Conceptual Model for the Edwards–Trinity (Plateau) Aquifer System, Texas


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Conceptual Model for the Edwards–Trinity (Plateau) Aquifer System, Texas

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Conceptual Model for the Edwards–Trinity (Plateau) Aquifer System, Texas
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Aquifers of the Edwards Plateau
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Anaya, Roberto
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Aquifers of the Edwards Plateau, Vol. 360 (2004).

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21Chapter 2 Conceptual Model for the Edwards–Trinity (Plateau) Aquifer System, Texas Roberto Anaya1 Introduction The passage of Senate Bill 1 in 1997 establishe d a renewed public interest in the State’s water resources not experienced since the dr ought of the 1950s. Senate Bill 1 of 1999 and Senate Bill 2 of 2001 provide d state funding to initiate th e development of groundwater availability models for all of the major a nd minor aquifers of Texas. The development and management of Groundwater Availability Models (GAMs) has been tasked to the Texas Water Development Board (TWDB) to provide reliable and timely information on the State’s groundwater resources. TWDB st aff is currently developing a GAM for the Edwards–Trinity (Plateau) aquifer. An esse ntial task in the design of a numerical groundwater flow model is th e development of a conceptual model. The conceptual model is a generalized description of th e aquifer system that defines boundaries, hydrogeologic parameters, and hydrologic stress variables. The conceptual model helps to compile and organize field da ta and to simplify the real-wor ld aquifer flow system into a graphical or diagrammatical representation while retaining the complexity needed to adequately reproduce the system beha vior (Anderson and Woessner, 1992). The first step in the development of a concep tual model is to delin eate the study area and form an understanding of its physical landscap e with regard to th e physiography, climate, and geology. Another early step also involves the research an d investigation of previous aquifer studies. Intermediate steps bring toge ther all of the information for establishing the hydrogeologic setting which consists of the hydrostratigraphy, structural geometry, hydraulic properties, water levels and regiona l groundwater flow, recharge, interactions between surface water and groundwater, well discharge, and water quality. Assembling the information into organized descriptive te xt, maps, tables, and diagrams concludes the development of the conceptual model. The pur pose of this chapter is to document the development of the conceptual model for the Edwards–Trinity (Plateau) aquifer system in central-west Texas. 1 Texas Water Development Board

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22Geographic Setting The Edwards–Trinity (Plateau) aquifer extends over an area of about 35,000 square miles beneath all or parts of 38 counties (Ashwo rth and Hopkins, 1995) in central-west Texas (Figure 2-1). Most of the counties have re latively sparse populations concentrated in small towns, usually the county seats (F igure 2-2). The study area extends to the northwest to include the Cenozoi c Pecos Alluvium aquifer and to the southeast to include the Trinity aquifer because of their hydrauli c connection to the Edwards–Trinity (Plateau) aquifer (Figure 2-1). The Edwards–Trinity (Plateau) aquifer is also hydraulically connected to several other major and minor aqui fers of the state, which is discussed later in this chapter. The study area falls within five Regional Water Planning Areas (Far West Texas, Lower Colorado, Plateau, Region F, and South Central Texas), although the aquifer is located mostly within Region F a nd the Plateau region (Fi gure 2-3). In addition to two Priority Groundwater Management Areas, there are about 29 groundwater conservation districts in th e study area (Figure 2-3). Physiography Physiography describes the natural features of the landscape in th e context of topography, landforms, surface drainage, soils, and natu ral vegetation, all of which reflect upon the geologic and climatic history of the region. In the United States, natural regions have been hierarchically delineated into phys iographic divisions, pr ovinces, and sections (Fenneman, 1931; Thornberry, 1965). Under th is classification, the Edwards Plateau Section occupies the southern margin of th e Great Plains Province, which is located within the western portion of the Interior Plains Divi sion. The Bureau of Economic Geology (BEG) at The University of Texas at Austin has delineated the Edwards Plateau into one of seven physiographic provinces within the state of Texas (Wermund, 1996). The BEG further subdivides th e Edwards Plateau Province in to the Principal Edwards Plateau, the Pecos Canyons, and the Stockton Plateau sub-provinces (Figure 2-4). The LBJ School of Public Affairs delineated el even natural regions within the state for statewide and/or regional analysis (LBJ Sc hool of Public Affair s, 1978). Many of the eleven regions also consist of two or mo re sub-regions. The Edwards Plateau Region includes the Live Oak-Mesquite Savanna, Balcones Canyonlands, and Lampasas Cut Plain sub-regions. The 1957 revision of Erwi n Raisz’s “Landforms of the United States” remains a classic hand drawn map of natu ral features depicting landforms and physiographic regions of the country with re markable detail (Raisz, 1957). The landscape image on Figure 2-4 identifies natural landf orms and physiographic regions of the Edwards Plateau and adjacent landscape comp iled from the various classifications. The Edwards Plateau is defined here from the perspective of the laterally contiguous sediments of the Edwards–Trinity (Plateau) aquifer system. The Balcones Fault Zone, a system of stair-stepped faults essential to the development of the Edwards Plateau, has displaced the aquifer sediments and juxta posed them against younger and less resistant sediments of the Gulf Coastal Plains. The re sulting fault displacements have formed the Balcones Escarpment, a feature so prominent, that the effects on the landscape along the

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Figure 2-1: Areal extent of the Edwards–Tr inity (Plateau) aquifer and the study area.

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Figure 2-2: Population density from 2000 census.

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Figure 2-3: Boundaries of Regional Water Pl anning Areas (E = Far West Texas, F = Region F, J = Plateau, K = Lower Colorado, L = South Central Texas), Groundwater Conservation Districts, and Prio rity Groundwater Management Areas.

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Figure 2-4: Landscape image showing natu ral landforms and physiographic regions.

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27southern and southeastern margins of the pl ateau are visible from space (Figure 2-5). A pronounced removal of Edwards Group or uppe r layer of aquifer sediments by the headward erosion of major streams trans ecting the Balcones Escarpment formed the Balcones Canyonlands, more traditionally know n as the Hill Country. The plateau and the aquifer sediments then terminate to the east along a margin of exposed Paleozoic and Pre-Cambrian rocks in the Central Mineral Re gion, often referred to as the Llano Uplift. Alluvial deposits form the Lipan Flats where the Concho River has cut and filled its way onto the northern plateau. Beyond the Lipan Flats and the northern margin of the Edwards Plateau are the Osage Plains, more commonly called the Ro lling Plains, which extend west to the Caprock Escarpment of the High Plains. A thin drape of remnant Quaternary sand sediments, with playa lakes characteristic of the Llano Estacado (Staked Plains) region of the High Pl ains, extend down to cover a small area of the northeastern Edwards Plateau. These remnant sand sedi ments and the underlying Edwards–Trinity aquifer sediments terminate to the southwest along the southeast trending Mescalero Escarpment. The eastern flanks of the Rustle r Hills (east of the Delaware Mountains), and the Apache, Davis, Glass, and Santia go mountains of the Trans-Pecos Basin and Range form the western boundary of the Edward s Plateau. Thick alluvial deposits fill the Pecos Valley between the western plateau boundary and the Mescalero Escarpment. A local drainage area between the Davis Mountains and the Pecos River is often referred to as the Toyah Basin. The Edwards–Trinity (Pla teau) aquifer sediments extend beneath the Stockton Plateau located east of the Glass a nd Santiago mountains and west of the Pecos River Canyon and continue south into the western Big Bend region and across the Rio Grande into the northern area of the Mexican Chihuahua Desert. Topography and Landform The landform of the Edwards Plateau may be de scribed as a flat tabl eland gently sloping from the northwest at about 3,000 feet above se a level to the southeast at about 2,000 feet above sea level (Figure 2-6). The Edwards Plat eau is also one of the largest contiguous karst regions in the United States (Kastni ng, 1984). The karst mor phology exhibits poorly integrated solution features with a sponge-lik e pattern on the platea u proper, whereas the southern plateau margin has well defined co rridors of connected conduits aligned with faults and fractures of the Balcones Fault Z one (Kastning, 1984). The plateau is capped with thick Edwards Group limestone sedime nts that protect underlying less resistant Trinity and Paleozoic sediments from eros ion. The Edwards Plateau has been in a prevailing state of erosion since the form ation of the Balcones Escarpment. As the protective limestone cap is br eached along the plateau’s marg ins, streams easily carve deep canyons into the softer underlying se diments along the northern, eastern, and southern edges of the platea u. Topographic relief for the entire study area ranges from about 5,000 feet in the mountains of the Tran s-Pecos to about 500 f eet along the Lake Austin reach of the Colorado River where it flows across the Balcones Escarpment. The greatest local surface relief o ccurs in the mountainous Trans-Pecos region and the western Hill Country.

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Figure 2-5: Satellite image view of centr al-west Texas showing the Edwards Plateau and the Balcones Escarpment (Image source from University of Texas Center for Space Research).

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Figure 2-6: Topographic elevati ons of the Edwards Plateau.

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30Surface Drainage Permanent surface water is sparse to non-ex istent on the Edwards Plateau and occurs only along the spring-fed trib utaries that dissect the nor thern, eastern, and southern plateau margins. Streams draining the plateau ha ve a very dendritic or branch-like pattern characteristic of drainage patterns for fl at-lying geologic strata (Figure 2-7). Stream density (stream channel length per unit area) on the plateau is mostly influenced by local and regional surface-water gradients. Stream density increases with increasing surface water gradients and approaches zero as the topography beco mes flat where playa lakes may be the dominant surface-drainage features. However, there is also a distinct increase in stream density towards the east that may be attributed to both the spatial distribution of eastward-increasing mean annual precipitation and the southeastward regional outflow of groundwater draining from springs of the Edwa rds–Trinity (Plateau) aquifer. Tributary streams of the Colorado River such as the Concho, San Saba, Llano, and Pedernales rivers drain the northeastern portion of the Ed wards Plateau into the Lipan Flats and the Llano Uplift. The Blanco, Guadalupe, Medina, Sabinal, Frio, and Nueces rivers drain the southeastern and southern portion of the pl ateau through the Hill Country and across the Balcones Escarpment. The Pecos River and Devils River, both major tributaries to the Rio Grande, drain the entire southwestern ha lf of the study area. Although there are some small surface-water bodies (less than one square mile) in the central region of the plateau, the only noteworthy water bodies on the plateau are Big Lake (more often a dry lakebed) in Reagan County, Orient Reservoir in Pecos County, and Balmorhea Lake in Reeves County. Other much larger water bodies along the edge of the Edwards Plateau include Amistad Reservoir in Val Verde County, Twin Buttes and San Angelo Reservoirs in Tom Green County, and E. V. Spence Reservoir in Coke County. Soil Development The predominant soils for most of the Edward s Plateau are classified as Ustolls, a suborder of Mollisols that drain easily and develop under grass or savanna type vegetation in subhumid to semiarid climates (USDA, 1999) . In the northwestern-most portion of the plateau, remnant soils thicken into more sandy, loamy soils characteristic of the Staked Plains. These soils are classified as Aridisols and are based on the limited availability of soil moisture to sustain plant growth (USDA, 1999). The Aridisols also extend westward across the Pecos Valley into the Trans-Pecos Basin and Range, cover the southern portion of the Stockton Plateau, and continue south in to the Big Bend region. In the eastern most portion of the plateau where the Edwards Gr oup sediments have been removed, the soils have minimal soil horizon development and form on steep slopes of young geomorphic surfaces in a humid to subhumid climate (U SDA, 1999; University of Idaho, undated). These soils of the eastern Hill Country are classified as Inceptisols. Another soil order found on the plateau includes the Vertisols, wh ich are clay-rich and have high shrink and swell potential (USDA, 1999; University of Idaho, undated). These Ve rtisols are located in a small central portion of the plateau al ong the northwest to s outheast trending flat topographic divide of the Colorado River and Rio Grande and in another small area of southern Reeves and western Pecos counties (Figure 2-8). Pl eistocene paleo-soils that

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Figure 2-7: Drainage density and dendritic pattern of surface water drainage on the Edwards Plateau.

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Figure 2-8: Spatial distribution of so il order types on the Edwards Plateau.

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33formed about 0.73 to 2.0 million years before pr esent and often called “terra rossas” may be found scattered throughout the plateau, usua lly within caves and sinkholes where they have been protected from erosion (Young, 1986) . Following the last glacial maximum of the Late Pleistocene, the rate of soil eros ion on the plateau is t hought to have increased due to an increase in aridity and increased variability of seasonal precipitation (Cooke and others, 2003). However, this climate-driven event produced a rate of soil erosion an order of magnitude less than the more recen t human induced erosion of soil from the plateau (Cooke and others, 2003). Heavy graz ing and the suppression of natural grass fires during the past 150 years of European se ttlement have augmented the erosional state of the plateau and allowed the soils to de velop thin and stony characteristics (Mecke, 1996). The thin characteristic nature of soils of the Edwards Plateau is shown in Figure 29. Natural Vegetation Early Spanish explorers described the vege tation on most of the Edwards Plateau as being dominated by a diversity of mid to tall grasses with short gra sses covering the more arid western regions (Mecke , 1996). The grasslands have since been transformed by unsustainable landuse into a stunted scrubby sa vanna of oak, juniper, and grass in the north and east and desert shrub and woody mesquite brush in the southwest. The combined landuse effects of overgrazing a nd inhibiting the natural rejuvenation of grasses by fire allows invasive w oody species such as mesquite or ashe juniper, often referred to as cedar, to change the landscape. The loss of grasslands reduce the amount of effective rainfall available for groundwater recharge and increase soil erosion while the woody vegetation consume more of the effectiv e rainfall drying up natural springs. Salt cedar has invaded some stream valleys contributing to significant amounts of evapotranspiration. On the steeper canyon slope s, oak forests and oa k–juniper woodlands are common. Some recent range management and brush control projects such as the Bamberger Ranch in Blanco County have shown that restoring the na tive grasslands is not only possible, but also be neficial in recovering spri ngs dehydrated by the loss of grasslands to invasive and thirsty woody vegetation. Additional discussion of brush management on the plateau is pr ovided in a later chapter. Landuse Ranching of cattle, sheep, and goats along with wild game hunting leases are the primary forms of land use except for the northern porti on of the plateau where irrigated croplands of cotton and grain sorghum ar e the more dominant land use. Oil and gas production from deep underlying Permian Basin sediments is also common in the northern and western portions of the plateau. Hay, pasture, and sma ll grains are grown in some of the valleys along the southern and eastern margins of the plateau where surface water and rainfall is more readily available. Over a sufficient pe riod of time, mankind is capable of changing the natural vegetation, soils, hydrologic char acteristics, and consequently the natural physiographic features of a large region su ch as the Edwards Plateau through landuse alone.

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Figure 2-9: Spatial distribution of soil thickness on the Edwards Plateau. Note the thi nner soils of the plateau.

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35Climatic History The climate of the Edwards Plat eau was twice as wet during the Pleistocene as it is today according to studies of the “terra rossas ” found in central Texas (Young, 1986). At some point after the last major ice age, the climat e became more arid and variable (Cooke and others, 2003). The more recent climate of the Edwards Plateau ranges from subhumid in the eastern to semiarid in the western plat eau (Walker, 1979). The long-term mean annual precipitation for the study area ranges from about 34 inches in the east to about 12 inches in the west (Figure 2-10, for years 1895 thr ough 2000). On the eastern two thirds of the Edwards Plateau, precipitation occurs mostly during late spring and early fall as cool northern frontal air masses collide with warm moist Gulf air masses from the south. On the western third of the plateau, most of th e precipitation occurs dur ing July, August, and September. Figure 2-11 shows the signifi cance of orographic precipitation along the Balcones Escarpment that is evident by the we stward distortion of the thirty-year mean annual rainfall contours for 1961 to 1990 (Carr, 1967). Tropical disturbances occasiona lly find their way onto the plateau from the warm late summer waters of the Gulf of Mexico and cont ribute to variability in annual precipitation totals. The variability of rainfall generally increases towards the arid west (Bomar, 1983) and the variation of total monthly precipitat ion is greatest for the month of September throughout the plateau. Other variations in th e mean annual precipitation of the plateau may also be attributed to the cyclic in teraction between the Pacific Ocean and the atmosphere, known as the El Nino-Southern Os cillation. Precipitati on usually increases during the El Nino phase and decreases duri ng the La Nina phase with the greatest variations in rainfall oc curring during the fall and wi nter periods (Slade, 2001). Moreover, the statistical data analysis of st ream-flow and precipitation records suggest a slight apparent increasing tre nd in the general vari ability of precipitati on events over time (Slade, 2001). Rates of evaporation are hi gh throughout the plateau and range between 43 inches in the east (Walker, 1979) to 80 in ches in the west (Rees and Buckner, 1980). Droughts are common on the Edwards Plateau w ith about 10 moderate to severe droughts during the last 100 years. Th e drought of record occurred during the 1950s, consistent with the rest of the state. Drought on the plateau is disc ussed in more detail in a later chapter. Geologic History The Edwards–Trinity (Plateau) aquifer is com posed of Early Cretaceous age sediments of the Trinity, Fredericksburg, and Lower Wash ita groups (Figure 2-12). The Trinity Group sediments form the underlying Trinity portion of the aquifer while the Fredericksburg and Lower Washita Group sediments form the overl ying Edwards portion of the aquifer. The Edwards–Trinity aquifer sediments rest unc onformable on top of an uneven erosional surface of Pre-Cretaceous age sediments, mo stly folded and faulted Paleozoic age sediments. The following subsections a brie f historical account of the evolutionary development of the Edwards–Trinity (Plateau ) aquifer system (Figures 2-13 and 2-14).

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36 Figure 2-10: Long-term mean a nnual precipitation for 1895 to 2000. Annual Precipitation (1895-2000) Texas Climate Division 50 10 20 30 40 50 18951905191519251935194519551965197519851995Year Annual Precipitation (1895-2000) Texas Climate Division 60 10 20 30 40 50 18951905191519251935194519551965197519851995Year

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Figure 2-11: Mean annual pr ecipitation for 1961 to 1990 in Texas (in inches).

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38 Figure 2-12: Stratigraphic chart of the Ed wards–Trinity aquifer sediments (after Barker and Ardis, 1996).

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39 Figure 2-13: Structural elements affecti ng the depositional environments of the Edwards–Trinity sediments (from Barker and Ardis, 1996).

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40 Figure 2-14: Evolutionary development of th e Edwards–Trinity aquifer system (after Barker and others, 1994).

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41Paleozoic The Paleozoic Era ended with a tectonic event known as the Qu achita orogeny. This orogenic event resulted in the formation of a structural fold belt of sediments deposited during the Ordovician, Silurian, Devonian, a nd Mississippian periods. The sediments were uplifted, faulted, and folded into a late Paleozoic mountain range that extended from northern Mexico along the present day Ba lcones Escarpment up into the Quachita Mountains of Oklahoma and Ar kansas (Barker and Ardis, 1996). Before a final uplift during the Paleozoic Era, an arid and rest ricted shallow marine sea deposited Upper Permian sediments and evaporites into the Permian Basin of West Texas. Triassic During the Triassic Period, terrigenous clastic red beds were deposited over the Paleozoic rocks as the Dockum Group sediments. Th e area of the Edwards Plateau was then exposed to erosion during the Jurassic Peri od to form a rolling peneplain known as the Wichita Paleoplain (Barker and Ardis, 1996). By the end of the Jura ssic Period, the Gulf of Mexico had begun to open and tilting of th e peneplain towards th e southeast provided a structural base for the deposition of th e Cretaceous age Edwards–Trinity sediments. Cretaceous As the Gulf of Mexico continued to open and the Cretaceous seas advanced from the southeast, a broad continenta l shelf known as the Comanche Shelf began to form. The Llano Uplift, a tectonically active structural feature since the Pre-Cambrian, became a prominent structural shelf element for th e deposition of the Trinity Group sediments (Barker and Ardis, 1996). The Early Cretaceous seas advanced across the Pre-Cretaceous structural base in three cycl es of transgressive-regressive stages to deposit the Trinity Group sediments (Barker and others, 1994). The Stuart City Reef Trend began to form parallel to the ancestral Gulf of Mexico about 150 miles inland from the present Gulf Coast enabling the carbonate platform de posits of the Edwards Group sediments to accumulate to the northwest behind the protec tion of the reef. Other structural shelf elements that formed behind the Stuart City Reef Trend and controlled the depositional environments and lithologic characteristics of the Edwards Group formations include the Central Texas Platform, the San Marcos Arc h, the Devils River Reef Trend on the edge of the Maverick Basin, and the Fort Stockton Basin (Figure 2-13). Prior to the deposition of Upper Cretaceous Del Rio, Buda, Boquillas, and Austin Group sediments, much of the Central Texas Platform was sub-aerially expose d allowing for an initi al karstification of Lower Cretaceous carbonate sedi ments (Barker and others, 1994). Cenozoic Towards the end of the Cretaceous and beginni ng of the Tertiary Periods, the Laramide orogenic event and the di ssolution of Upper Permian sediment s, resulted in the structural collapse and erosion of overlying Triassic and Cretaceous sediments along the Pecos

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42River Valley (Barker and othe rs, 1994). These sediments we re then redeposited as the Cenozoic Pecos Alluvium throughout the Tert iary and into the Quaternary Periods. During the mid-Tertiary Period, regional uplif t and continued deposition of sediments into the Gulf of Mexico provided tensional st resses along the ancient Quachita fold belt. Consequently, the development of the Balc ones Fault Zone occu rred and displaced Cretaceous and Lower Tertiary sediments by 900 to 1,200 feet (Barker and others, 1994). The Ogallala sediments were deposited over a portion of the Edwards–Trinity sediments from the northern region of the plateau duri ng the late Te rtiary Period. The headward erosion of streams has reduced the plat eau into its current form throughout the Quaternary. Previous Investigations Previous studies on the Edwards–Trinity (P lateau) aquifer began with county-wide studies by the Texas Board of Water Engin eers, the Texas Water Commission, the Texas Department of Water Resources, the Texa s Water Development Board, and the U.S. Geological Survey. The Texas Department of Water Resources was the first to publish regional study reports on the Trans-Pecos (R ees and Buckner, 1980) and Plateau (Walker, 1979) portions of the Edward-Trinity aquifer. During the late 1980s, the U.S. Geological Survey (USGS) began a Regional Aquifer Sy stems Analysis (RASA) program for the Edwards–Trinity aquifer system that resulted in the publication of the most recent and comprehensive reports on the aquifer sy stem model (Bush, 1986; Kuniansky, 1989; Kuniansky, 1990; Barker and Ardis, 1992; Ar dis and Barker, 1993; Bush and others, 1993; Barker and others, 1994; Bush and others 1994; Barker and Ardis, 1996) as well as a single-layer finite element steady-state numerical groundwater model (Kuniansky and Holligan, 1994). The USGS has also publis hed a groundwater atlas for Oklahoma and Texas that includes a very informative executive summary of the Edwards–Trinity aquifer system (Ryder, 1996). Currently, the Texas Water Development Board is conducting a comprehensive study to develop a st ate-of-the-art two-la yer finite difference numerical groundwater model of the Edwards–Tr inity aquifer system with a final report due for publication in 2004. Information on this most recent modeling study is maintained at the following Web addr ess: http://www.twdb.state.tx.us/gam/. Hydrogeologic Setting The hydrogeologic setting provides for an unders tanding of the aquifer system’s physical parameters, stress variables, and their intera ctions. Physical parameters include aquifer characteristics that remain constant over relatively long periods of time such as the hydrostratigraphy, structural ge ometry, hydraulic properties, steady-state water levels, and regional groundwater flow. Stress vari ables include aquifer characteristics that fluctuate over time such as recharge, well discharge, and natural interactions between groundwater and surface water such as springs, streams, and lakes.

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43Hydrostratigraphy The hydrostratigraphy represents the vertical and lateral organization of the various hydrogeologic units of the Edwards–Trinity (Pla teau) aquifer system and is shown in the stratigraphic chart on Figure 2-12 and discussed below. Paleozoic The Hickory, the Ellenburger–San Saba, and the Marble Falls aqui fers are hydraulically connected to the Edwards–Trinity aquifer along the eastern margin of the plateau surrounding the Llano uplift. The Permian age Capitan and Rustler sediments are hydraulically connected to the Edwards–Trinit y sediments in the Trans-Pecos portion of the aquifer (Bush and others, 1994). In genera l, most of the underlying Paleozoic rocks provide for a relatively impermeable base for the Edwards–Trinity aquifer sediments (Barker and Ardis, 1992). Triassic The Dockum Group consists of the Lower (T ecovas Formation), Middle (Santa Rosa Formation), and Upper (Chinle Formation) members (Walker, 1979). Only where the Chinle Formation is missing, allowing for th e Basal Cretaceous sands to be in hydraulic communication with the underlying Santa Rosa Formation, is the Santa Rosa Formation considered to be an aquifer (Walker, 1979). Cretaceous The Trinity Group sediments are composed of the Lower, Middle, and Upper Trinity aquifer units in the south eastern portion of the plateau (Ashworth, 1983). The Lower Trinity consists of Hosston Sand (Sycamore Sand in the outcrop), Sligo Formation, and Hammett Shale. The Middle Trinity consists of the Cow Creek Limestone, Hensel Sand, and the lower member of the Glen Rose Li mestone. The Upper Trinity consists of the upper member of the Glen Rose Limestone (Mace and others, 2000). In the Trans-Pecos region of the plateau, the Trinity Group sediments are composed of the Yearwood Formation and the Cox Sandstone. Elsewhere on the plateau, the Trinity Group sediments are composed of the Basal Cretaceous Sand, the Glen Rose Limestone, and the Maxon Sand. The Basal Cretaceous Sand and Maxon Sa nd are sometimes lumped together and referred to as the Antlers Sand or Trinity Sa nds in the northern plateau area where the Glen Rose Limestone is absent. The Edwards Group and equivalent sediments consist of the Fredericksburg and Lower Washita Group sediments. The Fredericksburg Group consists of the Finlay Formation within the Fort Stockton Basin; the Fort Terrett Formation within the Central Texas Platform; the Devils River Formation within the Devils River Reef Trend; the West Nueces and McKnight formations within the Maverick Basin; and the Kainer Formation within the San Marcos Arch. The Lower Wa shita Group sediments are composed of the Boracho Formation within the Fort Stockton Basin; the Segovia Formation within the

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44Central Texas Platform; the Devils River Form ation within the Devils River Reef Trend; the McKnight and Salmon Peak formations w ithin the Maverick Basin; and the Person Formation within the San Marcos Arch. The Upper Cretaceous sediments include the upper most section of the Washita Group sediments (the upper confining Del Rio Clay and the Buda Limestone) along with the Eagal Ford Group (Boquillas Formation) and the Austin Group sediments. Cenozoic The Cenozoic Pecos Alluvium aquifer is hydra ulically connected to the Edwards–Trinity (Plateau) aquifer in the northwe stern edge of the aquifer. The Upper Tertiary Ogallala Formation is hydraulically connected only in the northern-most portion of the Edwards– Trinity (Plateau) aquifer. Structural Geometry The initial base depositional surface of the Cr etaceous sediments is generally flat and tilted towards the Gulf of Me xico. Consequently, the Edward s–Trinity sediments form a wedge that thickens from th e north and northwest towards the south and southeast. The exceptions to this structural trend are in the areas near the Lla no Uplift and the TransPecos Basin and Range. The Llano Uplift is a tectonic structural hi gh that has persisted throughout the geologic history of the region. Edwards–Trinity sediments were deposited over this structural high and later removed by erosion. The Trans-Pecos Basin and Range is the result of a more recent geologic even t, the Laramide Orogeny, which uplifted and block faulted the Edwards–Trinity sediment s into mountain ranges and graben basins. The wedge of Cretaceous sediments pinches ou t beneath the Ogallala sediments in the northern portion of the plateau (Barker a nd Ardis, 1996). The wedge of Cretaceous sediments is faulted and offset along the s outh and southeast by th e Tertiary Balcones Fault Zone system. Most of the Edwards Group sediments and portions of the Upper Trinity sediments have been removed in th e canyonland areas of the Texas Hill Country. A small portion of the Edwards–Trinity (Plateau) aquifer is confined in Val Verde and Kinney counties beneath the Upper Cretaceous Del Rio Clay. The semi-permeable Upper Cretaceous sediments of the Buda Limestone and Boquillas Formation form a thin cap over the Edwards Group in the central and so uthwestern portions of the aquifer. Data were collected from several sources and merged within a Geographic Information System (GIS) for analysis. By using geosta tistical techniques within the GIS framework, structural surfaces were developed for the tops and bottoms of both the Edwards and Trinity aquifer units (Figures 2-15 and 2-16). The base of the Trinity sediments shows a paleo-valley coincident with the lower P ecos River (Figure 2-15). In addition, a local high exists near the intersecti on of Schleicher, Menard, Sutt on, and Kimble counties. The base of the Edwards sediments also show s the same local high in addition to the Maverick Basin along the southwestern porti on of Val Verde County (Figure 2-16). The structural surfaces show a steep gradient al ong the Balcones Fault Zone that generalizes the fault

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Figure 2-15: Interpolation of the base of the Trinity aquifer unit.

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Figure 2-16: Interpolation of the ba se of the Edwards aquifer unit.

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47system rather than actually representing the ab rupt stair-stepped offs ets of the individual faults. Hydraulic Properties The Edwards–Trinity (Plateau) aquifer is hydra ulically connected to four major aquifers: (1) the Cenozoic Pecos Alluvium, (2) the Og allala, (3) the Trinity, and (4) the Edwards (Balcones Fault Zone). The Edwards–Trinity (Plateau) aquifer is also hydraulically connected to several minor a quifers: (1) the Dockum, (2) th e Capitan Reef Complex, (3) the Rustler, (4) the Hickory, (5) the Ellenburger-San Saba, (6 ) the Lipan, and, to a very small degree, (7) the Marble Falls. The saturated thickness of less than 100 feet to greater than 800 feet for the Edwards– Trinity (Plateau) aquifer system generally in creases from north to south and varies the greatest along the western margins of the a quifer. Gentle north-south trending ridges and troughs of the folded Paleozoic base deposit ional surface combined with the topographic influence on the water table control the variability in sa turated thickness (Barker and Ardis, 1996). The aquifer is mostly under wa ter table or unconfin ed conditions, although the Trinity unit of the aquifer may be semi-confined locally where relatively impermeable sediments of the overlying basal member of the Edwards Group exists (Ashworth and Hopkins, 1995). Transmissivity is a function of the conductivity of the aquife r sediments and the saturated thickness. The Edwards–Trinity (Plateau) aquife r generally has transm issivity values of less than 5,000 feet squared per day in the nor th and eastern portions of the aquifer and values between 5,000 and 50,000 feet squared per day in the southern and western portions of the aquifer with an average of less than 10,000 feet squared per day (Barker and Ardis, 1996). Except for areas of signi ficant karst induced permeability, the average hydraulic conductivity of the Edwards–Trinity a quifer sediments is about 10 feet per day based on transmissivity and saturated thickne ss distributions (Barke r and Ardis, 1996). Specific-capacity test data were collecte d from the Texas Commission on Environmental Quality (TCEQ) and analyzed to calculate hydraulic conductivity values for both the Edwards and Trinity aquifer units. The spatial lo cations of these specific capacity data are limited to the area of a standard two and one -half minute quadrangle. An assumption was made to locate the specific-capacity data to the center of their respective quadrangle. Pumping-test data were then collected from the TWDB groundwater database and analyzed to calculate hydraulic conductivity for both Edwards and Trinity aquifer units. In addition, one or more pumping tests were conducted in almost every county of the aquifer system by TWDB staff during 2000. The pumping test data had unique latitude and longitude coordinates assigned to them. The calculated conductivity data were then analyzed and spatially interpolated using geostatistical techniques (Figures 2-17 and 218). The geometric mean of the hydraulic conductivity for the Edwards aquifer unit was calculated at about 6.5 feet per day. The me dian hydraulic conductivity for the Trinity

PAGE 28

Figure 2-17: Estimated spatial interp olation of hydraulic conductivity fo r the Trinity aquifer sediments.

PAGE 29

Figure 2-18: Estimated spatial interp olation of hydraulic conductivity fo r the Edwards aquifer sediments.

PAGE 30

50aquifer unit was calculated at a bout 2 feet per day for the sout hern part of the aquifer and about 4.5 feet per day for the northern part of the aquifer. Two different median hydraulic conductivity values were calculated for the Tr inity aquifer unit because of the difference in sediment composition between the southern and northern parts of the aquifer unit. The southern part of the aquifer is composed of the shale, sand, and lim estone transgressiveregressive sequence of the Lower, Middle, a nd Upper Trinity whereas the northern part is composed of the Trinity Sands. Water Levels and Region al Groundwater Flow Although water levels of the Edwards–Trinity aquifer system are influenced by climate, they have remained fairly constant except in areas of the northern and western plateau where a general trend of declining water leve ls has occurred as a result of increased irrigation pumpage (Ashworth and Hopkins, 1995). Steady-state water levels for the Edwards–Trinity aquifer were analyzed to gain an understanding of the regional groundwater flow within the aquifer syst em. Water level data from the TWDB groundwater database were queried for the firs t winter measurements of each well record available for both the Edwards and Trinity aquifer units and excluding measurements taken during the 1930s and 1950s drought years. The water-level data were queried with the assumption that they would have minimal influence from climate and pumpage, thus representing steady-state aquife r conditions. The water-level data were then entered into a GIS for inspection against the structural su rfaces of their corres ponding aquifer units. The quality controlled water level data were then analyzed and interpolated into potentiometric or water level surfaces for both the Trinity and Edwards aquifer units using geostatistical technique s (Figures 2-19 and 2-20). The Pecos River has a very si gnificant influence on groundwater flow in the western half of the aquifer system. Some groundwater flow in this part of the a quifer occurs as crossformational flow from the eastern flanks of the Trans-Pecos mountains into the Cenozoic Pecos Alluvium Aquifer. Regional groundwater flow east of the Pecos River is generally from the northwest towards the southeast. A regional groundwater divi de coincident with the surface topography and trending from the northwest in Ector county to the southeast near the common boundary of Real, Kerr, and Edwards counties separates flow towards the Colorado River from flow towa rds the Pecos River and Rio Grande. Trinity water levels in the west show a steep gradient towards the Pecos River and towards the Rio Grande. An area of anomalous low water levels appears in the central part of the Reagan-Glasscock county boundary, historically an area of concentrated groundwater irrigation. Anomalous low water leve ls are also visible within an area of concentrated oil production in central west Midland County. To the north and east of the Pecos River, the Trinity water levels have a more gentle and subdued surface with only the lower Devils River in southern Val Verd e County having an apparent effect on the water level surface. Trinity water levels conti nue with the gentle surface gradient south towards the Balcones Fault Zone an d southeast into the Hill Country.

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Figure 2-19: Historical mean winter water levels for the Trinity aquifer unit.

PAGE 32

Figure 2-20: Historical mean winter water levels for the Edwards aquifer unit.

PAGE 33

53Edwards water levels show a steep gradient towards the Rio Grande in Terrell County as well as the southern part of the aquifer just to the north of the Blacones Fault Zone in Kinney and Uvalde counties. Anomalous low wa ter levels appear in southern Reagan County near Big Lake. The central reach of th e Devils River affects Edwards water levels in the southeastern corner of Crockett C ounty. A mounding of the wa ter-level surface is visible in the area near the common boundary of Real, Kerr, and Edwards counties forming a southwest-northeast trending saddl e shaped valley through northern Edwards and southeastern Kimble counties. Recharge Recharge rates vary with climate conditi ons, surface geology, surface topography, soils, vegetation, and landuse. Most re charge occurs from the infilt ration of precipitation over outcrops of the Edwards–Trinity aquifer, fr om surface runoff into sinkholes, and stream losses from intermittent streams. An uncertain amount of cross-formational flow from the Ogallala aquifer also provides recharge to the Edwards–Trinity aquifer system in the northwestern portion of the a quifer. Induced recharge occurs in Pecos and Reeves counties as a result of water-level declines due to irrigation pumpage from the Cenozoic Pecos Alluvium aquifer (Barker and Ardis, 1996). Rees and Buckner (1980) estimated recharge ov er the Trans-Pecos region of the plateau to be between about 0.3 and 0.4 inches per year. Kuniansky (1989) estimated recharge over the eastern portion of the plateau to range between 0.12 and 2.24 inches per year. The simple strategy of using four percent of the mean annual precipitation will be used as an initial estimate for recharge for the main approach of calibrati ng recharge within the numerical groundwater flow model. The range in estimated recharge values by other investigators in conjunction with reasonable values in aquifer conductivity estimates will be used as limits in the calibration process. Interactions of Ground water and Surface Water Natural discharge from the Edwards–Trinity aq uifer occurs mostly along the margins of the aquifer from springs where the water ta ble intersects canyons or surface topography to provide baseflow to streams. Springs also discharge groundwater along the eastern flanks of the Trans-Pecos mountains and th e lower Pecos River canyons. As water levels decline in the western portion of the aquifer due to increa sed irrigation pumpage, spring flows within those areas have also declin ed. In addition, many small springs that once flowed throughout the plateau have ceased flow ing as a consequence of native grasslands being replaced by woody vegetation that consum e high amounts of poten tial recharge and allow other excess rainfall to runoff before it is able to recharge the aquifers. Most of the intermittent streams high on th e plateau lose their flow to the underlying aquifer. The lower reaches of major stream s along the northern, eastern, and southern margins of the plateau become gaining, usually when their stream channel elevation falls below the base of the Edwards aquifer uni t. Phreatophytes, mostly along major stream valleys, discharge groundwater naturally as evapotranspiration where the water table is

PAGE 34

54shallow enough for the root networks. The Pecos River has perhaps the most prime example of extreme evapotranspi ration by invasive salt cedar. The Edwards–Trinity aquifer interacts with re servoirs or lakes only along the southern margin of the aquifer system. These water bodies initially lost water to the aquifers and raised water levels in their vicinity but have all reached a fairly steady-state condition since the late 1970s. The largest of them is Amistad Reservoir just below the confluence of the Devils River with the Rio Grande in Val Verde County. The remaining lakes are located in the Hill Country just north of the Balcones Escarpment and include Medina Lake on the Medina River in northern Me dina County, Canyon Lake on the Guadalupe River in Northern Comal County, and Lake Austin on the Colorado River in Travis County. Pumping The Edwards Group sediments provide most of the water in the central, southern, and eastern portions of the plateau while the Tr inity Group sediments provide much of the water for the northern and western areas of the plateau in additi on to the Hill Country region (Barker and Ardis, 1996). Over three fourths of the total groundwater pumpage from the Edwards–Trinity aquifer is used fo r irrigation, primarily in the northern and western portions of the aquifer (Figure 2-21) . Municipal water suppl iers account for the second most common groundwater use followe d by minimal use for industrial, mining, livestock, and rural domestic uses. Climate has a significant effect on the amount of groundwater pumpage from the Edwards–Trin ity aquifer within areas of irrigation. Water Quality Although water quality is typically hard, it is generally fresh except for areas in the Trans-Pecos where groundwater from Permian ev aporite sediments and/or oil field brines are able to mix with groundwater from the Edwards–Trinity aquifer (Rees and Buckner, 1980). Water quality is also affected by induced recharge from Pecos River stream losses (Barker and Ardis, 1996). East of the Pecos River, oil field brines and agricultural runoff have a significant effect on the groundwater quality of the northern portion of the Edwards–Trinity aquifer (Walker, 1979). Wate r quality aspects of the Edwards–Trinity (Plateau) aquifer system are discussed in more detail in later chapters. Conceptual Model The conceptual model is a simplified interp retation that provides our best understanding of the aquifer system and defines the hydrostr atigraphic units, descri bes the water budget, and illustrates flow system (Anderson and Woessner, 1992). The conceptual model for the Edwards-Trinity (Plateau) aquifer define s two basic hydrostratigraphic units (Figure 2-22). The lower unit represents the partiall y confined Trinity aquifer while the upper unit represents the unconfined Edwards aqui fer. In addition, the lower Trinity unit is contiguously extended to the southeast to in clude the Trinity (Hill Country) aquifer.

PAGE 35

55 Figure 2-21: Pumpage from the Edwa rds–Trinity aquifer for 1980, 1985, 1990, and 1995.

PAGE 36

Figure 2-22: Conceptual model diagram of the Edwards–Trinity (Plateau) aquifer system (modified from Kuniansky, 1994).

PAGE 37

57The Cenozoic Pecos Alluvium aquifer is includ ed in the conceptual model in the TransPecos region. The water budget incorporates recharge from precipitation as the primary input into the Edwards aquifer unit. However, most of the precipitation returns to the atmosphere as evapotranspiration or exits from the study area as runoff before it can become recharge. Up to four percent of the annual rainfall ente rs the aquifer system as diffuse recharge over aquifer outcrops or as di rect recharge from losing st reams on the aquifer outcrops. Some of the recharge that occurs over th e Edwards aquifer outcrop flows downward into the underlying Trinity aquifer unit. Yet, most of the recharge that en ters into the Edwards aquifer eventually exits the aquifer unit as (1) seeps and springs along the plateau margins to become the headwaters for tributar ies of major streams; (2) baseflow to the lower gaining reaches of major perennial streams; (3) evapotranspiration where vegetation is able to tap into the wa ter table; or (4) pumping from wells. The Trinity aquifer has few outcrops exposed for recharge and consequently receives much of its water from the overlying Edwa rds aquifer except in the Hill Country where the Edwards Group sediments have been removed by erosion. In the Hill Country area, recharge over the Trinity aquifer outcrops is about four to six percent of annual rainfall. The Trinity aquifer may also receive some wate r from the adjacent Ogal lala aquifer in the northwest as cross-formational flow. The Trin ity aquifer loses its wa ter to pumping wells mostly in the Llano Estacado and Hill Count ry areas. In the Hill Country, water also flows out of the Trinity aquife r as springs and baseflow to gaining streams and as crossformational flow to the Edwards (Balcones Fa ult Zone) aquifer. In the Trans-Pecos area, water exits both the Edwards and Trinity aquifers into the Cenozoic Pecos Alluvium aquifer. After the water levels in the re servoirs and lakes reached their capacity, groundwater generally flows from the aquifer units into the reservoirs and lakes. Lithologic characteristics are the principal control on the permeability of the aquifer units. The Edwards aquifer unit has relatively high vertical and horizontal permeability because of the mostly massive limestone composition of the Edwards Group. The Trinity aquifer unit has a much more variable lit hologic composition. The northern half of the Trinity aquifer is thinner and composed of sands. The southern portion of the aquifer is composed of a thick sequence of sand, shal e, and limestone. Consequently, the northern part of the Trinity aquifer unit has higher vertical and horizontal permeability than the southern part. Moreover, the southern part has significantly lower vertical permeability than horizontal permeability because of the stratified sequence of lithologic sub-units. Conclusions The Edwards-Trinity (Plateau) aquifer is loca ted beneath the Edwards Plateau of centralwest Texas, a region characterized as a tabl eland with thin soils, subhumid to subarid climate and vegetation, and a sparse population. The aquifer system is composed of Early Cretaceous age sediments of the Trinity Group and the overlying Edwards Group.

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58The TWDB is developing a GAM for the Edwards-Trinity (Plateau) aquifer. A conceptual model was developed as an in itial task in the design of a numerical groundwater flow model of the Edwards-Trinit y (Plateau) aquifer system. The conceptual model is based on characteristics of the hydrogeologic setting such as the hydrostratigraphy, structural ge ometry, hydraulic properties, water levels, and regional groundwater flow. The conceptual model de fines two hydrostratig raphic units for the aquifer system in which the lower unit repr esents the Trinity aquifer and the upper unit represents the Edwards aquife r. In addition, the conceptu al model incorporates the Edwards-Trinity (Plateau) aquifer with the Trinity aquifer in the Hill Country and Cenozoic Pecos Alluvium aquifer systems b ecause of their unique hydraulic connection. Interactions with vertically and laterally adjacent aquifer systems as well as aquifer stresses such as recharge, well discharge, and natural interacti ons between groundwater and surface-water features such as springs, stre ams, and lakes are also represented in the conceptual model. References Anderson, M. P. and Wossener, W. W. 1992, Applied Groundwater Modeling– Simulation of Flow and Advective Trans port: Academic Press, Inc., San Diego, 381 p. Ardis, A. F., and Barker, R. A., 1993, Hist orical saturated thic kness of the Edwards– Trinity aquifer system and selected con tiguous hydraulically connected units, WestCentral Texas: U.S. Geological Survey Water-Resources Investigation Report 924125, 2 plates. Ashworth, J. B., 1983, Ground-water availability of the Lower Cretaceous formations in the Hill Country of south-central Texas: Texas Department of Water Resources Report 273, 173 p. Ashworth, J. B., and Hopkins, J., 1995, Aquifers of Texas: Texas Water Development Board Report 345, 69 p. Barker, R. A., and Ardis, A. F., 1992, Confi guration of the base of the Edwards–Trinity aquifer system and hydrogeology of the unde rlying pre-Cretace ous rocks, West Central Texas: U.S. Geological Survey Water Resources Investigation Report 914071, 25 p. Barker, R. A., and Ardis, A. F., 1996, Hydr ogeologic framework of the Edwards–Trinity aquifer system, West-Central Texas: U.S. Geological Survey Professional Paper 1421-B, 61 p. with plates. Barker, R. A., Bush, P. W., and Baker, E. T., Jr., 1994, Geologic history and hydrogeologic setting of the Edwards–Trinit y aquifer system, West-Central Texas: U.S. Geological Survey Water Resour ces Investigation Report 94-4039, 50 p. Bluntzer, R. L., 1992, Evaluati on of Ground-water Resources of the Paleozoic and Cretaceous Aquifers in the Hill Country of Central Texas: Texas Water Development Board Report 339, 130 p.

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59Bomar, G. W., 1983, Texas Weather: University of Texas Press, 256 p. Bush, P. W, 1989, Planning Report for the Edwards–Trinity Regional Aquifer-System Analysis in Central Texas, Southeast Oklahoma, and Southwest Arkansas: U.S. Geological Survey Water-Resources Investigations Report 86-4343, 15 p. Bush, P. W., Ardis, A. F., and Wynn, K. H., 1993, Historical pot entiometric surface of the Edwards–Trinity aquifer system and contiguous hydraulically connected units, West-Central Texas: U.S. Geological Surv ey Water-Resources Investigations Report 92-4055, 3 sheets. Bush, P. W., Ulery, R. L., and Rittmaster, R. L., 1994. Dissolved-solids concentrations and hydrochemical facies in water of th e Edwards–Trinity aquifer system, WestCentral Texas: U.S. Geological Survey Water-Resources Investigations Report 944126, 29 p. Carr, J. T., Jr., 1967, The climate and physiography of Texas: Texas Water Development Board Report 53, 27 p. Cooke, M. J., Stern, L. A., Banner, J. L., Lawrence, E. M., Stafford, T. W., Jr., and Toomey, R. S., III, 2003. Precise timing and rate of massive late Quaternary soil denudation: Geology, v. 31, no. 10, p. 853-856. Fenneman, N. M., 1931, Physiography of Wester n United States (1st ed.): New York, McGraw-Hill, 534 p. Harbaugh, A. W., and McDonald, M. G ., 1996, User’s documentation for MODFLOW96, an update to the U.S. Geological Survey modular finite-difference ground-water flow model: U.S. Geological Surv ey Open-File Report 96-485, 56 p. Kastning, E. H., Jr., 1984, Hydrogeomorphic evolu tion of karsted plateaus in response to regional tectonism, in LaFleur, R. G., ed., Groundwater as a geomorphic agent: Proceedings of the Thirteenth Annual Ge omorphology Symposium, Troy, New York: London, George Allen and Unwin, p. 351-382. Kuniansky, E. L., 1989, Precipitation, streamflow , and baseflow, in West-Central Texas, December 1974 through March 1977: U.S. Geological Survey Water-Resources Investigations Report 89-4208, 2 sheets. Kuniansky, E. L., 1990, Potentiometric surface of the Edwards–Trinity aquifer system and contiguous hydraulically connected uni ts, West-Central Te xas, winter 1974-75: U.S. Geological Survey Water-R esources Report 89-4208, 2 sheets. Kuniansky, E. L., and Holligan, K. Q., 1994, Simulations of flow in the Edwards–Trinity aquifer system and contiguous hydraulically connected units, West-Central Texas: U.S. Geological Survey Water-Resources Investigations Report 93-4039, 40 p. LBJ School of Public Affairs, 1978, Preser ving Texas’ Natural Heritage: Research Project Report 31, The University of Texas at Austin, p 17. Mace, R. E., 2001, Estimating transmissivity us ing specific-capacity data: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular No. 01-2, 44 p.

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60Mace, R. E., Chowdury, A. H., Anay a, R., and Way, S.-C., 2000, Groundwater Availability of the Trinity Aquifer, Hill Country Area, Texas: Numerical Simulations through 2050: Texas Water Development Board Report 353, 117 p. McDonald, M. G., and Harbaugh, A. W., 1988 , A modular three-dimensional finitedifference ground-water flow model: U.S. Geological Survey, Techniques of WaterResources Investigations of the United States Geological Survey, Book 6: Model Techniques, Chapter A1. Mecke, M. B., 1996, Historical Vegetation Ch anges on the Edwards Plateau of Texas and the Effects Upon Watersheds: Watershed ’96, EPA Conference – Moving Ahead Together, Technical Conference and Exposition, Baltimore, Maryland: Session 26, p. 281-285. http://www.epa.gov/owow/w atershed/Proceed/mecke.html Raisz, E., 1957, Landforms of the United St ates: Raisz Landform Maps, Brookline, MA. Rees, R., and Buckner, A. W., 1980. Occu rrence and quality of ground water in the Edwards–Trinity (Plateau) aquifer in th e Trans-Pecos Region of Texas: Texas Department of Water Resources Report 255, 41 p. Rose, P. R., 1972, Edwards Group, surface a nd subsurface, Central Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 74, 198 p. Ryder, P. D., 1996, Groundwater Atlas of th e United States, Segment 4Oklahoma, Texas: U.S. Geological Survey Hydrologic Investigations Atlas 730-E, p. E19-E25. Slade, R. M., Jr., 2001, Temporal Trends in the Precipitation and Related Hydrologic Characteristics for Central Texas: in Woodruff, C. M., Jr., and Collins, E. W., Trip Coordinators: Guidebook 21, Austin Geol ogical Society, Austin, Texas, April 2001, p. 55-61. Slade, R. M., Jr., Bentley, J. T., and Mich aud, D., 2002, Results of streamflow gain-loss studies in Texas, with emphasis on gains fr om and losses to major and minor aquifer: U.S. Geological Survey Open-File Report 02-068, 49 p. Thornberry, W. D., 1965, Regional Geomorphol ogy of the United Stat es: John Wiley and Sons, New York, 609 p. University of Idaho, undated, The twelve soil orders: University of Idaho Soil and Land Resources Division Web Site, http://s oils.ag.uidaho.edu/soilorders/index.htm. USDA, 1999, Soil Taxonomy–A basic system of soil classification for making and interpreting soil surveys: United States Depa rtment of Agriculture, Natural Resources Conservation Service, Agriculture Handbook Number 436, 871 p. ftp://ftpfc.sc.egov.usda.gov/NSSC/Soil_Taxonomy/tax.pdf Walker, L. E., 1979, Occurrence, availability, and chemical quality of ground water in the Edwards Plateau Region of Texas: Texas Department of Water Resources Report 235, 337 p. Wermund, E. G., 1996, Physiographic Map of Texas: Bureau of Economic Geology, The University of Texas at Austin, Texas, 1 p., 1 map plate. Young, K., 1986, The Pleistocene Terra Rossa of Central Texas: in Abbott, P. L., and

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61Woodruff, C. M., Jr., eds., The Balcones Escarpment–Geology, Hydrology, Ecology, and Social Development of Central Texas: Geological Society of America Annual Meeting San Antonio, Texas, November, 1986, p. 63-70.

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