifer Ed wards aquifer r echarge zone (outcr op) Ed wards aquifer conf ined zone Line of 1,000 milligrams per liter dissolved solids concentration (Schultz, 1994) Study ar ea boundary Spring 0 3 04 5 60 MILES 15 Balcones fault zone San Marcos Springs Hueco Springs Comal Springs San Antonio Springs San Pedro Springs 0 140 210 280 MILES 70 Study area TEXAS S AN MA R COS P LA TF O R M D EVIL S RIVE R TREND M A VERI CK B A S I N Depositional provinces modified from Rose (1972, fig. 1) Base modified from U.S. Geological Survey digital data Scale 1:250,000 Universal T ransverse Mercator projection, Datum NAD 83 Zone 14 Figure 1. San Antonio segment of the Edwards aquifer, location of study area, and relation between Edwards and Trinity aquifers, south-central Texas. 2 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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of Guadalupe and Hays Counties in addition to northeastern Bexar County and southern Comal County. Purpose and Scope The purpose of this report is to describe major groundwater flow paths in the Edwards aquifer in northeastern Bexar and southern Comal Counties identified on the basis of geologic, hydrologic, and geochemical data. Hydrogeol ogy and geologic structure based on work done by Maclay and Small (1984), Small (1986), Small and Hanson (1994), Maclay (1995), and Stein and Ozuna (1994) and flow path work done by Maclay and Land (1988) and Groschen (1996) provided a basis for the initial selection of hypothesized flow paths in the study area. Flow paths were further defined using hydrologic data collected from water-level observation wells and springflow monitoring sites in the study area. Altitudes of the potentiometric surface within each flow path provided evidence of flow directions. Comparisons of water-level and spring-discharge hydrographs and statistical correlation of water levels and spring discharges were used to show relations between water levels at wells and spring discharges within flow paths. Continuous water-level data were collected at six observation wells from mid-March 2004 through September 2006. Discharge data were obtained from two springflow sites for the same time period. Ground-water chemistry and isotope data were compiled from samples collected from 76 wells and nine springs (and spring orifices of major springs) during 1996. Isotope data also were obtained from samples col lected at four rainfall sites during 1998. Description of the Edwards Aquifer and Study Area The San Antonio segment of the Edwards aquifer (here inafter, Edwards aquifer) comprises Lower Cretaceous-age rocks of the Edwards Group (Rose, 1972) and the Georgetown Formation. The Edwards Group in the study area comprises two stratigraphic units, the basal Kainer Formation and the upper Person Formation. Each of those units comprises several informal members. The basal member of the Person Forma tion is a laterally extensive marine deposit consisting of poorly permeable, dense, carbonate mudstone known as the regional dense member (Maclay, 1995). Most recharge to the Edwards aquifer occurs in the recharge zone (aquifer outcrop) west of Bexar County (fig. 1), where streams originating north of the aquifer flow across and lose most or all of their flows into highly faulted and fractured limestone. Additional recharge enters the aquifer through the recharge zone in Bexar, Comal, and Hays Counties. After the water enters the aquifer, it moves generally in an easterly direction to discharge points in Bexar County, mainly munici pal water-supply wells. Water not discharged to wells then continues generally toward the northeast along and parallel to northeast-trending faults in the study area to discharge points in Comal and Hays Counties, primarily Comal Springs in Comal County and San Marcos Springs in Hays County (fig. 1). The study area is in an extensively faulted section of Cretaceous strata known as the Balcones fault zone (fig. 1). The fault zone developed as a result of extensional faulting and is characterized by a network of en-echelon, high-angle, mostly down-to-the-coast normal faults along the northwest ern margin of the Gulf Coastal Plain (Maclay and Small, 1984; Maclay, 1995). The Cretaceous strata were vertically displaced, intensively fractured, and differentially rotated within a series of southwest-to-northeast trending fault blocks (Barker and Ardis, 1996). The fault blocks, and their subse quent erosion and dissolution, are major factors affecting flow in the aquifer. Maclay and Land (1988, fig. 22) defined four major flow units in the Edwards aquifer. The flow units originate in areas referred to as storage units in the recharge zone and are regions of confined flow that generally move water initially to the southwest and then to the east and northeast to discharge at major springs. The eastern flow unit described by Maclay and Land (1988) originates in the study area. The western-south ern, south-central, and north-central flow units coalesce in northeastern Bexar County and southern Comal County in the vicinity of Cibolo Creek and Interstate Highway 35. Maclay and Small (1984, p. 50) estimated transmissivities for the Edwards aquifer to range from 200,000 to 2,000,000 feet squared per day. Maclay and Small (1984) describe one of the most transmissive areas in the Edwards aquifer as occur ring within a narrow, northeast-trending band downgradient from the area of coalescence of the three southernmost flow units. This band of high transmissivity (fig. 2) (labeled “R” in Maclay and Small [1984, fig. 20]) is bounded on the north west and southeast by faults where less-permeable rocks of the upper confining unit of the aquifer are juxtaposed against rocks of the Edwards aquifer. Recharge to and flow within the Edwards aquifer in northeastern Bexar and southern Comal Counties are com plicated by the structure and stratigraphy of the rocks. The study area (fig. 1) is on the structural high known as the San Marcos Platform (Rose, 1972). The San Marcos Platform is extensively faulted in the study area. The thickness of the Edwards aquifer in the study area is about 450 feet (Small and Hanson, 1994, p. 5; Stein and Ozuna, 1994, p. 5). Three major faults, the Comal Springs fault and the Hueco Springs fault in southern Comal County and the Northern Bexar fault (Maclay and Land, 1988) in northeastern Bexar County, are potentially effective barriers to flow in the Edwards aquifer in the study area (fig. 2). The Comal Springs complex (fig. 3) issues from the Comal Springs fault, which has as much as 500 feet of offset (Maclay and Land, 1988, p. A42) and is juxtaposed against the younger and less permeable upper confining unit. The springs developed because a roughly north-south trending transverse fault east of New Braunfels completely offsets the Edwards aquifer in the downthrown block of the Comal Springs fault, Introduction 3

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Bea r Salado Cr eek Guadalupe River Leon Cr eek Cibolo Cr eek Canyon Lak e BLANCO COUNTY KEN D ALL COUNTY COMAL COUNTY HA YS COUNTY BEXAR COUNTY GU AD ALUPE COUNTY EXPLAN A TION T rinity aquifer Ed wards aquifer r echarge zone (outcr op) Ed wards aquifer conf ined zone Line of 1,000 milligrams per liter dissolved solids concentration (Schultz, 1994) Study ar ea boundary Fa ul t Spring 0 7 10.5 14 MILES 3.5 98' 98' 98' 98' 98' 29' 30' Comal Springs faul t Springs fault Hueco faul t Cave Ba t fault N o r t h e r n B e x a r fault Creek faul t Hidden Va lley I I Band of high transmissivity reported by Maclay and Small (1984, fig. 20) AREA ENLARGED IN FIGURE 3 San P edro Springs San Antonio Springs Hueco Springs Comal Springs creating a barrier to northeastward flow and forcing water upward along the Comal Springs fault (Klemt and others, 1979, fig. 8). About one-fourth of the springflow from the Comal Springs complex discharges from three large spring orifices (Comal 1, Comal 2, and Comal 3 on the west side of the complex) that are sourced in the upthrown block of the Comal Springs fault (Ogden and others, 1985; LBG-Guyton Associates, 2004). The remaining springflow is discharged from numerous spring orifices and seeps that are within and near the banks of Landa Lake; these springs and seeps are sourced in the downthrown block of the Comal Springs fault. Figure 2. Study area and relation between recharge zone and confined zone of the Edwards aquifer and the Trinity aquifer, northeastern Bexar and southern Comal Counties, Texas. 4 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Figure 3. Locations of major springs in the Comal Springs complex, southern Comal County, Texas. Introduction 5 Elizabeth To rrey Cali fornia Union Lakevie w Klingem ann Edgewater Col e Acorn Allen A ustin Libert y Housto n Chicago Boonevill e M a r y m o n t Hay s el t on Marsh Dallas Oa k w oo d Fred er i cksburg L a n d a P a r k Baden Bell P l a y g r o u n d Mu l berr y C o u r s e Oak T ree Mount Jo y M o n u m e n t Te x as Country Club Dallas Liberty To r rey M ul b er r y Landa Lak e Golf Comal 1 Comal 3 Comal 2 Comal 5 Comal 7 Comal—Spring Island 98'15" 98' 98'45" 29'45" 29' 29'15 " 0 0.1 0.15 0.2 MILE 0.05 EXPLAN AT IO N Ed wards aquifer re charge zone (outcr op) Ed wards aquifer conf ined zone Comal Springs comple x Line of 1,000 milligrams per liter dissolve d solids concentration (Schultz, 1994 ) Spring Base from StreetMap USA, 2004

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Hueco Springs issues from the Hueco Springs fault (fig. 2), which has 380 to 400 feet of offset in the vicinity of the springs (William F. Guyton and Associates, 1979, p. 25) and exposes much of the less-permeable upper confining unit on the downthrown block. The series of fault blocks immediately adjacent to and southeast of the Hueco Springs fault in southern Comal County are oriented mainly down-tothe-northwest (fig. 4). Consequently, this section of Edwards aquifer between the Hueco Springs fault and the Comal Springs fault is in a graben that is tilted opposite to the prevail ing structure in the area. The fault blocks near Hueco Springs fault generally contain the entire thickness of the Edwards aquifer and parts of the upper confining unit, whereas the fault blocks near Comal Springs fault generally contain incomplete, unconfined sections of the Edwards aquifer exposed at the surface. The fault blocks contain numerous transverse faults that possibly impede ground-water flow. Northwest of Hueco Springs fault are faults that probably are not effective barriers to flow, including Bat Cave, Bear Creek, and Hidden Valley faults (fig. 2). Acknowledgments Thanks are extended to SAWS and other well own ers who granted permission to have their wells sampled for water quality and measured for water levels. Special thanks are extended to Hanson Aggregates, Inc., and New Braunfels Utility (NBU) for access to their wells for use as real-time, continuous water-level observation sites for this study. Methods of Investigation Data Collection and Sample Analysis Six wells were used for the collection of continuous water-level data (fig. 5; table 1). Two real-time, continuous water-level data-collection sites—DX (Hanson) and DX (Solms)—were established to collect hourly ground-water-level data for this study (appendix 1.1, 1.2). Water-level data also were compiled on an hourly basis from four existing USGS real-time, continuous ground-waterlevel observation wells—AY (HCV), AY– 203 (Bexar), DX (NBU–LCRA), and DX– 208 (Bracken). Collection of continuous water-level data at the observation wells began in mid-March 2004 and continued through September 2005 at wells HCV and Bracken and through September 2006 at the four other wells. USGS-com puted springflow data for Comal Springs and Hueco Springs also were compiled for mid-March 2004 through September 2006. All water-level and spring-flow data are in the USGS National Water Information System for Texas (U.S. Geologi cal Survey, 2006a). Ground-water-chemistry and isotope data were collected and compiled from 76 wells and nine springs (and spring ori fices from major springs) during 1996 (fig. 5; table 1). Data were collected for this study during 2003 (and during 2006 at selected sites). Additional data were collected during 1996 as part of the National Water Quality Assess ment (NAWQA) program (U.S. Geological Survey, 2006c). Two wells and two springs provided data from both sampling periods. Sixty-three wells and two springs provided NAWQA data only. Eleven wells and five springs provided data only for this study. Samples were collected for dissolved gases (methane, carbon dioxide, oxygen, nitrogen, and argon), sulfur hexafluo ride (SF 6 ), and tritium ( 3 H) and helium-3 ( 3 He) concentrations from 13 wells and seven springs during 2003 (table 1; appen dix 1.3). 3 H/ 3 He ratios were computed and used to estimate an apparent age (year sampled minus recharge year) of the water. Four wells (DX, DX, DX– 304, and DX) and six springs (Comal 1, Comal 3, Comal 5, Comal 7, Comal-Spring Island, and Hueco A) were resampled in 2006 for 3 H/ 3 He because samples collected in 2003 resulted in either inconclusive or questionable age dates for these wells and springs. The 2006 sample for well DX– 22 resulted in an inconclusive age date. Isotope data were collected and compiled from four rainfall sites (fig. 5, table 1). Two sites provided only data col lected in 1998 as part of the NAWQA program (U.S. Geologi cal Survey, 2006c). The remaining two sites provided both NAWQA data and 2003 data. Water-chemistry data included field properties (water temperature, pH, specific conductance, and dissolved oxygen), major ions, trace elements, nutrients, and alkalinity. Water samples were collected, processed, and preserved using stan dard USGS protocols as described in Wilde and others (1999, 2003, and 2004). The concentrations of major ions, trace elements, and nutrients in the water samples were measured by the USGS National Water Quality Laboratory in Denver, Colo., using approved methods (Fishman and Friedman, 1989; Patton and Truitt, 1992, 2000; Faires, 1993; Fishman, 1993; American Public Health Association, 1998; Garbarino and others, 2006). The ratios of naturally occurring, stable isotopes of hydrogen ( 2/1 H) and oxygen ( 18/16 O) were measured by the USGS Stable Isotope Laboratory in Reston, Va., using approved methods (U.S. Geological Survey, 2005). Results for stable isotope analysis are reported as delta deuterium ( D) and delta 18-oxygen ( 18 O), which represent the relative dif ference in parts per thousand (per mil) between the sample iso tope ratio and the isotope ratio of a known standard (Kendall and McDonnell, 1998). Dissolved gases and SF 6 samples were analyzed by the USGS Chlorofluorocarbon Laboratory in Reston, Va., using approved methods (U.S. Geological Survey, 2006b). 3 H/ 3 He samples collected in 2003 were analyzed by personnel at the Nobel Gas Laboratory of Lamont-Doherty Earth Observatory in Palisades, N.Y., and samples collected in 2006 were 6 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Figure 4. Idealized block diagram of the Edwards aquifer between Comal Springs fault and Hueco Springs fault, southern Comal County, Texas. Upper confining unit Edwards aquifer Lower confining unit EXPLANA TION Hueco Springs Co mal Springs Co mal Springs fault Hueco Springs fault Nor thwestern trough Southeastern trough Bracken gap SOUTHWEST NOR THEAST NOT TO SCALE Methods of Investigation 7

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( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Sala do Cr eek eek Bl a nco River Leon Cr eek eek San Antonio Rive r Guada lupe River Canyon Lak e Cibol o Cr eek San Antonio New Braunfel s Bulverde Rainfall site 2 Rainfall site 1 DX DX AY DX DX DX AY AY AY DX DX 1 DX– 206 DX 2 DX DX 9 DX DX DX 5 DX DX 0 DX DX DX DX DX DX 0 DX DX DX DX DX AY 9 AY AY 0 AY AY AY AY AY AY AY AY AY AY AY AY AY 4 AY AY Hueco Spr ings San Antonio Sp r ings San P edro Spr ing s BEXA R CO UNTY KEN D ALL COUNTY COMAL COUNTY HA YS COUNTY 08168000 ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( Comal R ive r DX DX DX DX DX DX DX Rainfall site 4 Rainfall site 3 DX DX Comal Sp ri ngs ! 08168710 ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( Salado Cr ee k Leo n Cr ee k A Y AY AY A Y A Y A Y A Y A Y A Y AY AY A Y AY A Y AY A Y AY AY A Y A Y A Y AY AY AY A Y AY A Y AY AY AY A Y AY San Antonio 0 5 7.5 10 MILES 2.5 EXPLANA TION T rinity aquife r Ed wa rd s aquifer rechar ge zone (outcr op ) Ed wa rd s aquifer confined zone Stud y area boundary Line of 1,000 milligrams per liter dissolved solids concentration (Schultz , 1994) Spring 98' 98' 98' 29' 30' ! ! ! ! ! Monitoring site s Gr ound-water le vel and identifier Gr ound-water quality and identifier Gr ound-water le vel and quality and Rainfall isotopes and identifier Spring dischar ge and identifier A Y A Y DX Rainfall site 4 08168710 identifie r Figure 5. Locations of wells and spring orifices from which water-quality samples were collected and locations of continuous water-level observation wells, northeastern Bexar and southern Comal Counties, Texas . 8 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data were collected and compiled for this report—Continued. USGS site number State well number (fig. 5) USGS name Aquifer Total depth (feet) Open interval (feet) Comple tion type Data type Period of record Ground-water sites (wells, springs, and spring orifices) 293746098265401 AY NA Edwards 290 225 P QW, I 1998 293508098375101 AY NA Edwards 270 190 P QW, I 1998 293429098373801 AY NA Edwards 260 190 P QW, I 1998 293516098362801 AY NA Edwards 320 250 P QW, I 1998 293518098332601 AY NA Edwards 435 162 X QW, I 2005 293504098332601 AY NA Edwards 485 273 X QW, I 1996 293530098343401 AY NA Edwards 281 201 P QW, I 1998 293516098325501 AY NA Edwards 300 220 P QW, I 1998 293635098302301 AY NA Edwards 300 197 P QW, I 1998 293535098304101 AY NA Edwards 241 170 P QW, I 1998 293611098311901 AY NA Edwards 280 190 P QW, I 1998 293350098355801 AY NA Edwards 310 224 P QW, I 1998 293425098350801 AY NA Edwards 310 240 P QW, I 1998 293348098334101 AY NA Edwards 305 245 P QW, I 1998 293340098344701 AY NA Edwards 302 202 P QW, I 1998 293436098343001 AY NA Edwards 261 181 P QW, I 1998 293439098324101 AY NA Edwards 261 181 P QW, I 1998 293408098331301 AY NA Edwards 280 179 P QW, I 1998 293451098313201 AY NA Edwards 500 40 X QW 2005 293459098321401 AY NA Edwards 260 200 P QW, I 1998 293111098340901 AY NA Edwards 685 420 X QW 2005 293133098303201 AY NA Edwards 640 528 X QW, I 1997 293042098305201 AY NA Edwards 787 448 X QW 2005 293522098291201 AY HCV Edwards 547 90 X WLC 2004 293512098291701 AY NA Edwards 600 197 X QW, I 1996 293559098284801 AY NA Edwards 260 160 P QW, I 1998 293534098282801 AY NA Edwards 241 160 P QW, I 1998 293528098274301 AY NA Edwards 201 120 P QW, I 1998 293504098270901 AY NA Edwards 222 142 P QW, I 1998 293520098254101 AY NA Edwards 222 101 P QW, I 1998 293537098262401 AY NA Edwards 180 120 P QW, I 1998 293643098264001 AY NA Edwards 261 180 P QW, I 1998 293615098262301 AY NA Edwards 257 188 P QW, I 1998 Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data were collected and compiled for this report. [Datum for open interval is land surface; USGS, U.S. Geological Survey; NA, not applicable; P, perforated or slotted; QW, water chemistry; I, deuterium and oxygen isotopes; X, open hole; WLC, continuous water level; DG, dissolved gas; SF, sulfur hexafluoride; THE, tritium and helium isotopes; W, walled; S, screen; --, unknown; R, wire-wound; DIS, discharge] Methods of Investigation 9

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Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data were collected and compiled for this report—Continued. USGS site number State well number (fig. 5) USGS name Aquifer Total depth (feet) Open interval (feet) Comple tion type Data type Period of record Ground-water sites (wells, springs, and spring orifices)—Continued 293551098244801 AY NA Edwards 295 56 X QW, I 1996 293359098290301 AY NA Edwards 710 380 X QW 2004 293456098280201 AY NA Edwards 181 111 P QW, I 1998 293358098231101 AY NA Edwards 811 359 X QW, I, DG, SF, THE 2003 293120098285801 AY NA Edwards 848 408 X QW 2004 293055098264301 AY NA Edwards 612 209 X QW, I, DG, SF, THE 2003 293149098224201 AY NA Edwards 784 360 X QW, I, DG, SF, THE 2003 293026098241401 AY NA Edwards 845 423 X QW, I, DG, SF, THE 2003 293100098225401 AY NA Edwards 655 493 X QW, I 1997 293145098224201 AY NA Edwards 870 409 X QW 2004 293525098213701 AY NA Edwards 710 314 X QW, I 1997 293440098195201 AY NA Edwards 777 427 X QW, I, DG, SF, THE 2003 293119098211201 AY NA Edwards 877 530 X QW 2004 292648098303701 AY NA 1 Edwards NA NA NA QW, I 1997 292648098303401 AY NA 1 Edwards NA NA NA QW, I 1997 292944098292301 AY NA Edwards 1050 476,050 X QW 2004 292931098274601 AY NA Edwards 557 390 X QW 2005 292845098255401 AY Bexar Edwards 874 491 X WLC 2004 292808098230101 AY NA Edwards 685 600 X QW, I 1997 292522098291901 AY NA Edwards 1114 774,114 X QW 2004 292643098241801 AY NA Edwards 1150 1012,150 X QW 2004 295458098143001 DX NA Trinity 180 59 W QW, I 1996 295352098071201 DX NA Edwards 240 239 S QW, I 1996 295013098255201 DX NA Trinity 252 123 X QW, I 1996 294743098291801 DX NA Trinity 500 40 X QW, I 1996 294650098265801 DX NA Trinity 580 100 W QW, I 1996 294739098075301 DX NA Edwards 375 220 W QW, I 1996 294533098082301 DX Hueco A 1 Edwards NA NA NA QW, I, DG, SF, THE 2003 294533098082401 DX Hueco B 1 Edwards NA NA NA QW, I, DG, SF, THE 2003 294604098060801 DX NA Edwards 400 200 W QW 2005 294344098253801 DX NA Trinity 464 --QW, I 1996 294437098235201 DX NA Trinity 500 160 X QW, I 1996 293937098182501 DX NA Edwards 405 --QW, I, DG, SF, THE 2003 293807098155301 DX NA Edwards 255 148 X QW 2005 293823098153801 DX NA Edwards 204 --QW, I, DG, SF, THE 2003 10 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Table 1. Ground-water sites (wells, springs, and spring orifices), springflow sites, and rainfall sites, northeastern Bexar and southern Comal Counties, Texas, from which data were collected and compiled for this report—Continued. USGS site number State well number (fig. 5) USGS name Aquifer Total depth (feet) Open interval (feet) Comple tion type Data type Period of record Ground-water sites (wells, springs, and spring orifices)—Continued 293745098162001 DX NA Edwards 450 400 X QW, I 1996 293838098161601 DX Hanson Edwards 248 188 X WLC 2004 294440098132701 DX NA Edwards 450 --QW, I, DG, SF, THE 2003 294323098115101 DX NA Edwards ---QW, I 1996 294300098080001 DX Comal 1 1 Edwards NA NA NA QW, I, DG, SF, THE 1997 294249098080301 DX NA Edwards 1045 586,045 X QW, I 1997 294239098081401 DX NBU–LCRA Edwards 1061 406,061 X WLC, QW, I, DG, SF, THE 1998 294248098081201 DX Comal 3 1 Edwards NA NA NA QW, I, DG, SF, THE 2003 294255098080501 DX Comal 7 1 Edwards NA NA NA QW, I, DG, SF, THE 2003 294304098075501 DX Comal-Spring Island 1 Edwards NA NA NA QW, I, DG, SF, THE 2003 294314098074101 DX Comal 5 1 Edwards NA NA NA QW, I, DG, SF, THE 2003 294054098104601 DX NA Edwards 210 25 X QW, I, DG, SF, THE 2003 294054098103501 DX Solms Edwards 448 46 X WLC 2004 294019098114701 DX NA Edwards 215 119 X QW 2005 294225098080301 DX NA Edwards 365 92 X QW 2004 294206098090101 DX NA Edwards 790 149 P 239 X QW, I, DG, SF, THE 2003 294137098093201 DX NA Edwards ---QW, I 1996 293949098141001 DX NA Edwards 217 --QW, I, DG, SF, THE 2003 294428098063701 DX NA Edwards ---QW, I 1996 293653098201501 DX NA Edwards 230 --QW, I, DG, SF, THE 2003 293636098190901 DX Bracken Edwards 292 220 R WLC 2004 272 P 293729098173101 DX NA Edwards 660 185 X QW 2005 Springflow sites 08168710 NA Comal Springs at New Braunfels, Tex. 1 NA NA NA NA DIS 2004 08168000 NA Hueco Springs near New Braunfels, Tex. 1 NA NA NA NA DIS 2004 Rainfall sites 295025098243200 NA Rainfall site 1 NA NA NA NA I 1998 295104098171900 NA Rainfall site 2 NA NA NA NA I 1998 294230098093300 NA Rainfall site 3 NA NA NA NA I 1998 294233098093300 NA Rainfall site 4 NA NA NA NA I 1998 1 Spring or spring orifice. Methods of Investigation 11

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analyzed by USGS personnel at the USGS Noble Gas Labora tory in Denver, Colo., using approved methods (U.S. Geologi cal Survey, 2006b). Quality Control and Quality Assurance of Geochemical Samples Duplicate samples for major ions and trace elements were collected from one of the 13 wells (DX) sampled for this study (appendix 1.3). Duplicate samples for SF 6 were collected from all 13 wells sampled for this study. Duplicate samples for major ions, trace elements, and nutrients collected during 1996 as part of the NAWQA program (U.S. Geo logical Survey, 2006c) were available for seven of the 65 wells supplying existing data for this study. Duplicate samples for major ions, trace elements, and nutrients were collected at one of the seven springs (DX) sampled for this study (appendix 1.3). Duplicate samples for SF 6 were collected from all seven springs sampled for this study. Duplicate samples were used to evaluate the methods used by field and laboratory personnel to collect and analyze a given sample with consistent results. The nonzero relative percent differences between environmental and duplicate samples collected for major ions, trace elements, and nutrients for this study ranged from 0.41 percent for nickel in the spring sample to 54.5 percent for molybdenum in the spring sample with a median value of 3.1 percent. The greatest relative percent difference between the environmental and duplicate samples collected for SF 6 for this study was 10.3 percent in well DX. Methods of Data Analysis Hydrologic Data Water levels measured in wells in the Edwards aquifer during October 30–November 3, 2000, in a multi-agency effort headed by the Edwards Aquifer Authority (Hamilton and Schindel, 2006) were used to construct a potentiometric-sur face map for the study area. Water-level contours within each flow path were examined to determine flow directions in the Edwards aquifer. Hydrographs of water levels at the six observation wells and discharge at Comal Springs and Hueco Springs were compared. Similarities in the water-level hydrographs might indicate either that the wells are responding to a wide-spread recharge event or that they are in the same flow path. Similari ties in hydrographs for the spring discharge and water levels in wells could indicate that the spring and wells share the same flow path. Statistical correlations between datasets of water lev els for the six observation wells and discharge for Comal Springs and Hueco Springs were analyzed using Pearson’s r (Helsel and Hirsch, 1995). Pearson’s r (or linear correlation coefficient) is a measure of the linear association between two variables. Pearson’s r was calculated for each of the 28 combinations of datasets to indicate the strength of the linear association (correlations) between datasets. The assumption was that a strong linear correlation indicates a higher probabil ity of a shared or common flow path than a nonlinear correla tion or no correlation. Hydrograph recession-curve analysis was done on spring-discharge hydrographs to identify the number and type of flow regimes (diffuse, fracture, and conduit) that charac terize the ground-water flow path contributing to the spring. Hydrographs of spring discharge at Comal Springs and Hueco Springs were graphed, and recession curves (sections of the hydrograph where the discharge is falling after a sudden rise) spanning the study period were examined. Methods in Milanovich (1981), Bonacci (1993), Padilla and others (1994), Shevenell (1996), and Baedke and Krothe (2001) were used to examine the recession curves for breaks in the recession slope, which are indicative of a change from one flow regime to another within the karst continuum. A recession coeffi cient ( ) was calculated for each part of the karst continuum. The value of relates to the rate of release of water from the aquifer. In general, higher values of indicate a steeper slope in the recession curve, and therefore, a release of water from conduit-type features in the aquifer, whereas lower values of indicate a gentler recession slope and release of water from diffuse (matrix) features of the aquifer. Geochemical Data Geochemical data collected and compiled from selected wells and springs in the Edwards and Trinity aquifers in the study area were used to assess major-ion chemistry and appar ent ground-water age (table 1). Ground-water flow paths were analyzed using geochemical and isotopic data. Additional 18/16 O and 2/1 H isotopic concentrations were estimated from rainfall samples to determine the local meteoric water line. Dissolved gases and SF 6 were used to determine appar ent age (year sampled minus recharge year) of ground-water samples. Apparent age is determined by comparing the con centration of SF 6 in the water sample to an established annual atmospheric concentration (Busenberg and Plummer, 2000). SF 6 is a trace gas in the atmosphere that accumulates in rain fall that eventually becomes recharge to the ground-water sys tem. Mainly an anthropogenic compound, SF 6 also can occur naturally in fluid inclusions in some minerals and igneous rocks and in some volcanic and igneous fluids. Apparent age derived using this method does not take into account the mix ing of young and old waters. The addition of excess air into the ground-water system, which can occur when air bubbles are dissolved during a rapid rise of the water table, increases the SF 6 concentration in the ground water to levels greater than the air-water equilibrium concentration. If the existence of 1 2 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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excess air is not considered in the computation of the recharge year, then the apparent age will be too young. Apparent ages also were obtained using a method that involves measuring the relative abundance of tritium ( 3 H) and helium-3 ( 3 He) in a water sample. Tritium is a radioactive iso tope of hydrogen that decays to 3 He with a half-life of 12.43 years. Tritium was introduced into the ground-water system in a series of peaks beginning in 1952, caused by atmospheric testing of nuclear devices, and reached a maximum concentra tion during 1963 (Schlosser and others, 1988). Although tritium concentrations in rainfall generally have decreased since this mid-1960s peak, an annual atmospheric concen tration remains. Using a helium isotope mass balance, the amount of 3 He from the decay of 3 H (tritiogenic 3 He denoted as 3 He*) is measured with the remaining 3 H in the water sample (Plummer and others, 2003). The values are added to determine the amount of 3 H that was present in the water sample at the time of recharge to the ground-water system. This calculated recharge concentration of 3 H then is compared to the established annual atmospheric concentration to deter mine an apparent age or recharge year for the water sample. This method of age-dating a water sample has proved to be effective for waters recharged within about the past 30 years and takes into account the mixing of young and old waters. Problems arise with this method when large amounts of ter rigenic helium (derived from crustal or mantle sources) are present in a sample, such as in aquifers with host rock contain ing uranium or thallium, or in samples where young water has mixed with relatively old water containing terrigenic helium sources. In these cases, the ratio of 3 He/ 4 He for the terrigenic helium must be known within about 1 percent to determine an apparent age. If this ratio cannot be defined within the 1-percent limit, a range in age can be evaluated for a range in the terrigenic helium ratio (Schlosser and others, 1988). Major-ion and stable isotope data were graphically evaluated to determine relations among constituents that could distinguish differences between flow paths in the study area. Piper trilinear diagrams were used to visually categorize the principal water compositions for each flow path (Hem, 1992). Scatterplots of constituents and ratios of constituents were constructed to graphically indicate variations in water samples collected from wells from different flow paths. Geologic and Hydrologic Identification of Flow Paths Southern Comal Flow Path The southern Comal flow path (SCFP) (fig. 6) is bounded on the northwest by the Comal Springs fault and on the southeast by the freshwater/saline-water interface (threshold of 1,000-milligrams per liter (mg/L) dissolved solids concentra tion [Schultz, 1994]) in the Edwards aquifer (fig. 1). Although the interface is not an actual physical barrier to flow, previous studies (Maclay and Small, 1984; Groschen, 1994; Lindgren and others, 2004) have indicated that flow in the transition zone (zone in which dissolved solids concentration ranges from 1,000 to 10,000 mg/L) is considerably more sluggish than flow in the immediately adjacent freshwater zone, likely because of relatively low permeability and transmissivity in the transition zone. The 1,000-mg/L threshold was selected as the boundary because that concentration historically has been considered the separation between the freshwater and salinewater zones of the aquifer and is a well-documented marker within the aquifer. The potentiometric-surface map constructed from data collected in fall 2000 in a multi-agency effort organized by the Edwards Aquifer Authority indicates that water within the SCFP flows from southwest to northeast from areas of higher water-level altitude to areas of lower water-level altitude (fig. 7). Recharge to the SCFP is primarily regional, occurring in the Edwards aquifer recharge zone west of the study area. The Comal Springs fault fails to offset the entire thickness of the Edwards aquifer near the area where the boundaries of Bexar, Comal, and Guadalupe Counties come into contact. Maclay and Land (1988) referred to this area as the Bracken gap (fig. 6). Subsurface inflow to the SCFP can occur as ground water spills over from north of Comal Springs fault across the Bracken gap. The permeable sections of the aquifer in the SCFP, although faulted, are juxtaposed in a manner that promotes the flow of ground water in the study area from the center of Bexar County through southern Comal County. The ground water then encounters a transverse fault northeast of New Braunfels (fig. 6) that forms a barrier to ground-water flow and is forced up along the Comal Springs fault and through the overlying gravels to form a majority of the springs and seeps in the Comal Springs complex. The SCFP gradu ally narrows as it nears Comal Springs because saline water encroaches as the freshwater is discharged at the springs. The transmissivity in the freshwater zone of the Edwards aqui fer southeast of the Comal Springs fault in southern Comal County is greater than in any other part of the aquifer (Maclay and Small, 1984). Water-level hydrographs for wells Bexar, Bracken, Solms, and NBU–LCRA and the discharge hydrograph for Comal Springs (fig. 8) provided evidence of patterns of ground-water flow in the SCFP. The hydrographs for the four wells and Comal Springs fluctuated in a similar manner, pos sibly indicating a common flow path for ground water flowing past the wells and discharging at Comal Springs. Wells Bexar and Bracken showed larger magnitudes of fluctuation than did wells Solms and NBU–LCRA. This indicates that amplitude of the pressure wave moving through the confined section of the aquifer likely is decreased by the substantial increase in horizontal hydraulic conductivity of the aquifer in the SCFP near Comal Springs. The relatively flat potentiometric surface near Comal Springs (fig. 7) also provides evidence of higher horizontal hydraulic conductivity at the northeastern end of the SCFP. Geologic and Hydrologic Identification of Flow Paths 1 3

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Figure 6. Locations of hypothesized ground-water flow paths, northeastern Bexar and southern Comal Counties, Texas. 1 4 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas Canyon Lake COMAL COUNTY BLANCO COUNTY KEN D ALL COUNTY BEXAR COUNTY HA YS COUNTY Bracken ga p Alamo Heights hors t 0 1 0 1 5 20 MILE S 5 S Norther n Comal flow path and inferr ed dir ection of flow Central Comal flow path and inferr ed dir ection of flow Souther n Comal flow path and inferr ed dir ection of flow Ed wards aquifer r echarge zone (outcr op) Study ar ea boundary Line of 1,000 milligrams per liter dissolved solids concentration (Schultz, 1994) Inferr ed flow path barrier Inferr ed normal fault Normal fault Unspecif ied fault Spring and identif ier Wa ter -quality site and identif ie r EXPLAN A TION Comal Springs S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S 98' 98' 98' 29' 30' Cibolo Cr eek San An t onio River Leon Cr eek Salado Cr eek B l an c R r G u a d a l u p e R i v e r New Braun fels San P edro Springs San Antonio Springs Hueco Springs Comal Springs S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S S o C S S S S ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( AY (HCV) DX (Solms ) AY (Bexar ) DX (Hanson) DX (Bracken) DX (NBU-LCRA) Hueco Springs near New Braunfel s Comal Springs at New Braunfel s ! ( AY (HCV) Comal Springs faul t Springs fault Hueco fault Cav e Ba t fault N o r t h e r n B e x a r fault Cr e ek faul t Hidden Va lley Bear

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BEXAR COUNTY COMAL COUNTY 98' 98' 98' 98' 29' 29' 29' 30' 0 1 01 5 20 MILES 5 EXPLANA TION W ater -le vel altitude , in feet abo ve NA VD 88 596 to 630 630 to 664 664 to 698 698 to 732 732 to 766 766 to 800 800 to 834 834 to 868 868 to 902 902 to 936 Nor thern Comal flo w path boundar y Central Comal flo w path boundar y Southern Comal flo w path boundar y ! 745 Inferred fl ow path internal barrier Line of 1,000 milligrams per liter dissolved solids concentration (Shultz, 1994) Stu dy area boundary Gr ound-water -le vel site and water le vel altitude , in feet ab ov e NA VD 88 Spring ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 708 69 6 69 2 68 4 68 4 68 3 69 3 69 3 69 8 683 708 71 1 69 7 69 5 69 4 69 8 69 4 69 7 69 7 70 0 70 0 706 74 5 69 2 69 4 70 7 74 0 77 1 772 72 4 76 2 770 73 1 706 72 1 78 3 749 78 1 819 762 74 3 80 0 78 5 768 78 9 843 82 8 823 79 5 83 4 861 927 872 778 78 2 68 9 62 5 68 0 67 9 67 1 62 9 65 4 667 65 1 65 9 63 7 63 0 63 5 65 1 662 66 5 69 7 68 6 63 1 60 4 70 5 68 4 65 0 64 6 64 1 64 4 62 5 62 6 62 2 73 3 72 5 792 84 8 79 2 69 7 69 3 68 4 Comal Sp ri ngs Figure 7. Potentiometric surface of Edwards aquifer in northeastern Bexar and southern Comal Counties, Texas, fall 2000. Geologic and Hydrologic Identification of Flow Paths 1 5

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620 640 660 680 700 720 740 760 3/14/2004 6/22/2004 9/30/2004 1/8/2005 4/18/2005 7/27/2005 11/4/2005 2/12/2006 5/23/2006 8/31/2006 DAT E WATER LEVEL, IN FEET ABOVE NGVD 29 0 100 200 300 400 500 600 SPRING DISCHARGE, IN CUBIC FEET PER SECON D HCV (AY) Hanson (DX) Bexar (AY ) Bracken (DX) Solms (DX ) NBU–LCRA (DX) Comal Springs (08168710) Hueco Springs (08168000 ) Figure 8. Hydrographs of water levels from six wells and discharge for Comal Springs and Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 2004–September 2006. 1 6 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Central Comal Flow Path The central Comal flow path (CCFP) (fig. 6) is bounded on the northwest by the Hueco Springs fault and on the southeast by the Comal Springs fault. The potentiometricsurface map indicates that water within the CCFP flows from southwest to northeast (fig. 7). As water moves into central Bexar County from the west it is bifurcated into two flow paths by the Alamo Heights horst (Maclay and Land, 1988) (fig. 6) in the subsurface southwest of the recharge zone in central Bexar County. Water that flows north of the horst becomes constrained northeast of the Comal Springs fault, which completely offsets the Edwards aquifer east of the horst, and becomes flow within the CCFP, whereas water that flows south of the horst becomes flow within the SCFP. Local recharge to the CCFP occurs in northeastern Bexar County as water infiltrates into the Edwards aquifer outcrop northwest of the Northern Bexar fault (fig. 6). The water then moves through the subsurface to the southeast until it encounters the Northern Bexar fault. The Northern Bexar fault acts as a barrier to flow at its northeastern end (Small, 1986, fig. 8), causing water to move to the southwest along the fault trace. The displacement along the Northern Bexar fault decreases southwestward along its trace until it is no longer a barrier to flow (Small, 1986, fig. 9), which allows water to enter the con fined section of the aquifer. The local recharge remains north of Comal Springs fault as it merges with the northeastwardmoving water of the horst-induced CCFP. Additional inflow to the CCFP might occur in the subsurface when water from north of the Hueco Springs fault in Comal County crosses over the Hueco Springs fault in areas where the fault fails to fully offset the Edwards aquifer (Small, 1986, figs. 4). As the water in the CCFP continues flowing toward the northeast, structural features in the Edwards aquifer affect the patterns of flow. In eastern Bexar County, the Bracken gap conveys water from the CCFP into the SCFP (fig. 6). Beyond the Bracken gap, the aquifer containing the CCFP is heavily faulted and partially unconfined in the area directly northwest of Comal Springs. In the confined section of the CCFP in this area, the flow is funneled into two hydraulically con nected troughs that follow roughly parallel grabens formed in the fault blocks between Comal Springs fault and Hueco Springs fault (fig. 4). The southeastern trough is unconfined at its northeastern end and receives recharge from direct infiltration of rainfall and streamflow losses. The northwest ern trough receives subsurface inflow across sections of the Hueco Springs fault that fail to offset the entire thickness of the Edwards aquifer. Although flow within this section of the CCFP is complicated by the complexity of the fault ing, in general, much of the water in the northwestern trough likely discharges at Hueco Springs and most of the water in the southeastern trough and in the unconfined sections of the aquifer in the CCFP discharges at springs Comal 1, Comal 2, and Comal 3 at the Comal Springs complex. Water that is not discharged at either Hueco or Comal Springs continues north eastward toward the San Marcos Springs (fig. 1). Comparison of the water-level hydrographs for wells HCV and Hanson and discharge hydrographs for Comal Springs and Hueco Springs (fig. 8) provide insight into the movement of ground water in the CCFP. The hydrograph for well HCV shows little, if any, similarity to the hydrographs for well Hanson or either of the springs, indicating a lack of shared flow paths between well HCV and the other sites in the CCFP. Well HCV is in a heavily faulted section of the CCFP that is most likely locally isolated from the rest of the flow path. Well Hanson is in the unconfined section of the south eastern trough of the CCFP. In general, the hydrograph for well Hanson followed the fluctuation patterns of the discharge hydrographs of Comal Springs and Hueco Springs in number and relative vertical displacement for water levels greater than about 700 feet above NGVD 29. In this area, the regional dense member of the Person Formation within the Edwards aquifer acts as a confining unit. Well Hanson is completed below the regional dense member (approximate altitude 700 feet above NGVD 29). When water levels in well Hanson rose above the regional dense member, the large magnitude of fluctuations in the hydrograph reflected confined properties similar to those at Comal Springs and Hueco Springs. When water levels in well Hanson fell to levels near or below the regional dense member, the Hanson well hydrograph tended to lose much of the vertical displacement and actually resembled the hydrographs of wells Solms and NBU–LCRA. As water levels fell in well Hanson and thus in that section of the CCFP, conditions changed from confined to unconfined. Furthermore, confined flow from the SCFP, under extreme pressure, might have pushed upward across Bracken gap into the CCFP to locally influence water levels in the CCFP. The Hueco Springs hydrograph (fig. 8) provided evidence that the CCFP might not be the only source of water to the Hueco Springs. An increase in discharge at Hueco Springs in August 2005 had no corresponding increase at Comal Springs, indicating that the pulse of water causing the increased dis charge at Hueco Springs either was not sourced in the CCFP or did not reach Comal Springs. Northern Comal Flow Path The northern Comal flow path (NCFP) (fig. 6) is in the Edwards aquifer recharge zone north of Hueco Springs fault and south of Bat Cave fault in Comal County. The potentio metric-surface map indicates that water in the NCFP flows from southwest to northeast (fig. 7). Recharge to the NCFP occurs from direct infiltration of rainfall and streamflow losses to the Edwards aquifer exposed at the surface in the NCFP. Additional inflow likely comes from the Trinity aquifer in the subsurface where the Bat Cave fault juxtaposes the Trinity aquifer against the Edwards aquifer. An undetermined amount of water from the NCFP might flow into the confined sec tions of the CCFP across Hueco Springs fault in areas where the fault does not completely offset the Edwards aquifer. The Geologic and Hydrologic Identification of Flow Paths 1 7

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water remaining north of the Hueco Springs fault flows toward San Marcos Springs. Correlation of Water Levels Statistical correlation between water levels for the six observation wells and spring discharges for Comal Springs and Hueco Springs provides additional evidence for the hypothesized flow paths. Figure 9 graphically demonstrates the relations involving each of the 28 combinations of paired datasets. The Pearson’s r correlation coefficient indicates the strength of the linear correlation between dataset pairs and, thus, between water levels or spring discharges, or both. The larger the absolute value of r between -1 and 1, the stronger the linear correlation. The strongest correlations (r-values ranging from .991 to .999) were observed between the datasets for wells NBU–LCRA and Solms (SCFP) and well Hanson (CCFP), providing evidence for the hydraulic connection between the SCFP and the CCFP across the Bracken gap. The dataset for well Bracken, the well closest to the Bracken gap, is not as strongly correlated with datasets for wells NBU-LCRA, Solms, and Hanson. The well Bracken might be in a less transmissive part of the aquifer than the other three wells, allowing for greater fluctuations in water levels in the well Bracken dataset, which could account for weaker correlations with datasets from those wells. The dataset for well HCV displayed the weakest correlations with the other datasets, indicating a lack of shared flow paths with water at the other wells, Comal Springs, or Hueco Springs. The dataset for Comal Springs was strongly correlated with those for wells Bexar, NBU–LCRA, Hanson, and Solms (r-values ranging from .961 to .983) and fairly strongly correlated with the data set from well Bracken (r-value of .954). The dataset for Hueco Springs did not correlate linearly with any of the datasets from the other wells or Comal Springs but did have monotonic rela tions (an increase in one variable corresponds to an increase in the other) with the datasets from wells Bexar, NBU–LCRA, Hanson, and Solms and Comal Springs. One reason for the lack of linear correlation between Hueco Springs and the other Edwards aquifer sites could be the influence of inflow from the Trinity aquifer. In addition, the northwestern trough of the CCFP, which provides most of the flow from the Edwards aquifer to Hueco Springs, could be less hydraulically con nected to the rest of the CCFP during periods of low spring discharge than during periods of high spring discharge. Analysis of Hydrograph Recession Curves Hydrographs of discharge from Comal Springs and Hueco Springs were analyzed. Recession curves representing a range of flows were selected from each hydrograph for use in the analyses of hydrograph recession curves. The reces sion curves were examined for breaks (inflection points) in the recession slope, which are indicative of a change from one flow regime to another within the karst continuum. A reces sion coefficient ( ) was calculated for each section of hydro graph between inflection points using Milanovich’s (1981) equation, Q = Qe t0 tt 0 , where Q 0 is the initial discharge at time t 0 , the beginning of each recession slope, and Q t is the discharge at time t. Solving for yields the equation = ln (Q t /Q 0 ) _____________________ _ (t-t 0 ) . For the hydrographs of this study (fig. 9), an of about 0.18, corresponding to a steep (nearly vertical) slope, indicates con duit-driven drainage of a karst aquifer. An of about 0.008, corresponding to a more horizontal slope, indicates the diffuse drainage of the primary porosity in the matrix of the aquifer. An of about 0.02.09, corresponding to an intermediate slope, can indicate either drainage of the fractures in the aqui fer or a mixture of conduit and diffuse drainage. Figure 10 illustrates discharge magnitude and variability for Comal Springs and Hueco Springs from mid-March 2004 through August 2006. Comal Springs discharge was con sistently greater and more variable than discharge at Hueco Springs. In general, increases in discharge at Hueco Springs were smaller and shorter in duration than increases at Comal Springs. Discharge-recession curves for Comal Springs were used to identify the flow regimes in the aquifer contributing to the springs. Flow varied from 202 to 509 cubic feet per second (ft 3 /s) during 2004 with the highest discharges occurring during November 21, 2004. The recession curve for the highest-discharge period indicates that fracture flow, or a mixture of conduit flow and diffuse flow, first dominated the ground-water system supplying water to the springs, followed by conduit flow and then fracture flow, or a mix ture of conduit and diffuse flow, again. This pattern indicates that, during periods of high-discharge recession at the Comal Springs complex, the ground-water system supplying the springs might include upper and lower layers of the aquifer dominated by fractures or a mixture of conduits and aquifer matrix, with a middle section of the aquifer dominated by conduits. The lowest discharges occurred near the end of the study period (August 2006). During the lowest-discharge period, July 2005–August 2006, the recession curves for Comal Springs discharge reflected mostly an of about 0.09, indicating fracture flow or a mixture of conduit and diffuse flow. The majority of the recession slopes for the Comal Springs hydrograph for the 2-year period had an of about 0.09, indicating that the overall flow through the aquifer to Comal Springs is fracture flow or a mixture of conduit and diffuse flow. During the 2-year period, values less than 0.02 were not observed for Comal Springs, indicating that dif fuse flow never dominated the flow system supplying Comal Springs. 1 8 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Figure 9. Correlations between water levels for six wells and discharge for Comal Springs and Hueco Springs, northeastern Bexar and southern Comal Counties, Texas, March 2004–September 2006. AY Bexar DX NBU–LCRA DX Hanson DX Solms AY HCV DX Bracken 0816871 0 Comal Spring s 08168000 Hueco Springs SITE NUMBER AND NAM E Pearson's r equal to or greater than .9 9 Pearson's r equal to or greater than .96 but less than .99 Pearson's r equal to or greater than .90 but less than .96 Pearson's r less than .90 EXPLANATIO N Geologic and Hydrologic Identification of Flow Paths 1 9

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DATE DISCHARGE, IN CUBIC FEET PER SECOND Hueco Springs (08168000) Comal Springs (08168710) = 0.02, frac ture flow or mix ture of conduit flow and diffuse flow (with diffuse dominant) = 0.18, conduit flo w = 0.008, diffuse flo w = 0.09, frac ture flow or mix ture of conduit flow and diffuse flow (with conduit dominant) 3/14/2004 6/22/2004 9/30/2004 1/8/2005 4/18/2005 7/27/2005 11/4/2005 2/12/2006 5/23/2006 8/31/2006 0 100 200 300 400 500 November 21, 2004 November 2004 July 2005–August 2006 Figure 10. Hydrographs of discharge at Comal Springs and Hueco Springs, southern Comal County, Texas, March 2004–September 2006, showing recession coefficients for recession curves. 2 0 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Discharge-recession curves for Hueco Springs were ana lyzed to identify the flow regimes in the aquifer contributing to the springs and to attempt to obtain additional evidence of contribution from the Trinity aquifer. Flow at Hueco Springs varied from 5.3 to 148 ft 3 /s during 2004 with the highest discharges occurring during November 2004. The recession curve for the highest-discharge period indicates that fracture flow or a mixture of conduit and diffuse flow first dominated the ground-water system supplying water to the springs. Fol lowing this period of fracture or mixed drainage, conduit flow controlled the release of water to the springs, followed by another period of fracture or mixed flow. The second period of fracture or mixed drainage was followed by periods in which diffuse flow became increasingly dominant and eventually prevailed. The lowest discharges occurred near the end of the study period (August 2006). During the lowest-discharge period, July 2005–August 2006, the increases in discharge at Hueco Springs were characterized by sudden high increases with recession curves that were dominated by conduit flow followed by fracture or conduit/diffuse mixed flow and ending with diffuse flow. During 2004, the recession slopes with an of about 0.18 represented the greatest number of slopes for the Hueco Springs hydrograph, indicating that initial drain age of the aquifer to Hueco Springs following a recharge event is dominated by a fast-draining conduit system. Following the drainage of the conduits, the system typically is characterized by either fracture flow or a mixture of conduit and diffuse flow, or diffuse flow. Although the Comal Springs complex and Hueco Springs are only about 3 miles apart, the hydrograph recession slope analysis for the two springs reinforces the fact that the springs are sourced from a complex karst system. The initial recession slopes for Comal Springs predominately represent a mixture of conduit and fracture flow, or a mixture of conduit and diffuse flow, whereas the initial recession slopes for Hueco Springs predominately represent conduit flow. The initial mixed flow from Comal Springs likely represents the conduit flow contribution from the SCFP, which provides most of the flow from the springs, and the fracture flow contribution from the CCFP. The initial conduit flow from Hueco Springs provides evidence that the northwestern trough within the CCFP in Comal County might be highly transmissive, allow ing for rapid movement of water through the aquifer to Hueco Springs. Geochemical Identification of Flow Paths Major-Ion Chemistry Trilinear diagrams constructed using major-ion chemis try from samples collected at wells in and springs emerging from the SCFP, CCFP, and NCFP (fig. 11) indicate that all three flow paths are dominated by calcium-bicarbonate type water. Relatively little variation in major-ion chemistry was observed. The water samples from wells and springs in the CCFP showed more variability in major-ion chemistry than those from the two other flow paths. The samples from wells in the NCFP consistently had higher percentages of bicarbon ate and lower percentages of sulfate and chloride than the samples collected from sites in the SCFP. The comparatively high sulfate and chloride concentrations in the SCFP samples might indicate that the water in the flow path was mixing with small amounts of saline water from the freshwater/saline-water transition zone. A graph of the relation between calcium concentra tion and the ratio of magnesium to calcium concentrations provided further evidence for the hypothesized flow paths (fig. 12). Samples from wells in the SCFP generally yielded low calcium concentrations coupled with moderate Mg/Ca ratios (thus moderate magnesium concentrations). The highest calcium concentrations and lowest Mg/Ca ratios (contributed to by low magnesium concentrations) were in samples col lected from the CCFP. The samples from the CCFP yielded data points that plotted in a grouping overlapping the data points for the SCFP. The high concentrations of magnesium in the samples collected from the recharge zone in Comal County (NCFP) might be evidence that water from the Trinity aquifer is entering the Edwards aquifer in the subsurface. The calcium concentration of water from Hueco Springs was lower when the spring discharge was low than when the spring discharge was high, indicating that during periods of low discharge Hueco Springs might receive a larger contribution from the Trinity aquifer than during periods of high discharge. Stable Isotopes The relation between D and 18 O in water samples from the Edwards aquifer, by flow path, is shown in figure 13. The graph shows that, except for samples collected from wells in the Edwards aquifer recharge zone in northeastern Bexar County (CCFP) following an unusually intense rain storm produced by a tropical system in October 1998, there was not much variation in stable isotope values among the flow paths. In the study area D in ground water ranged from -36.00 per mil (well AY in the CCFP) to -20.89 per mil (well DX in the SCFP) and 18 O ranged from -6.03 per mil (well AY in the CCFP) to -3.70 per mil (spring Hueco A in the CCFP). Excluding ground-water samples collected from wells in the recharge zone of north eastern Bexar County following the October 1998 storm, the D range in the study area was -33.00 to -20.89 per mil and the 18 O range was -4.60 to -3.70 per mil. The local meteoric water line (LMWL; D=8.8032 18 O+17.825) was calculated using rainfall isotope data col lected in the study area. All ground-water samples, except one, plotted below the LMWL. The plotting positions indicate that the water evaporated, to varying degrees, before entering the aquifer as recharge. Geochemical Identification of Flow Paths 2 1

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Figure 11. Trilinear diagrams showing composition of ground water in the Edwards aquifer in the southern, central, and northern Comal flow paths, northeastern Bexar and southern Comal Counties, Texas. PER C EN T MILLIEQ UIV ALENTS SOUTHERN COMAL FL OW P AT H CAL C IU M 100 80 60 40 20 0 0 20 40 60 80 100 M AG NESI U M 0 20 40 60 80 100 SO DI U M PL U S PO TA S SI U M CH LO R ID E, FLU O R IDE, N ITR ITE P LU S N IT R A TE 0 20 40 60 80 100 100 80 60 40 20 0 C ARBO N AT E PLU S BICAR BO N AT E 100 80 60 40 20 0 SUL FA TE 0 20 40 60 80 100 SU LF A TE PLU S C HL O R ID E 0 20 40 60 80 100 C AL C IU M PL U S MA G NESIUM 100 80 60 40 20 0 100 80 60 40 20 0 100 0 20 40 M AG N PERCENT MILLIEQUIV ALENTS CENTRAL COMAL FLO W P AT H CALCIUM 80 60 40 20 0 60 80 100 ESI U M 0 20 40 60 80 100 SO D IU M PL U S PO TA S SI U M CHLORIDE, FLUORIDE , NITRITE PLUS NI TRA TE 0 20 40 60 80 100 100 80 60 40 20 0 C ARBO N AT E PLU S BICA R BO N AT E 100 80 60 40 20 0 SULF AT E 0 20 40 60 80 100 SU LF A TE PLU S C H LO R ID E 0 20 40 60 80 100 CALC IU M PL U S M AG N ES I U M 100 80 60 40 20 0 100 80 60 40 20 0 CALCIU M 10 0 80 60 40 20 0 0 20 40 60 80 10 0 MA G N ESI U M 0 20 40 60 80 100 SO D IUM PL U S PO TA SS I UM CHLORI D E, FLUORIDE , NITRIT E PLUS NITRA T E 0 20 40 60 80 100 100 80 60 40 20 0 C ARBO N A TE PLU S BICAR BO N AT E 100 80 60 40 20 0 SU LF AT E 0 20 40 60 80 100 SU LF A TE PLU S C HL O R ID E 0 20 40 60 80 100 C ALCI U M PL U S M AG NESI U M 100 80 60 40 20 0 10 0 80 60 40 20 0 NOR THER N COMAL FL OW PA TH PE R C EN T MILLIEQ UIV ALENTS 22 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 60 70 80 90 100 110 120 130 140 150 CALCIUM, IN MILLIGRAMS PER LITER MAGNESIUM/CALCIUM Southern Comal flow path Central Comal flow path Northern Comal flow path Hueco Springs during high discharge Hueco Springs during low discharge Trinity aquifer Sample, by sourc e Figure 12. Relation of calcium concentration to magnesium/calcium ratio in samples collected from wells and springs in the Edwards aquifer and wells in the Trinity aquifer, northeastern Bexar and southern Comal Counties, Texas. Geochemical Identification of Flow Paths 23

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-60.00 -50.00 -40.00 -30.00 -20.00 -10.00 0 -10 -9 -8 -7 -6 -5 -4 -3 DELTA OXYGEN-18, IN PER MIL DELTA DEUTERIUM, IN PER MIL Northern Comal flow path Central Comal flow path Southern Comal flow path Trinity aquife r Global meteoric water line (Craig, 1961) Rainfal l Local meteoric water line Sample, by sourc e -40.00 -35.0 0 -30.0 0 -25.0 0 -6.1 -5.6 -5.1 -4.6 -4.1 -3.6 DELTA OXYGEN-18, IN PER MI L DELTA DEUTERIUM, IN PER MI L -20.0 0 Area of graph shown below Figure 13. Relation between delta oxygen-18 and delta deuterium for selected wells and springs, northeastern Bexar and southern Comal Counties, Texas, 1996, with insert showing same plot with axes extended to range of rainfall samples. 24 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Samples collected from wells and springs in the SCFP plotted in a relatively small area on the graph (fig. 13) with D values ranging from -24.40 to -20.89 per mil and 18 O val ues ranging from -4.28 to -3.92 per mil. The largest range in isotopic composition was observed in samples collected from the CCFP, with D values ranging from -36.00 to -21.39 per mil and 18 O values ranging from -6.03 to -3.70 per mil. CCFP samples with D values greater than about -4.3 per mil overlap the grouping of SCFP samples. The samples from wells in the NCFP plot in a narrow area that spans the area occupied by the SCFP samples with D values ranging from -24.40 to -22.44 per mil and 18 O values ranging values ranging from -4.48 to -3.86 per mil. Ground-Water Ages Apparent ages of ground water in samples collected in 2003 derived from SF 6 analysis (fig. 14) ranged from about 21 years in well DX in the NCFP to present day (less than 0.5 year or recharged in 2003) at spring Comal 7 in the SCFP. Ages ranged from about 17 years (well AY) to present day (spring Comal 7) in the SCFP, with the oldest water in the upgradient section of the SCFP and the youngest water from a spring at the downgradient end of the SCFP. The young apparent age of the Comal 7 spring might be evidence that young water from the upthrown block of Comal Springs fault is supplying additional recharge to the spring. The SF 6 method of age dating water does not take into account the mixing of young and old waters and, therefore, reflects the age of the young water. In the CCFP, ages ranged from about 16 years (well DX) to about 1 year (well DX), with both the oldest and youngest water in the upgradient section of the flow path. Water in the NCFP likely receives inflow from the subsurface as a result of inter-aquifer hydraulic connection with the Trin ity aquifer, which has substantially lower transmissivity (thus, possibly water of older apparent age) than does the Edwards aquifer. The apparent age of the water in well DX might reflect the influence of the Trinity aquifer on the NCFP. The apparent ages of the samples collected in 2003 from the various orifices of the Comal Springs complex and Hueco Springs, computed using SF 6 analysis, provide evidence of the complex nature of the karst aquifer in which the springs are sourced. The samples from the orifices at the Comal Springs complex range in age from about 6 years to present day (less than 0.5 year or recharged in 2003). The samples from Hueco Springs provided ages that range from about 7 to 8 years. The recent ages observed from the orifices at Comal Springs indicate an inflow of water near the springs complex, possibly from the unconfined section of the Edwards aquifer at the northeastern end of the southeastern trough of the CCFP in Comaal County. The relatively old water at Hueco Springs indicates a possible inflow of water into the ground-water system supplying the springs from a source of water yielding relatively older water than the Edwards aquifer, possibly the Trinity aquifer. Apparent ages of ground-water samples collected in 2003 derived from 3 H/ 3 He analysis (fig. 15) ranged from greater than 45 years in well DX in the NCFP to present day (less than 0.5 year or recharged in 2003) in well DX in the CCFP. Ages ranged from 21.3 to 8.8 years in the SCFP. In general, the samples from wells in Bexar County (upgradient section of the SCFP) showed younger ages than wells and springs in the downgradient section of the SCFP. The samples from a spring in the Comal Spring complex, Spring Island, indicated ages similar to those for samples from the wells in Bexar County. A possible explanation for the anomalously young age of water in the Comal Springs complex is that there is a local source for the spring. Well DX in the SCFP had a younger age than the wells in Bexar County. This well is on the escarpment formed along the Comal Springs fault and might be receiving inflow from the unconfined CCFP on the upthrown side of the fault. In the CCFP, ages ranged from 17.3 years to present day (less than 0.5 year or recharged in 2003) with the oldest water in well AY near the Bexar-Comal County line in the central section of the flow path and the youngest water in the central section of the flow path where the Edwards aquifer is exposed, allowing recharge from infiltration of rainfall and streamflow losses to occur. The data from samples collected at Hueco Springs indicate very young water (apparent age less than 0.5 year) mixed with very old water. The oldest water was in well DX in the NCFP. Water in this well is tritium depleted, indicating that the water has undergone complete tritium decay. The apparent age of the water is not attainable by the 3 H/ 3 He method, but based on the tritium decay rate the water is estimated to have been recharged more than 45 years ago. A possible source for this old water could be inflow from the underlying Trinity aquifer. Apparent ages for ground-water samples collected in 2006 ranged from 41.1 years at spring Comal 7 in the SCFP to present day (less than 0.5 year or recharged in 2006) in wells DX and DX in the CCFP (fig. 15). In general, samples collected in 2003 yielded younger apparent ages than samples collected during 2006. Water levels in the Edwards aquifer were higher during the sampling in 2003 than they were during the sampling period in 2006, likely result ing in faster flow rates in the aquifer because of increased hydraulic gradients. Many of the samples contained a mixture of young and old water, especially those collected from areas near the downgradient ends of the flow paths. The samples collected from Hueco Springs in 2006, like those collected in 2003, showed a mixture of very young water (apparent age less than 0.5 year) with very old water (possibly sourced from the Trinity aquifer). Geochemical Identification of Flow Paths 25

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Figure 14. Apparent ages of ground water in 2003 samples from selected wells and springs in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas, based on sulfur hexafluoride concentration (modified from Busenberg and Plummer, 2000, fig. 6A). 26 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas DX 4 AY DX DX DX DX DX Hueco A AY DX Hueco B AY DX Comal 3 DX Comal-Spring Islan d DX DX Comal 1 DX DX Comal 5 DX DX Comal 7 W ell or spring and identifier — Refer to table 1 Southern Comal flow path Central Comal flow path Northern Comal flow path AY 4 SULFUR HEXAFLUORIDE CONCENTRA TION, IN P ARTS PER TRILLION BY VOLUME 0 1 2 3 4 5 6 1980 1985 1990 1995 2000 2005 YEAR 20 15 10 5 0 APPARENT AGE

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! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( AY– 68 AY 3 AY AY 0 DX 0 DX 5 DX 0 DX 1 DX 2 DX (Hueco A) DX (Hueco B) DX (Comal 5) DX (Spring Island) DX (Comal 7) DX (Comal 3) DX (NBU) DX (Comal 1) DX 4 <0.5 <0.5 28.2 30.3 41.1 29.1 33.8 <0.5 29.7 06 91 2 MILE S 3 Sample y ear 2006 98' 98' 98 ' 29' 30 ' ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( ! ( AY AY– 68 AY– 68 AY 0 DX 0 DX 5 DX 0 DX 1 DX 2 DX (Hueco A) DX (Hueco B) DX (Comal 5) DX (Spring Island ) DX (Comal 7) DX (Comal 3) ) DX (NBU) DX (Comal 1) DX 4 >4 5 8. 8 <0.5 <0.5 17.3 11.7 1 12.0 8 16.0 6 17.3 9 <0.5 10.7 5 14.5 5 21.3 5 19.8 2 98' 98' 98 ' 29' 30 ' Sample y ear 2003 06 91 2 MILE S 3 AY 16.06 DX 1 11.71 DX 4 >45 > < EXPLANA TIO N Stud y area boundar y ! ( Southern Comal flo w path well , ! ( ! ( identifier , and age of water in year s Central Comal flo w path well , identifier , and age of water in year s No rt hern Comal fl ow path well, identifier , and age of water in year s = greater tha n = less than BE X AR COUNT Y COMA L COUNTY BE X AR COUNT Y COMA L COUNTY Figure 15. Apparent age of water in selected wells in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas, based on tritium/helium-3 ratio. Geochemical Identification of Flow Paths 27

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Summary The U.S. Geological Survey (USGS), in cooperation with the San Antonio Water System, conducted a 4-year study dur ing 2002 to identify major flow paths in the Edwards aqui fer in northeastern Bexar and southern Comal Counties (study area). Geologic structure, surfaceand ground-water data, and geochemistry were used to identify flow paths. Knowledge of geologic structure and previous flow path analyses conducted by the USGS provided a basis for the initial selection of hypothesized flow paths in the study area. Historical and new data were used in analyses. Continu ous water-level data were collected at six observation wells from mid-March 2004 through September 2006. Discharge data were obtained from two springflow sites for the same time period. Ground-water chemistry and isotope data were compiled from samples collected from 76 wells and nine springs (and spring orifices of major springs) during 1996– 2006. Isotope data also were obtained from samples collected at four rainfall-sampling sites during 1998. The southern Comal flow path (SCFP) is bounded on the northwest by the Comal Springs fault and on the southeast by the freshwater/saline-water transition zone in the Edwards aquifer. Most of the water in this flow path enters the aquifer west of the study area. Additional inflows occur as spillover from north of the Comal Springs fault at the Bracken gap, an area near the Bexar-Comal-Guadalupe County line intersec tion where the fault has not completely offset the entire aquifer thickness. The SCFP gradually narrows as it nears Comal Springs because saline water encroaches as the freshwater is discharged at the springs. The central Comal flow path (CCFP) is bounded on the northwest by the Hueco Springs fault and on the southeast by the Comal Springs fault. Water that is diverted north by the Alamo Heights horst in central Bexar County flows north of Comal Springs fault, accounting for the regional source to this flow path. Local recharge from infiltration of rainfall and streamflow leakage is supplied to the flow path from the Edwards aquifer recharge zone in northeastern Bexar County. These mixed waters then flow northeast through a highly fractured and faulted section of the aquifer. The complex fault ing produces a hydraulically connected, two-trough system in the aquifer that is confined in some areas and unconfined in others. The troughs channel flow to discharge points at both Comal Springs and Hueco Springs. Water that does not dis charge at either of the springs continues flowing northeastward toward San Marcos Springs. The northern Comal flow path (NCFP) is in the Edwards aquifer recharge zone in Comal County and is bounded on the north by the Bat Cave fault and on the south by the Hueco Springs fault. Recharge to this flow path occurs from the Edwards aquifer recharge zone northwest of Bat Cave fault in Comal County from direct infiltration of rainfall and stream flow losses to the Edwards aquifer exposed at the surface. Additional inflow likely comes from the Trinity aquifer in the subsurface where the Bat Cave fault juxtaposes the Trinity aquifer against the Edwards aquifer. Flow also might occur between the CCFP and the NCFP in central Comal County through a section of the Hueco Springs fault that does not completely offset the entire thickness of the Edwards aquifer. A potentiometric-surface map derived from synoptic water-level measurements made in fall 2000 was used to iden tify the generally southwest to northeast flow directions within the flow paths. Statistical correlations between water levels for six observation wells and discharges from Comal Springs and Hueco Springs (28 combinations of paired datasets) yielded additional evidence for the hypothesized flow paths. Strong linear correlations were evident between the datasets from wells and springs within the same hypothesized flow path and the datasets from wells in areas where flow between flow paths was suspected. Hydrograph recession slope analysis for Comal and Hueco Springs reinforces the fact that the springs are sourced from a complex karst system. The initial recession slopes for Comal Springs predominately represent a mixture of con duit and fracture flow, or a mixture of conduit and diffuse flow, whereas the initial recession slopes for Hueco Springs predominately represent conduit flow. The initial mixed flow from Comal Springs likely represents the conduit flow contri bution from the SCFP, which provides most of the flow from the springs, and the fracture flow contribution from the CCFP. The initial conduit flow from Hueco Springs provides evi dence that the northwestern trough within the CCFP in Comal County might be highly transmissive, allowing for rapid move ment of water through the aquifer to Hueco Springs. Geochemical data (major ions, stable isotopes, sulfur hexafluoride, and tritium and helium) were used in graphical analyses to obtain evidence of the flow path from which wells or springs derive water. Major-ion geochemistry in samples from selected wells and springs showed relatively little varia tion. Samples from the SCFP were characterized by relatively high sulfate and chloride concentrations possibly indicating that the water in the flow path was mixing with small amounts of saline water from the freshwater/saline-water transition zone. Samples from the CCFP yielded the most varied majorion geochemistry of the three hypothesized flow paths. CCFP samples were characterized, in general, by high calcium concentrations and low magnesium concentrations. Samples from the NCFP were characterized by relatively low sulfate and chloride concentrations and high magnesium concentra tions. The high magnesium concentrations characteristic of NCFP samples from the recharge zone in Comal County might indicate water from the Trinity aquifer is entering the Edwards aquifer in the subsurface. A graph of the relation between the stable isotopes D and 18 O showed that, except for samples collected from wells in the Edwards aquifer recharge zone in northeastern Bexar County following an unusually intense rain storm produced by a tropical system, there was not much variation in stable isotope values among the flow paths. In the study area D in ground water ranged from -36.00 to -20.89 per mil and 18 O 28 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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ranged from -6.03 to -3.70 per mil. Excluding samples col lected from wells in the recharge zone of northeastern Bexar County following the intense rain storm, the D range in the study area was -33.00 to -20.89 per mil and the 18 O range was -4.60 to -3.70 per mil. Two ground-water age-dating techniques, sulfur hexa fluoride concentrations and tritium-helium-3 isotope ratios, were used to compute apparent ages (time since recharge occurred) of water samples collected from wells and springs in the study area. In general, the apparent ages computed by the two methods do not seem to indicate direction of flow. Appar ent ages computed for water samples in northeastern Bexar and southern Comal Counties do not vary greatly except for some very young water in central Comal County. Additional recharge from direct infiltration of precipitation and stream flow losses into the exposed Edwards aquifer in central Comal County might account for younger age dates in the middle section of the CCFP. References American Public Health Association, 1998, Standard methods for the examination of water and wastewater (20th ed.): Washington, D.C., American Public Health Association, American Water Works Association, and Water Environ ment Federation, p. 3-37-43. Baedke, S.J., and Krothe, N.C., 2001, Derivation of effec tive hydraulic parameters of a karst aquifer from discharge hydrograph analysis: Water Resources Research, v. 37, no. 1, p. 13. Barker, R.A., and Ardis, A.F., 1996, Hydrogeologic frame work of the Edwards-Trinity aquifer system, west-central Texas: U.S. Geological Survey Professional Paper 1421–B, 61 p. Bonacci, O., 1993, Karst springs hydrographs as indicators of karst aquifers: Hydrological Sciences-Journal-des Sciences Hydrologiques, v. 33, no. 1, p. 51. Busenberg, Eyrybaides, and Plummer, L.N., 2000, Dating young groundwater with sulfur hexafluoride—Natural and anthropogenic sources of sulfur hexafluoride: Water Resources Research, v. 36, no. 10, p. 3,011,030. Craig, H., 1961, Isotopic variation in meteoric water: Science, v. 133, p. 1,702,703. Edwards Aquifer Authority, 2007, Edwards aquifer optimiza tion program/reports: accessed December 5, 2007, at http:// www.edwardsaquifer.org/pages/research_optimization.htm Faires, L.M., 1993, Methods of analysis by the U.S. Geologi cal Survey National Water Quality Laboratory—Determina tion of metals in water by inductively coupled plasma-mass spectrometry: U.S. Geological Survey Open-File Report 92, 28 p. Fishman, M.J., and Friedman, L.C., 1989, Methods for determination of inorganic substances in water and fluvial sediments: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. A1, 545 p. Fishman, M.J., ed., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory— Determination of inorganic and organic constituents in water and fluvial sediments: U.S. Geological Survey OpenFile Report 93, 217 p. Garbarino, J.R., Kanagy, L.K., and Cree, M.E., 2006, Determi nation of elements in natural-water, biota, sediment and soil samples using collision/reaction cell inductively coupled plasma-mass spectrometry: U.S. Geological Survey Tech niques and Methods, book 5, sec. B, chap. 1, 88 p. Groschen, G.E., 1994, Analysis of data from test-well sites along the downdip limit of freshwater in the Edwards aqui fer, San Antonio, Texas, 1985: U.S. Geological Survey Water-Resources Investigations Report 93, 92 p. Groschen, G.E., 1996, Hydrogeologic factors that affect the flowpath of water in selected zones of the Edwards aquifer, San Antonio region, Texas: U.S. Geological Survey WaterResources Investigations Report 96, 73 p. Hamilton, J.M., and Schindel, G.M., 2006, Edwards Aquifer Authority synoptic water level program 1999 report: Edwards Aquifer Authority Report No. 06, 97 p. Helsel, D.R., and Hirsch, R.M., 1995, Statistical methods in water resources—Studies in environmental science 49: New York, Elsevier, 529 p. Hem, J.D., 1992, Study and interpretation of the chemical characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p. Kendall, C., and McDonnell, J.J., 1998, Isotope tracers in catchment hydrology: New York, Elsevier, 839 p. Klemt, W.B., Knowles, T.R., Elder, G.R., and Sieh, T.W., 1979, Ground-water resources and model applications for the Edwards (Balcones fault zone) aquifer in the San Anto nio region: Texas Department of Water Resources Report 239, 88 p. LBG-Guyton Associates, 2004, Evaluation of augmentation methodologies in support of in-situ refugia at Comal and San Marcos Springs, Texas: Report prepared for Edwards Aquifer Authority, 181 p. Lindgren, R.J., Dutton, A.R., Hovorka, S.D., Worthington, S.R.H., and Painter, Scott, 2004, Conceptualization and simulation of the Edwards aquifer, San Antonio region, Texas: U.S. Geological Survey Scientific Investigations Report 2004, 143 p. Maclay, R.W., 1995, Geology and hydrology of the Edwards aquifer in the San Antonio area, Texas: U.S. Geological Survey Water-Resources Investigations Report 95, 64 p. References 29

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Maclay, R.W., and Land, L.F., 1988, Simulation of flow in the Edwards aquifer, San Antonio region, Texas, and refine ment of storage and flow concepts: U.S. Geological Survey Water-Supply Paper 2336–A, 48 p. Maclay, R.W., and Small, T.A., 1984, Carbonate geology and hydrology of the Edwards aquifer in the San Antonio area, Texas: U.S. Geological Survey Open-File Report 83, 72 p. Milanovich, P.T., 1981, Water regime in deep karst—Case study of the Ombla Spring drainage area, in Yevjevich, V., ed., Karst Hydrology and Water Resources Publication: Littleton, Colo., p. 165. Ogden, A.E., Spinelli, A.J., and Horton, J., 1985, Hydrologic and hydrochemical data for the Edwards aquifer in Hays and Comal Counties, October 1981 to September 1983: San Marcos, Tex., Southwest Texas State University, Edwards Aquifer Research and Data Center, Report R1, 102 p. Padilla, A., Pulido-Bosch, A., and Mangin, A., 1994, Relative importance of baseflow and quickflow from hydrographs of karst springs: Ground Water, v. 32, p. 267. Patton, C.J., and Truitt, E.P., 1992, Methods of analysis by the U.S. Geological Survey National Water Quality Labo ratory—Determination of total phosphorus by a Kjeldahl digestion method and an automated colorimetric finish that includes dialysis: U.S. Geological Survey Open-File Report 92, 39 p. Patton, C.J., and Truitt, E.P., 2000, Methods of analysis by the U.S. Geological Survey National Water Quality Labora tory—Determination of ammonium plus organic nitrogen by a Kjeldahl digestion method and an automated photomet ric finish that includes digest cleanup by gas diffusion: U.S. Geological Survey Open-File Report 00, 31 p. Plummer, L.N., Bohkle, J.K., and Busenberg, Eurybiades, 2003, Approaches for ground-water dating, in Lindsey, B.D., Phillips, S.W., Donnelly, C.A., Speiran, G.K., Plummer, L.N., Bohkle, J.K., Focazio, M.J., Burton, W.C., and Busenberg, Eurybiades, Residence times and nitrate transport in ground water discharging to streams in the Chesapeake Bay Watershed: U.S. Geological Survey WaterResources Investigations Report 03, p. 12. Rose, P.R., 1972, Edwards Group, surface and subsurface, cen tral Texas: Austin, University of Texas, Bureau of Economic Geology Report of Investigations 74, 198 p. Schlosser P., Stute, M., Dorr, H., Sonntag, C., and Munnich, K.O., 1988, Tritium/ 3 He dating of shallow groundwater: Earth and Planetary Science Letters, v. 89, p. 353. Schultz, A.L., 1994, 1994 review and update of the position of the Edwards aquifer freshwater/saline-water interface from Uvalde to Kyle, Texas: Edwards Underground Water District Report 94, 31 p. Shevenell, L., 1996, Analysis of well hydrographs in a karst aquifer—Estimates of specific yields and continuum trans missivities: Journal of Hydrology, v. 174, p. 331. Small, T.A., 1986, Hydrogeologic sections of the Edwards aquifer and its confining units in the San Antonio area, Texas: U.S. Geological Survey Water-Resources Investiga tions Report 85, 52 p. Small, T.A., and Hanson, J.A., 1994, Geologic framework and hydrogeologic characteristics of the Edwards aquifer outcrop, Comal County, Texas: U.S. Geological Survey Water-Resources Investigations Report 94, 10 p. Stein,W.G., and Ozuna, G.B., 1994, Geologic framework and hydrogeologic characteristics of the Edwards aquifer out crop, Bexar County, Texas: U.S. Geological Survey WaterResources Investigations Report 95, 8 p. U.S. Geological Survey, 2005, Quality assurance plan of the Reston Stable Isotope Laboratory—Reston Stable Isotope Laboratory Isotope Fractionation Project: accessed June 14, 2005, at http://isotopes.usgs.gov/Quality.htm U.S. Geological Survey, 2006a, National Water Information System (NWISWeb) [for Texas] data available on the World Wide Web at http://waterdata.usgs.gov/tx/nwis/nwis U.S. Geological Survey, 2006b, The Reston Chlorofluoro carbon Laboratory—Dissolved gas analysis, SF 6 analysis, 3 H/ 3 He analysis: accessed January 17, 2006, at http://water. usgs.gov/lab U.S. Geological Survey, 2006c, USGS National Water Quality Assessment Data Warehouse data available on the World Wide Web: accessed March 2006, at http://infotrek.er.usgs. gov/tranverse/f?p=NAWQA:HOME:2081027407802063 Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 1999, Collection of water samples: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A4, accessed February 15, 2003, at http://pubs.water. usgs.gov/twri9A4/ . Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 2003, Cleaning of equipment for water samples (ver sion 1.2): U.S. Geological Survey Techniques of WaterResources Investigations, book 9, chap. A3, accessed February 15, 2003, at http://pubs.water.usgs.gov/twri9A3/ . Wilde, F.D., Radtke, D.B., Gibs, Jacob, and Iwatsubo, R.T., 2004, Processing of water samples (version 2.1): U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A5, accessed February 15, 2003, at http://pubs.water.usgs.gov/twri9A5/ . William F. Guyton and Associates, 1979, Geohydrology of Comal, San Marcos, and Hueco Springs: Texas Department of Water Resources Report 234, 85 p. 3 0 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix Appendix 1—Water-Level ( 2 00 4 –0 6 ) and Chemical Data ( 2 00 3 –0 6 )

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Appendix 1.1. Daily mean depth to water at well DX, Comal County, Texas, 2004—Continued. Water year 2004 (Oct. 2003–Sept. 2004) daily mean values Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 1 ------------109.05 106.24 106.52 98.11 99.03 104.68 2 ------------109.10 106.11 106.84 97.78 --104.87 3 ------------109.06 105.97 107.44 97.75 --104.86 4 ----------109.37 108.94 105.95 107.85 97.66 --104.77 5 ----------109.32 108.75 106.01 107.97 97.68 --104.63 6 ----------109.30 108.64 106.05 108.07 97.85 --104.48 7 ----------109.28 108.58 105.96 108.20 98.24 --104.15 8 ----------109.23 108.51 105.86 108.34 98.50 --104.08 9 ----------109.30 108.42 105.70 108.32 98.53 --104.05 10 ----------109.36 108.38 105.53 106.48 98.58 --104.08 11 ----------109.40 108.24 105.44 102.91 98.63 --104.23 12 ----------109.42 107.99 105.42 100.11 98.73 --104.24 13 ----------109.35 107.93 105.52 99.56 98.85 103.36 104.27 14 ----------109.27 107.83 105.57 99.98 98.97 103.34 104.31 15 ----------109.11 107.75 105.40 100.60 99.13 103.33 104.23 16 ----------109.11 107.77 105.14 101.16 99.28 103.41 104.19 17 ----------109.11 107.74 104.97 101.65 99.33 103.57 104.28 18 ----------109.11 107.60 104.91 102.05 99.40 103.87 104.43 19 ----------109.10 107.49 104.99 102.48 99.58 104.18 104.41 20 ----------109.05 107.43 105.15 102.66 99.99 104.14 104.40 21 ----------109.05 107.38 105.15 102.81 100.31 104.23 104.50 22 ----------109.03 107.35 105.11 103.02 100.40 104.14 104.70 23 ----------108.99 107.38 105.06 103.22 --104.16 104.90 24 ----------108.94 107.35 105.12 103.39 100.20 104.22 105.06 25 ----------108.94 107.06 105.28 103.56 100.26 104.29 105.04 26 ----------108.96 106.73 105.55 103.41 99.98 104.33 104.92 27 ----------108.90 106.53 105.83 103.34 99.81 104.30 104.88 28 ----------108.83 106.38 106.12 102.89 99.66 104.24 105.01 29 ----------108.82 106.25 106.31 101.68 99.41 104.18 105.07 30 ----------108.88 106.19 106.32 99.86 99.21 104.22 105.10 31 ----------108.99 --106.37 --98.99 104.46 --Mean ------------107.79 105.62 103.88 ----104.56 Maximum ------------109.10 106.37 108.34 ----105.10 Minimum ------------106.19 104.91 99.56 ----104.05 Appendix 1.1. Daily mean depth to water at well DX, Comal County, Texas, 2004. [In feet below land surface; ---, not collected or computed] Appendix 33

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Appendix 1.1. Daily mean depth to water at well DX, Comal County, Texas, 2004—Continued. Water year 2005 (Oct. 2004–Sept. 2005) daily mean values Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 1 105.75 98.49 80.83 --89.78 89.41 88.23 92.37 95.10 ----106.10 2 105.67 98.18 81.73 --89.85 89.34 88.13 92.71 94.87 ----106.51 3 103.63 98.07 82.56 --89.93 89.47 88.10 93.38 94.77 ----106.60 4 101.08 98.13 83.22 --89.96 89.41 88.10 93.80 94.75 ----106.57 5 100.38 98.37 83.84 --89.86 89.31 88.14 94.04 94.78 ----106.55 6 100.37 98.85 84.39 --89.75 88.86 88.35 93.76 94.83 ----106.63 7 100.47 99.26 85.50 --89.67 88.32 88.82 93.63 94.95 ----106.99 8 100.61 99.55 86.30 --89.42 87.65 89.05 93.63 95.12 ----107.24 9 100.71 99.76 87.16 --89.27 87.28 89.18 93.55 95.27 ----107.34 10 100.79 99.94 88.13 --89.14 87.05 89.06 93.60 95.40 --103.79 107.18 11 101.08 100.31 88.66 --88.99 87.02 89.13 93.91 95.46 --103.19 107.11 12 101.43 100.66 ----88.86 87.15 89.23 94.06 95.56 --103.04 107.02 13 101.83 101.00 ----88.84 87.30 89.39 93.92 95.71 --103.03 106.89 14 102.05 101.14 --88.48 88.85 --89.95 93.92 95.91 --103.07 106.79 15 102.10 100.83 ----88.87 87.45 90.36 93.95 96.17 --103.17 106.98 16 102.35 100.61 ----88.98 87.44 90.31 94.02 96.45 --103.33 106.94 17 102.57 99.19 ----89.19 --90.28 94.15 96.68 --103.49 106.87 18 102.68 96.19 ----89.49 --90.27 94.28 96.87 --103.59 106.84 19 102.66 94.01 ----89.57 --90.30 94.38 97.10 --103.67 106.90 20 102.92 92.47 ----89.34 --90.41 94.38 97.43 --103.88 107.01 21 103.07 91.56 ----89.30 --90.60 94.40 98.19 --104.00 107.16 22 103.26 89.80 ----89.30 86.86 90.79 94.47 98.49 --104.22 107.31 23 103.28 84.60 ----89.27 87.01 90.99 94.58 98.48 --104.78 107.38 24 102.32 81.94 ----89.37 87.31 91.11 94.78 98.67 --105.14 107.45 25 99.48 ----89.69 89.50 87.57 91.09 95.04 ----105.09 107.59 26 97.45 ----90.12 89.57 87.51 91.24 95.31 ----105.08 107.76 27 97.00 ----89.97 89.53 87.54 91.58 95.55 ----105.14 108.00 28 97.07 79.72 --89.71 89.47 87.57 92.15 95.90 ----105.26 108.24 29 97.32 79.88 --89.65 --87.79 92.08 95.48 ----105.40 108.54 30 97.95 80.35 --89.63 --87.99 92.22 95.26 ----105.57 108.75 31 98.45 ----89.70 --88.10 --95.22 ----105.80 --Mean 101.22 ------89.39 --89.95 94.24 ------107.17 Maximum 105.75 ------89.96 --92.22 95.90 ------108.75 Minimum 97.00 ------88.84 --88.10 92.37 ------106.10 34 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.1. Daily mean depth to water at well DX, Comal County, Texas, 2004—Continued. Water year 2006 (Oct. 2005–Sept. 2006) daily mean values Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 1 108.88 107.83 ----110.85 112.18 114.66 117.79 120.01 123.79 125.97 128.54 2 109.00 107.79 ----110.83 112.28 114.58 117.82 120.16 123.90 126.07 128.59 3 109.11 107.85 ----110.87 112.37 114.57 117.80 120.27 124.01 126.17 128.64 4 109.10 108.12 ----110.92 112.52 114.64 117.81 120.36 124.07 126.27 128.69 5 108.39 108.14 --109.49 110.94 112.75 114.71 117.81 120.49 123.98 126.37 128.73 6 107.78 108.11 --109.57 111.03 112.88 114.79 117.71 120.64 123.80 126.48 128.71 7 107.87 108.09 --109.74 111.07 113.01 114.94 117.50 120.77 123.71 126.61 128.70 8 107.86 108.12 --109.77 111.13 113.10 115.07 117.35 120.92 123.62 126.75 128.66 9 107.79 108.19 ----111.25 113.20 115.15 117.28 121.08 123.55 126.85 128.59 10 107.74 108.28 ----111.38 113.32 115.22 117.15 121.25 123.55 126.93 128.49 11 107.76 108.33 --109.97 111.49 113.42 115.35 117.09 121.39 123.57 127.05 128.39 12 107.74 108.34 --109.99 111.50 113.50 115.47 117.04 121.58 123.66 127.09 128.34 13 107.64 108.35 --110.08 111.49 113.61 115.60 117.03 121.79 123.74 127.15 128.18 14 107.53 108.34 --110.17 111.54 113.76 115.76 117.05 121.99 123.85 127.23 128.06 15 107.42 108.37 --110.20 111.66 113.91 115.85 117.11 122.21 123.98 127.29 127.95 16 107.34 108.45 --110.26 111.76 114.04 115.97 117.17 122.40 124.10 127.36 127.78 17 107.32 108.51 --110.42 111.87 114.17 116.11 117.30 122.56 124.22 127.44 127.59 18 107.30 108.73 ----111.94 114.28 116.33 117.41 122.62 124.38 127.53 127.39 19 107.32 108.71 ----111.92 114.37 116.58 117.55 122.65 124.55 127.63 127.22 20 107.39 108.67 ----111.89 114.43 116.81 117.72 122.54 124.74 127.72 127.04 21 107.49 108.68 ----111.88 114.48 116.95 117.91 122.48 124.88 127.82 126.85 22 107.54 ------111.90 114.54 117.02 118.11 122.58 125.02 127.92 126.68 23 107.57 --------114.63 117.07 118.33 122.64 125.13 128.03 126.53 24 107.62 109.07 ----112.08 114.68 117.13 118.54 122.74 125.27 128.14 126.38 25 107.62 109.08 --110.68 112.09 114.70 117.23 118.79 122.83 125.35 128.21 126.28 26 107.66 ----110.72 112.10 114.68 117.33 119.04 122.96 125.44 128.28 126.20 27 107.74 ----110.79 112.07 114.67 117.41 119.25 123.13 125.52 128.35 126.10 28 107.83 108.99 --110.79 112.10 114.67 117.49 119.45 123.31 125.61 128.41 126.02 29 107.89 108.96 --110.74 --114.64 117.57 119.61 123.47 125.70 128.46 125.94 30 107.88 ----110.79 --114.64 117.68 119.74 123.64 125.78 128.48 125.88 31 107.85 ----110.84 --114.70 --119.86 --125.87 128.50 --Mean 107.84 --------113.81 116.03 117.97 121.92 124.46 127.37 127.57 Maximum 109.11 --------114.70 117.68 119.86 123.64 125.87 128.50 128.73 Minimum 107.30 --------112.18 114.57 117.03 120.01 123.55 125.97 125.88 Appendix 35

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Appendix 1.2. Daily mean depth to water at well DX-68-23-502, Comal County, Texas, 2004. Water year 2004 (Oct. 2003–Sept. 2004) daily mean values Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 1 ------------32.40 30.44 31.13 27.62 27.93 29.71 2 ------------32.36 30.25 31.50 27.36 28.13 29.75 3 ------------32.14 30.29 31.75 27.04 28.38 29.76 4 ------------31.99 30.34 31.88 26.79 28.51 29.69 5 ------------31.97 30.32 31.77 26.70 28.64 29.43 6 ------------31.99 30.32 31.76 26.88 28.75 29.37 7 ------------31.92 30.30 32.00 26.96 28.57 29.43 8 ------------31.88 30.08 32.09 26.96 28.48 29.49 9 ------------31.80 29.94 31.63 26.99 28.58 29.52 10 ------------31.60 30.01 30.35 26.82 28.74 29.55 11 ------------31.38 30.08 30.02 26.69 28.84 29.59 12 ------------31.36 30.03 29.66 26.76 28.94 29.48 13 ------------31.43 29.97 29.31 26.91 29.04 29.60 14 ------------31.41 29.89 29.31 26.97 28.95 29.67 15 ------------31.36 29.67 29.35 27.09 28.89 29.58 16 ------------31.36 29.47 29.33 27.19 29.07 29.55 17 ------------31.19 29.54 29.34 27.14 29.29 29.59 18 ----------32.29 31.04 29.68 29.37 27.07 29.44 29.61 19 ----------32.29 31.14 29.70 29.27 27.26 29.48 29.53 20 ----------32.14 31.22 29.79 29.22 27.53 29.55 29.62 21 ----------32.01 31.25 29.91 29.38 27.75 29.56 29.75 22 ----------32.15 31.28 29.82 29.58 27.92 29.44 29.78 23 ----------32.28 31.29 29.78 29.60 27.95 29.51 29.82 24 ----------32.28 31.18 29.97 29.52 27.76 29.58 29.83 25 ----------32.27 30.96 30.18 29.46 27.67 29.57 29.76 26 ----------32.28 30.84 30.34 29.25 27.61 29.60 29.69 27 ----------32.12 30.85 30.50 29.03 27.80 29.65 29.79 28 ----------32.00 30.78 30.67 28.93 27.92 29.58 29.90 29 ----------32.16 30.72 30.68 28.73 27.99 29.35 30.00 30 ----------32.29 30.62 30.68 28.10 28.04 29.50 30.02 31 ----------32.37 --30.84 --27.97 29.66 --Mean ------------31.42 30.11 30.05 27.33 29.07 29.66 Maximum ------------32.40 30.84 32.09 28.04 29.66 30.02 Minimum ------------30.62 29.47 28.10 26.69 27.93 29.37 Appendix 1.2. Daily mean depth to water at well DX, Comal County, Texas, 2004. [In feet below land surface; ---, not collected or computed] 36 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.2. Daily mean depth to water at well DX, Comal County, Texas, 2004—Continued. Water year 2005 (Oct. 2004–Sept. 2005) daily mean values Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 1 30.04 26.48 17.83 19.59 21.61 21.43 20.70 23.46 24.96 28.97 29.80 31.46 2 29.71 26.35 17.81 19.65 21.57 21.38 20.61 23.63 24.86 29.10 29.93 31.52 3 29.11 26.22 17.83 19.72 21.59 21.40 20.57 23.78 24.81 29.08 30.07 31.46 4 28.98 26.15 17.67 19.85 21.65 21.42 20.74 23.93 24.78 29.21 30.25 31.33 5 28.93 26.09 17.55 20.02 21.46 21.24 20.87 24.06 24.68 29.54 30.26 31.31 6 28.85 25.95 17.69 20.09 21.32 20.92 21.02 24.20 24.75 29.80 30.16 31.45 7 28.80 25.77 17.83 20.20 21.42 20.76 21.11 24.23 24.90 29.98 30.13 31.51 8 --25.84 17.88 20.19 21.53 20.67 21.23 24.13 25.01 30.13 30.32 31.52 9 28.58 25.92 17.91 20.13 21.44 20.59 21.16 24.15 25.09 30.16 30.46 31.58 10 28.37 25.94 18.03 20.27 21.52 20.50 21.08 24.28 25.15 30.15 30.44 31.54 11 28.42 26.00 17.94 20.48 21.42 20.44 21.34 24.31 25.25 30.45 30.31 31.29 12 28.52 26.06 17.84 20.54 21.26 20.27 21.53 24.37 25.27 30.71 30.28 31.20 13 28.57 26.05 18.13 --21.04 20.10 21.65 24.36 25.42 30.87 30.21 31.21 14 28.51 25.86 18.38 --21.17 20.20 21.80 24.34 25.66 30.94 29.95 31.13 15 28.45 25.76 18.45 --21.26 20.30 21.93 24.23 25.85 31.02 30.04 31.03 16 28.38 25.76 18.55 --21.27 20.27 21.89 24.30 26.01 30.87 30.15 31.02 17 28.25 24.84 18.67 --21.33 20.20 21.86 24.41 26.16 30.43 30.13 31.03 18 28.31 24.15 18.62 --21.38 20.18 22.03 24.46 26.31 30.23 30.18 30.96 19 28.41 23.81 18.57 --21.22 20.06 22.22 24.52 26.36 30.17 30.25 31.17 20 28.46 23.40 18.67 --21.14 19.94 22.33 24.60 26.65 30.11 30.25 31.32 21 28.56 22.90 18.83 --21.30 20.01 22.39 24.65 26.93 30.03 30.21 31.39 22 28.58 21.49 18.99 --21.46 20.20 22.43 24.61 27.17 29.98 30.39 31.42 23 28.28 19.38 19.12 --21.47 20.27 22.49 24.80 27.38 29.89 30.55 31.46 24 27.95 18.83 19.04 --21.50 20.31 22.46 24.97 27.60 29.66 30.69 31.47 25 27.52 18.38 19.00 21.53 21.59 20.36 22.61 25.16 27.78 29.77 30.86 31.46 26 27.32 17.96 19.08 21.61 21.47 20.27 22.77 25.26 27.82 29.87 30.97 31.72 27 27.19 17.90 19.20 21.65 21.27 20.22 23.02 25.30 28.08 29.90 31.03 31.95 28 27.08 17.69 19.37 21.59 21.35 20.34 23.21 25.16 28.34 29.89 30.99 32.11 29 27.01 17.71 19.57 21.51 --20.47 23.36 24.91 28.53 29.83 31.13 32.19 30 26.89 17.83 19.71 21.39 --20.52 23.47 24.83 28.74 29.83 31.28 32.26 31 26.68 --19.65 21.47 --20.59 --24.94 --29.72 31.35 --Mean --23.42 18.50 --21.39 20.51 21.86 24.46 26.21 30.01 30.42 31.45 Maximum --26.48 19.71 --21.65 21.43 23.47 25.30 28.74 31.02 31.35 32.26 Minimum --17.69 17.55 --21.04 19.94 20.57 23.46 24.68 28.97 29.80 30.96 Appendix 37

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Appendix 1.2. Daily mean depth to water at well DX, Comal County, Texas, 2004—Continued. Water year 2006 (Oct. 2005–Sept. 2006) daily mean values Day Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. 1 32.28 31.04 32.73 32.62 33.55 34.87 36.45 38.99 41.46 45.04 46.20 47.72 2 32.26 31.09 32.79 32.71 33.59 34.97 36.38 --41.42 44.66 46.32 47.64 3 32.38 31.20 32.78 32.88 33.71 35.11 36.58 39.19 41.41 44.54 46.47 47.31 4 --31.28 32.78 33.04 33.80 35.21 36.69 39.23 41.39 44.41 46.66 47.30 5 33.48 31.23 32.87 33.17 33.80 35.24 36.84 39.07 41.63 43.74 46.61 47.42 6 31.33 31.20 32.79 33.25 33.86 35.49 36.92 38.67 41.97 43.51 46.34 47.03 7 30.42 31.42 32.78 33.23 33.96 35.62 37.02 38.20 42.18 43.38 46.32 46.82 8 30.30 31.56 32.71 33.12 34.03 35.73 37.11 38.03 42.40 43.19 46.45 46.72 9 30.28 31.64 32.80 33.25 34.09 35.78 37.08 37.97 --43.11 46.53 46.39 10 30.34 31.72 32.74 33.28 34.13 --37.29 37.97 --43.31 46.64 46.06 11 30.35 31.77 32.62 33.26 34.16 --37.50 38.04 --43.62 46.83 46.03 12 30.23 31.77 32.67 33.35 34.06 --37.70 38.08 --43.94 46.82 45.85 13 30.17 31.73 32.71 33.47 34.20 --37.85 38.18 --44.23 46.61 45.51 14 30.16 31.91 32.73 33.41 34.36 --38.01 38.19 --44.48 46.84 45.32 15 30.11 32.02 32.78 33.44 34.46 36.80 38.19 38.35 --44.47 47.10 45.19 16 30.05 32.11 32.72 33.49 34.49 36.79 38.21 38.59 --44.46 47.28 44.97 17 30.21 32.17 32.65 33.54 34.61 36.88 38.43 38.90 --44.78 47.41 44.72 18 30.28 32.20 32.53 33.54 34.66 36.89 38.71 39.19 --45.24 47.55 44.54 19 30.35 32.18 32.52 33.50 34.55 36.83 38.94 39.41 --45.47 47.43 44.39 20 30.41 32.10 32.60 --34.55 36.82 39.15 39.62 --45.61 47.30 44.27 21 30.51 32.22 32.63 33.54 34.60 36.83 39.08 39.74 43.68 45.70 47.41 44.15 22 30.56 32.40 32.67 33.38 34.61 36.80 38.83 40.12 43.78 45.68 47.60 44.12 23 30.52 32.50 32.51 33.42 34.67 36.81 38.74 40.47 43.96 45.41 47.60 44.01 24 30.71 32.43 32.44 33.60 34.73 36.80 38.92 40.76 44.00 --47.64 43.73 25 30.76 32.37 32.39 33.66 34.71 36.71 39.09 41.04 43.97 45.64 47.72 43.73 26 30.84 32.40 32.43 33.61 34.60 36.55 39.17 41.29 44.19 45.71 47.62 43.84 27 30.90 32.34 32.56 33.65 34.68 36.59 39.26 41.53 44.53 45.84 47.32 43.84 28 31.00 32.50 32.70 33.60 34.81 36.57 39.23 41.46 44.76 45.95 47.39 43.85 29 31.02 32.64 32.78 33.45 --36.48 39.13 41.25 44.99 45.92 47.63 43.89 30 30.86 32.69 32.71 33.43 --36.45 38.91 41.23 45.22 45.75 47.52 43.85 31 30.98 --32.63 33.53 --36.48 --41.39 --45.94 47.57 --Mean --31.93 32.67 --34.29 --38.05 ------47.06 45.34 Maximum --32.69 32.87 --34.81 --39.26 ------47.72 47.72 Minimum --31.04 32.39 --33.55 --36.38 ------46.20 43.73 38 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. Site type State well number (USGS name) Date Time Temperature, water (C) pH (standard units) Specific conductance (S/cm) Dissolved oxygen (mg/L) Calcium (mg/L) Magne sium (mg/L) Southern Comal flow path W AY 7/28/2003 1200 22.5 7 608 -100 13.5 7/28/2003 dup 1201 ------W AY 7/28/2003 1100 24.5 7.1 515 -73.2 15.9 7/28/2003 dup 1101 ------W AY 7/28/2003 1000 25 7.2 493 -68.2 16.1 7/28/2003 dup 1001 ------W DX 6/18/2003 1400 24 7.2 558 2.5 79.9 17 (NBU–LCRA) 6/18/2003 dup 1401 ------3/1/2006 1330 24.5 7.1 533 4.5 84.6 16.5 Sp DX 7/22/2003 1230 23.9 7.1 547 -84.7 17 (Comal 7) 7/22/2003 dup 1231 ------3/8/2006 1430 23.5 7.1 561 5.4 77.7 16.4 Sp DX 8/7/2003 1030 ----79.4 17.8 (Comal-Spring Island) 8/7/2003 dup 1100 ----98.7 18.5 3/8/2006 1400 23.5 7.1 562 5.1 82.5 15.6 Sp DX 7/22/2003 1330 24 7.1 548 -86.8 17.6 (Comal 5) 7/22/2003 dup 1331 ------3/8/2006 1330 23.5 7.1 565 4.9 77.5 16.1 W DX 7/24/2003 1100 23 7 544 -83.2 13.5 7/24/2003 dup 1101 ------Central Comal flow path W AY 7/28/2003 0900 23.5 7 555 -82.9 17.4 7/28/2003 dup 0901 ------W AY 7/17/2003 1100 26 7 527 5.5 74.9 16.7 7/17/2003 dup 1101 ------Sp DX 3/5/2003 1400 20 6.7 613 6.7 109 9.81 (Hueco A) 3/5/2003 dup 1401 ------3/16/2006 1300 20.5 7.1 594 5.5 86 17.9 Sp DX 3/6/2003 1100 20 6.6 615 6.5 107 9.71 (Hueco B) 3/6/2003 dup 1101 ------Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003. [USGS, U.S. Geological Survey; C, degrees Celsius; S/cm, microsiemens per centimeter at 25 C; mg/L, milligrams per liter; W, well; --, not analyzed for or not detected; dup, duplicate; Sp, spring; R, Rainfall; CaCO 3 , calcium carbonate; <, less than; E, estimated; g/L, micrograms per liter; pptv, parts per trillion by volume; D, delta deuterium; per mil, parts per thousand; 18 O, delta oxygen-18; 3 H, tritium; TU, tritium units; 3 He, helium-3; M, presence of material verified but not quantified] Appendix 39

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. Site type State well number (USGS name) Date Time Temperature, water (C) pH (standard units) Specific conductance (S/cm) Dissolved oxygen (mg/L) Calcium (mg/L) Magne sium (mg/L) Central Comal flow path—Continued W DX 7/11/2003 1000 23 6.8 535 6.7 88.3 11.2 7/11/2003 dup 1001 ------3/14/2006 1300 22.8 7.1 544 5.8 98.6 6.56 W DX 7/15/2003 1100 22.5 6.9 476 6.9 81 10.5 7/15/2003 dup 1101 ------3/16/2006 1100 21.5 7.2 534 6.3 85.8 11 Sp DX 7/22/2003 1030 23.5 7 543 -85.3 16 (Comal 1) 7/22/2003 dup 1031 ------3/1/2006 1110 23 7.2 558 5.3 80.9 17.1 Sp DX 7/22/2003 1130 23.5 7 544 -87.1 16.5 (Comal 3) 7/22/2003 dup 1131 ------3/1/2006 1150 23 7.2 556 5.2 81.9 16.2 W DX 7/24/2003 1200 23.5 7 563 -83.9 14.2 7/24/2003 dup 1201 ------W DX 7/15/2003 1200 23 6.7 588 7.5 110 8.75 7/15/2003 dup 1201 ------3/14/2006 1200 22.3 7.1 591 6.4 109 7.99 3/14/2006 dup 1230 22.5 7.1 591 6.4 109 7.53 W DX 7/17/2003 1000 22 6.7 664 6.3 115 8.2 7/17/2003 dup 1001 ------Northern Comal flow path W DX 7/9/2003 1300 23.5 7 527 4.3 62.4 32.6 7/9/2003 dup 1301 ------Rainfall sites R Rainfall site 3 6/5/2003 900 ------7/8/2003 1400 ------R Rainfall site 4 6/5/2003 900 ------4 0 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Potassium (mg/L) Sodium adsorption ratio Sodium (mg/L) Sodium (percent) Alkalinity (mg/L as CaCO 3 ) Bicarbonate (mg/L) Carbonate (mg/L) Chloride (mg/L) Fluoride (mg/L) Silica (mg/L) Southern Comal flow path AY 2.26 0.3 10.3 7 257 313 0 15.7 0.19 12.3 ----------AY 1.24 .3 10.6 8 210 256 0 17.7 .2 12.8 ----------AY 1.16 .3 10.1 8 194 237 0 19.8 .2 12.6 ----------DX 1.42 .3 10.9 8 234 285 0 17.7 .26 12.3 (NBU–LCRA) ----------1.41 .3 9.61 7 244 296 <1 15.4 .26 12.7 DX 1.28 .3 10.6 8 220 268 0 17.6 .23 12.9 (Comal 7) ----------1.57 .3 10.4 8 242 295 <1 16.9 .27 12.5 DX 1.47 .3 9.78 7 228 278 0 15.6 .2 12.7 (Comal-Spring Island) 1.58 .2 9.48 6 226 276 0 16.9 .21 12.8 1.46 .3 9.53 7 250 305 <1 15.3 .25 12.2 DX 1.33 .3 10.9 8 220 268 0 18.2 .23 12.7 (Comal 5) ----------1.46 .3 10.1 8 238 290 <1 16.6 .27 12.7 DX 1.18 .2 8.27 6 236 288 0 13.7 .22 12.7 ----------Central Comal flow path AY 1.29 .3 10.6 8 225 275 0 16.2 .21 12.7 ----------AY 1.41 .3 10.8 8 196 239 0 20.9 .22 12.8 ----------DX 1.07 .2 7.98 5 268 327 0 12.4 .15 9.9 (Hueco A) ----------1.41 .2 9.29 7 272 331 <1 14.1 .3 11 DX 1.07 .2 7.92 5 266 324 0 13.3 .15 9.9 (Hueco B) ----------Appendix 4 1

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Potassium (mg/L) Sodium adsorption ratio Sodium (mg/L) Sodium (percent) Alkalinity (mg/L as CaCO 3 ) Bicarbonate (mg/L) Carbonate (mg/L) Chloride (mg/L) Fluoride (mg/L) Silica (mg/L) Central Comal flow path—Continued DX 0.81 0.2 5.66 4 246 300 0 10.2 <0.17 12 ----------.75 .2 6.11 5 267 325 <1 10.3 .11 11.4 DX .83 .2 5.57 5 204 248 0 9.78 <.17 10.7 ----------.89 .2 7.85 6 233 284 <1 11.5 .15 10.7 DX 1.24 .2 9.47 7 218 265 0 15.8 .22 12.6 (Comal 1) ----------1.48 .3 10.5 8 237 288 <1 17.3 .3 12.6 DX 1.34 .3 9.82 7 223 272 0 15.9 .22 12.6 (Comal 3) ----------1.39 .3 9.49 7 244 297 <1 15.7 .24 12.9 DX 1.24 .2 8.82 7 247 301 0 14.1 .21 12.7 ----------DX .97 .1 4.49 3 283 345 0 8.76 <.17 12.9 ----------1 .1 4.27 3 299 365 <1 7.65 .14 12.6 1.03 .1 4.14 3 299 364 0 7.51 .13 12.5 DX 1.73 .3 13.3 8 242 295 0 22.8 <.17 12.7 ----------Northern Comal flow path DX 1.21 .2 5.94 4 254 310 0 8.21 .45 12.9 ----------Rainfall sites Rainfall site 3 --------------------Rainfall site 4 ----------42 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Sulfate (mg/L) Residue (sum of constituents) (mg/L) Dissolved ammonia plus organic nitrogen (mg/L) Dissolved ammonia nitrogen (mg/L) Dissolved nitrite plus nitrate nitrogen (mg/L) Dissolved nitrite nitrogen (mg/L) Dissolved orthophosphate phosphorus (mg/L) Dissolved phosphorus (mg/L) Aluminum (g/L) Southern Comal flow path AY 25.7 342 <0.10 <0.04 1.66 <0.008 E0.01 <0.04 E1.2 ---------AY 21.2 287 <.10 <.04 1.84 <.008 <.02 <.04 <1.6 ---------AY 17.1 270 <.10 <.04 1.91 <.008 <.02 <.04 E.9 ---------DX 24.4 312 <.10 <.04 1.81 .036 <.02 <.04 E1.1 (NBU–LCRA) ---------22.9 309 ------2.1 DX 23.5 308 <.10 <.04 1.91 <.008 <.02 <.04 <1.6 (Comal 7) ---------25.3 306 ------<1.6 DX 22 304 <.10 <.04 1.78 <.008 <.02 <.04 E1.1 (Comal-Spring Island) 21.9 324 <.10 <.04 1.78 <.008 E.01 <.04 E.8 21.9 309 ------2.8 DX 23.7 312 <.10 <.04 1.85 <.008 <.02 <.04 <1.6 (Comal 5) ---------25 302 ------E1.4 DX 17.1 300 <.10 <.04 1.89 <.008 E.01 <.04 2.7 ---------Central Comal flow path AY 29.2 314 <.10 <.04 1.76 <.008 <.02 <.04 <1.6 ---------AY 22.9 287 <.10 <.04 1.89 <.008 <.02 <.04 <1.6 ---------DX 17.6 333 E.06 <.04 .92 <.008 <.02 <.04 <1.6 (Hueco A) ---------29.2 332 ------<1.6 DX 17.9 330 E.06 <.04 .91 <.008 <.02 <.04 <1.6 (Hueco B) ---------Appendix 43

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Sulfate (mg/L) Residue (sum of constituents) (mg/L) Dissolved ammonia plus organic nitrogen (mg/L) Dissolved ammonia nitrogen (mg/L) Dissolved nitrite plus nitrate nitrogen (mg/L) Dissolved nitrite nitrogen (mg/L) Dissolved orthophosphate phosphorus (mg/L) Dissolved phosphorus (mg/L) Aluminum (g/L) Central Comal flow path—Continued DX 6.53 291 <0.10 <0.04 1.8 <0.008 <0.02 <0.04 <1.6 ---------8.97 303 ------E1.3 DX 9.09 261 <.10 <.04 2.55 <.008 <.02 <.04 <1.6 ---------27.9 295 ------E.8 DX 20.9 301 <.10 <.04 1.94 <.008 <.02 <.04 <1.6 (Comal 1) ---------25.5 307 ------<1.6 DX 21.1 307 <.10 <.04 1.96 <.008 <.02 <.04 <1.6 (Comal 3) ---------23.2 307 ------E1.0 DX 19.1 310 E.07 <.04 1.83 <.008 <.02 <.04 <1.6 ---------DX 4.65 328 <.10 <.04 1.72 <.008 <.02 <.04 <1.6 ---------5.13 327 ------<1.6 5.17 326 ------<1.6 DX 21.1 387 <.10 <.04 10.5 <.008 <.09 <.04 <1.6 ---------Northern Comal flow path DX 9.76 293 <.10 <.04 1.59 <.008 <.02 <.04 E1.2 ---------Rainfall sites Rainfall site 3 ------------------Rainfall site 4 ---------44 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Antimony (g/L) Barium (g/L) Beryllium (g/L) Cadmium (g/L) Chromium (g/L) Cobalt (g/L) Copper (g/L) Lead (g/L) Manganese (g/L) Molybdenum (g/L) Southern Comal flow path AY <0.30 35 <0.06 <0.04 <0.8 0.201 1.9 1.77 <0.2 0.6 ----------AY <.30 43 <.06 <.04 <.8 .128 .8 1.02 <.2 .7 ----------AY <.30 53 <.06 <.04 <.8 .118 1.4 1.39 E.1 .7 ----------DX <.30 56 <.06 <.04 <.8 .189 .7 <.08 2.6 .8 (NBU–LCRA) ----------<.20 50 <.06 <.04 .24 .118 .6 .54 .5 .7 DX <.30 51 <.06 <.04 <.8 .174 .3 <.08 <.2 .6 (Comal 7) ----------<.20 57 <.06 <.04 .35 .125 1.6 .2 E.1 .7 DX <.30 45 <.06 <.04 <.8 .177 .5 <.08 E.2 .8 (Comal-Spring Island) <.30 45 <.06 <.04 <.8 .177 .5 <.08 E.2 1.4 <.20 52 <.06 <.04 .3 .12 E.3 E.08 <.2 .6 DX <.30 53 <.06 <.04 <.8 .173 .4 <.08 <.2 .6 (Comal 5) ----------<.20 55 <.06 <.04 .33 .118 1.8 .18 <.2 .7 DX <.30 37 <.06 E.03 <.8 .252 1.8 1.45 <.2 .6 ----------Central Comal flow path AY <.30 33 <.06 <.04 1.3 .148 1.1 1.39 <.2 .7 ----------AY <.30 53 <.06 <.04 <.8 .138 .8 .96 <.2 .7 ----------DX <.30 34 <.06 E.03 <.8 .26 .6 E.07 .2 .5 (Hueco A) ----------<.20 33 <.06 <.04 .08 .273 .7 .14 <.2 1 DX <.30 34 <.06 <.04 <.8 .26 .7 .35 .4 .5 (Hueco B) ----------Appendix 45

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Antimony (g/L) Barium (g/L) Beryllium (g/L) Cadmium (g/L) Chromium (g/L) Cobalt (g/L) Copper (g/L) Lead (g/L) Manganese (g/L) Molybdenum (g/L) Central Comal flow path—Continued DX <0.30 30 <0.06 0.05 <0.8 0.275 1.1 5.47 0.2 0.5 ----------<.20 33 <.06 .09 .06 .39 2.5 3.76 .6 E.2 DX <.30 27 <.06 E.02 <.8 .196 .4 1.45 <.2 .4 ----------<.20 29 <.06 <.04 .08 .277 2.5 .7 .2 .6 DX <.30 44 <.06 <.04 <.8 .179 .6 .11 <.2 .6 (Comal 1) ----------<.20 55 <.06 <.04 .24 .12 .4 <.08 1.1 .7 DX <.30 45 <.06 <.04 <.8 .179 .3 <.08 <.2 .6 (Comal 3) ----------<.20 50 .09 <.04 .27 .113 .5 .32 <.2 .6 DX <.30 38 <.06 <.04 E.4 .248 3.2 .79 <.2 .6 ----------DX <.30 38 <.06 E.04 <.8 .247 1.4 1.3 <.2 E.2 ----------<.20 36 <.06 <.04 .11 .414 3.3 .31 <.2 <.4 <.20 37 <.06 <.04 .11 .437 3.2 .29 <.2 <.4 DX <.30 37 <.06 E.02 <.8 .283 1.3 .38 3.8 .9 ----------Northern Comal flow path DX <.30 58 <.06 .07 <.8 .087 1.6 1.8 <.2 2.7 ----------Rainfall sites Rainfall site 3 --------------------Rainfall site 4 ----------46 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Nickel (g/L) Silver (g/L) Zinc (g/L) Uranium (g/L) Sulfur hexafluoride (pptv) D (per mil) 18 O (per mil) 3 H (TU) 3 He, tritiogenic (TU) Southern Comal flow path AY 3.01 <0.2 3.2 0.8 4.08 -23.09 -4.25 2.45 0 ----3.98 ----AY 2.07 <.2 2.3 .75 3.9 -22.83 -4.03 2.23 -23.53 ----3.52 ----AY 2.01 <.2 2.8 .69 1.57 -24.03 -4.22 1.99 -32.14 ----1.77 ----DX 1.47 <.2 6.8 .84 5.08 -24.14 -4.09 1.63 -57.05 (NBU–LCRA) ----4.13 ----1.68 <.2 1.2 .78 --23.83 -4.14 1.22 5.04 DX 2.59 <.2 E.7 .77 5.36 -23.31 -4.08 1.85 -38.23 (Comal 7) ----5.49 ----1.53 <.2 4.5 .77 --22.67 -4.12 1.35 12.24 DX 2.47 <.2 <1.0 .82 4.29 -23.02 -4.23 2.13 -26.43 (Comal-Spring Island) 2.46 <.2 <1.0 .81 4.18 ----1.52 <.2 2 .74 --24.27 -4.13 1.26 5.67 DX 2.64 <.2 E.5 .78 4.66 -23.27 -4.05 2.03 -61.35 (Comal 5) ----4.51 ----1.6 <.2 3.9 .76 --23.52 -4.15 1.33 7.57 DX 2.1 <.2 4 .68 2.98 -23.16 -4.14 2.21 .40 ----3.82 ----Central Comal flow path AY 2.35 <.2 3 .81 15.69 -20.89 -3.92 2.13 -3.38 ----7.69 ----AY 2.34 <.2 1.6 .78 6.33 -23.2 -4.12 1.93 -28.98 ----6.31 ----DX 5.22 <.2 2.1 .88 3.7 -25.87 -4.54 1.98 -6.06 (Hueco A) ----3.47 ----2.83 <.2 2.7 .93 --22.95 -3.96 1.53 -3.93 DX 4.89 <.2 3.1 .88 3.81 -24.46 -4.46 2.14 -6.03 (Hueco B) ----3.87 ----Appendix 47

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Appendix 1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003—Continued. State well number (USGS name) Nickel (g/L) Silver (g/L) Zinc (g/L) Uranium (g/L) Sulfur hexafluoride (pptv) D (per mil) 18 O (per mil) 3 H (TU) 3 He, tritiogenic (TU) Central Comal flow path—Continued DX 2.92 <0.2 642 0.65 1.96 -23.82 -4.28 2.30 -2.67 ----1.77 ----.28 <.2 796 .6 --23.2 -4.29 1.42 -2.64 DX 1.78 <.2 1.7 .63 2.88 -24.26 -4.18 2.21 .92 ----2.75 ----2.77 <.2 4.3 .69 --22.83 -4.33 1.60 -.08 DX 2.79 <.2 E.6 .74 4.33 -24.22 -4.13 2.03 -23.80 (Comal 1) ----4.32 ----3.47 <.2 20.6 .84 --24.12 -4.16 1.25 4.84 DX 2.68 <.2 <1.0 .76 4.19 -23.36 -4.2 1.12 -49.90 (Comal 3) ----4.19 ----1.42 <.2 1.5 .78 --23.22 -4.13 1.23 5.30 DX 2.07 <.2 10.6 .72 6.08 -22.66 -4.14 2.25 -14.22 ----4.45 ----DX 2.11 <.2 1.7 .71 2.42 -25.3 -4.5 .34 .73 ----2.07 ----3.95 <.2 5 .61 --25.14 -4.58 1.23 .02 2.98 <.2 4.9 .61 -----DX 4.4 <.2 375 .78 4.64 -24.63 -4.2 2.15 -.01 ----5.16 ----Northern Comal flow path DX 1.51 <.2 21.4 1.02 1.16 -22.44 -3.86 .00 -15.28 ----1.14 ----Rainfall sites Rainfall site 3 ------35.76 -4.92 --------38.04 -6.02 --Rainfall site 4 ------41.7 -6.1 --48 Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer . . . Texas

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Prepared by the USGS Lafayette Publishing Service Center. Information regarding water resources in Texas is available at http: //tx.usgs.gov/


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