Conceptualization of groundwater flow in the Edwards Aquifer through the Knippa Gap hydrogeologic constriction, Uvalde County, Texas

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Conceptualization of groundwater flow in the Edwards Aquifer through the Knippa Gap hydrogeologic constriction, Uvalde County, Texas

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Conceptualization of groundwater flow in the Edwards Aquifer through the Knippa Gap hydrogeologic constriction, Uvalde County, Texas
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NCKRI Symposium 2: Proceedings of the Thirteenth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst
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Adkins, Jennifer
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The Balcones Fault Zone Edwards aquifer (Edwards aquifer) is one of the major regional karst aquifers in the United States, with an average withdrawal of 950 million liters per day (L/d). This study focuses on the connection between the Uvalde pool and the San Antonio pool of the Edwards aquifer, west of the San Antonio metropolitan area in Uvalde County, Texas. This area is known as the Knippa Gap and is located north of the community of Knippa. The Knippa Gap is a major zone controlling the flow from the Uvalde pool to the San Antonio pool. The San Antonio pool is the primary source of water for the greater San Antonio water supply. The Knippa Gap is a restriction where the aquifer narrows to a width estimated to be approximately 4 km, is bounded by northeast trending faults of the Balcones Fault Zone on the north, and uplift from the Uvalde salient and igneous intrusive plugs to the south. (Green et al., 2006). The hydrogeology in the Knippa Gap has been a topic of major interest among researchers in this area for numerous years, yet the exact location, nature of boundaries, and karst hydrogeology are not well defined, and the flow through this area is in need of refinement to improve the aquifer water balance. This study integrates recent research by other scientists with field studies conducted during the summer of 2012 as part of an M.S. thesis. This paper is limited to a discussion of the water quality as it relates to the southern flow boundary of the Knippa Gap near the Devils River Trend of the Uvalde salient. Water-quality data constrain a revised conceptual model of the flow and karstification in this critical area of recharge to the San Antonio pool, and provide specific lateral boundaries and vertical karstification zones which are being tested in the more comprehensive M.S. thesis. Although current interpretations are tentative, it appears this conceptual model will be readily convertible into a digital model that can test 2hypotheses relating a much broader suite of calibration data, including water levels, water budgets, and spring discharges. -- Authors
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Description
The Balcones Fault
Zone Edwards aquifer (Edwards aquifer) is one of the major
regional karst aquifers in the United States, with an average
withdrawal of 950 million liters per day (L/d). This study
focuses on the connection between the Uvalde pool and the San
Antonio pool of the Edwards aquifer, west of the San Antonio
metropolitan area in Uvalde County, Texas. This area is known
as the Knippa Gap and is located north of the community of
Knippa. The Knippa Gap is a major zone controlling the flow
from the Uvalde pool to the San Antonio pool. The San Antonio
pool is the primary source of water for the greater San Antonio
water supply. The Knippa Gap is a restriction where the aquifer
narrows to a width estimated to be approximately 4 km, is
bounded by northeast trending faults of the Balcones Fault Zone
on the north, and uplift from the Uvalde salient and igneous
intrusive plugs to the south. (Green et al., 2006). The
hydrogeology in the Knippa Gap has been a topic of major
interest among researchers in this area for numerous years, yet
the exact location, nature of boundaries, and karst
hydrogeology are not well defined, and the flow through this
area is in need of refinement to improve the aquifer water
balance. This study integrates recent research by other
scientists with field studies conducted during the summer of
2012 as part of an M.S. thesis. This paper is limited to a
discussion of the water quality as it relates to the southern
flow boundary of the Knippa Gap near the Devils River Trend of
the Uvalde salient. Water-quality data constrain a revised
conceptual model of the flow and karstification in this
critical area of recharge to the San Antonio pool, and provide
specific lateral boundaries and vertical karstification zones
which are being tested in the more comprehensive M.S. thesis.
Although current interpretations are tentative, it appears this
conceptual model will be readily convertible into a digital
model that can test 2hypotheses relating a much broader suite
of calibration data, including water levels, water budgets, and
spring discharges. --
Authors



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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 hypotheses relating a much broader suite of calibration data, including water levels, water budgets, and spring discharges. Introduction in the world, providing more than 950 million liters of water to more than 2 million people on an average day. In addition, this aquifer is home to more than 40 aquatic subterranean species, several of which are endangered, agricultural, industrial, recreational, and domestic water with some depths extending up to 1,000 m. The north south extent of the aquifer ranges between 10 to 60 kilometers (Figure 1). Recharge to the Edwards aquifer occurs from the capture of surface water originating from the contributing zone (allogenic recharge), direct precipitation on the recharge zone (autogenic recharge), both above and below the Edwards Limestone. Discharge Regionally, the structure of the aquifer is exceedingly complex, owing to the extensive faulting associated with the Balcones Fault Zone. The faulting in the Balcones Fault Zone is primarily en echelon normal faulting that Abstract The Balcones Fault Zone Edwards aquifer (Edwards United States, with an average withdrawal of 950 million between the Uvalde pool and the San Antonio pool of the Edwards aquifer, west of the San Antonio metropolitan area in Uvalde County, Texas. This area is known as the The San Antonio pool is the primary source of water for the greater San Antonio water supply. The Knippa width estimated to be approximately 4 km, is bounded by northeast trending faults of the Balcones Fault Zone on the north, and uplift from the Uvalde salient for numerous years, yet the exact location, nature of improve the aquifer water balance. This study integrates recent research by other scientists 2012 as part of an M.S. thesis. This paper is limited to a discussion of the water quality as it relates to quality data constrain a revised conceptual model which are being tested in the more comprehensive M.S. thesis. Although current interpretations are tentative, it appears this conceptual model will be readily convertible into a digital model that can test CONCEPTUALIZATION OF GROUNDWATER FLOW IN THE EDWARDS AQUIFER THROUGH THE Jennifer Adkins 5695 W. Michael Cole Dr., Fayetteville, Arkansas, 72704, USA, jadkins@uark.edu 343

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE aerial exposure, burial (middle Cretaceous), faulting In the catchment area of the aquifer (Figure 1), dominant karst processes are epigenic. This means dissolution is produced primarily by descending recharge and horizontal groundwater movement. morphological forms such as vertical shafts, scallops, thought to delineate the northwestern boundary of the activity occurred, which includes igneous intrusions and uplift. This event bowed the overlying sediments, much shallower depths (Mosher et al., 2006), and resulted in the structural feature known as the Uvalde salient of the Devils River Trend. This feature dips into the Maverick Basin toward the southwest (Figure 2). Lithologically, the Edwards aquifer in the area of the San Antonio pool comprises as many as 8 members carbonates and evaporates that were deposited in the latter part of the Early Cretaceous period (Clark, 344 Figure 1. Location of the major hydrogeologic zones of the Edwards aquifer in south-central Texas, by county. The study area of this project is the Knippa Gap in Uvalde County, shown in purple between the towns of Uvalde and Sabinal on the map [Modified from Edwards Aquifer Authority webpage].

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 Problem Statement The Edwards aquifer has been intensively studied, but many important questions remain unanswered. One (Figure 4). This part of the Edwards aquifer represents northeast trending faults of the Balcones Fault Zone. Water discharges from the Uvalde Pool on the west into the San Antonio Pool in the east. Southeast of the Knippa and minimal well yields in this part of the aquifer and cupulas, many researchers conclude that hypogenic an important role in the karst development of the systems in the Edwards aquifer resurge as large springs where groundwater is returned to the surface from depth. Permeability derived by this upward water hydrocarbon storage within the rock unit (Schindel et al., 2008). into three regional zones, the recharge zone, the contributing zone, and the artesian zone (Figure 1). The aquifer to occur. This zone is also where contamination of the aquifer is most likely to occur, primarily as a little to no soil cover. The recharge zone is dominated by vertical faulting of the Balcones Fault Zone, and is way to the artesian zone. Entryways are predominantly point and line sources where streams and rivers cut across this zone of faulting. The artesian zone is the southern and easternmost part of the aquifer where 345 Figure 2. Location of the Devils River Trend, Maverick Basin, Uvalde Salient and the San Marcos Platform [Adapted from Green et al., 2006]. Figure 3. Stratigraphic column from southwest to northeast across Uvalde County in the study area, Knippa Gap. Karstified focused flow zones of the Edwards are shown in blue [Modified from Green, 2009].

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE gravels of the Leona River, and the Trinity aquifers are features have been studied throughout Uvalde County, such as the Uvalde salient (resulting from crustal uplift, faulting, and igneous activity that elevates the Edwards aquifer to the surface across the central region of the county), and the Balcones fault zone (a tensional structure area aligned southwest to northeast across the study area). Preliminary interpretation of that acts as a barrier, separating the Uvalde pool from the San Antonio pool under Medina, Bexar, and Comal Counties. It is described as being a narrow opening in an extensive system of barrier faults. (McClay and Land, 1988) Although 2.4 x 10 11 annually, the constriction causes water levels to build up conclude that the Uvalde salient has several prominent through topographic saddles between the high points. They also note the large amounts of recharge from the Frio and Dry Frio Rivers that are contributing to the cities of San Antonio, New Braunfels, and San Marcos. Recharge of the aquifer is greatly impacted by periodic Methods and Approach The hydrogeology and eleven wells described herein boundaries in the conceptual model. The conceptual model (Figure 7) incorporates samples compositions of these samples. These data allow visualization of geochemically related waters, and the an understanding of the geochemical processes evolution of water type in the aquifer. These should the south through subcrops to the Leona gravels. More the water budget for the San Antonio Pool and more the Uvalde pool. Objective and Scope constriction in Uvalde County, Texas, known as the related to water quality; however, this paper is only a small part of a much broader M.S. study of the karst hydrogeology of the region. Study Area The study area is shown in the shaded region of Figure 4. An expanded but secondary area of interest surrounds the main study area, encompassing contiguous portions Edwards aquifer in Uvalde County is predominantly composed of Lower Cretaceous carbonate (dolomitic limestone) of the Devils River Formation within the Devils River trend in the northeast, transitioning into the West Nueces, McKnight, and Salmon Peak Formations in the Maverick Basin in the southwest. These carbonate rocks were formed in evolving environments that ranged across a variety of tectonic and depositional conditions. The Devils River Trend current energy, whereas the West Nueces, McKnight, and Salmon Peak Formations were restricted to open The upper units of the Devils River Trend along with the upper unit of the Salmon Peak Formation are the most Throughout the study area there are numerous Upper Cretaceous or Lower Tertiary igneous rocks that intrude through the stratigraphic units composing the Edwards minor groundwater resources from a thick sequence of sedimentary rocks. The Edwards is by far the most of the county from west to east. The Buda, Austin Chalk, 346

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 m temperatures (26.6 to 24.7 o C) that occur in wells within the Uvalde salient (QW Sites 2, 5, 7). Waters west (QW o C). QW Site 8 represents the least mineralized of all wells sampled, not of the lowest concentrations of dissolved chloride and dissolved sulfate. Various degrees of mixing of waters from different sources are present in these latter wells, Conclusions The conceptual model (Figure 7) allows visualization not be used alone to delineate the gap, but they are a good conceptual start to test alternative hypotheses. Considering the complex faulting in the immediate area, they are consistent with a structural basis for Results Table 1 shows the water quality and dissolved constituents in water from wells located within the study area. The Well ID in Table1 is referenced to Figure 5, and the QW Sites to Figure 7. Figure 5 includes 2 sample sites (QW range of 5% error. Table1 and Figures 6 and 7 indicate the presence of high sulfate and high chloride waters with higher 347 QW Site Well ID Date TDS Temp o C Cond pH Ca +2 Mg +2 Na + K + ALK Cl SO 4 -2 1 606 481 7.20 82.1 10.4 10.8 1.07 19.9 2 1210 26.6 1605 6.98 277 27 28.1 2.91 169 72.9 260 471 7.25 79.9 0.974 188 20.6 11 4 477 7.49 86.8 9.28 11.1 1.1 20.2 11.7 5 877 24.7 1274 7.24 168 21.9 77.8 5.62 241 158 196 6 24 502 7.27 11.7 206 12 7 24.7 701 7.16 17.8 25.6 200 55.5 55.9 8 25.1 428 6.9 0.097 179 14.1 11.5 9 6942606 502 7.21 8.22 11.7 199 10.6 10 601 88.8 9.19 0.962 212 42.8 18 11 448 7.19 9.57 24 1.09 215 51.1 19.2 Table 1. Selected water quality and dissolved constituents in water from wells in the study area. QW Site number is referenced to Figure 4. Chemical parameters are in mg/L. [QW, water quality; TDS, total dissolved solids, in mg/L; Cond, specific conductance, in S/cm]

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 348 Figure 5. Geology of the Edwards aquifer in the study area, including areal geology, faulting associated with the Balcones fault zone (red lines), exposures of igneous intrusives associated with the Devils River Trend of the Uvalde salient (in red), and sampling sites of wells used to measure water levels and collect groundwater samples. The numbers refer to the sampled wells discussed in Table 1. [Map modified from multiple sources, including Clark, 2003; Green, 2006, and personal communications with Vic Hilderbran, Uvalde County Water Conservation District and Rob Esquilin, Edwards Aquifer Authority]. Figure 4. Location of key components of the Knippa Gap, the expanded study area, and other relevant hydrogeologic features in Uvalde County [Modified from Green, 2009]. NCKRI SYMPOSIUM 2 Figure 4. Location of key components of the Knippa Gap, the expanded study area, and other relevant hydrogeologic features in Uvalde County [Modified from Green, 2009]. Figure 5.Geology of the Edwards aquifer in the study area, including areal geology, faulting associated with the Balcones fault zone (red lines), exposures of igneous intrusives associat ed with the Devils River Trend of the Uvalde salient (in red), and sampling sites of wells used to measure water levels and collect groundwater samples. The numbers refer to the sampled wells discussed in Table 1. [Map modified from multiple sources, inc luding Clark, 2003; Green, 2006, and personal communications with Vic Hilderbran, Uvalde County Water Conservation District and Rob Esquilin, Edwards Aquifer Authority]. Expanded area of this study including Uvalde Pool on the west, and San Antonio Pool on the east

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 secondary permeability than the freshwater portions of the aquifer. These factors result in greater salinity levels and distinctive Stiff diagrams. The conceptual model (Figure 7) shows that (QW) sites 5 and 7 plot along a mixing line of meteoric water and down gradient water similar in chemical composition to well 2. As indicated by the curved blue lines on the model, these QW sites have mixing components that are inconsistent with Most of the wells with these attributes overlie the Uvalde salient, and because of the structural uplift, the aquifers proximity may contribute to slightly higher temperatures although this needs to be investigated further. Well yields dissolution of the highly soluble evaporates) through this part of the aquifer. Well 11 is an exception to this, but inasmuch as it lies on the boundary of this study and its explanation at this point is not obvious. Data from the remaining QW sites have Stiff diagrams as the main geochemical process. These QW sites diagram (Figure 6) as well, and have calculated TDS Future Work In addition to the geochemical analysis discussed in this paper, the larger M.S. study will incorporate the compilation of a complete table of wells, geophysical aquifer tests within the study area. The completed table of wells represents sites with multiple names and aliases, and will aid in future investigations in accessible digital format. The table will involve historic published well data, and unpublished records from drillers, water managers, and hydrogeologists in of wells which will be conducted during the summer superimposed on Figure 5, (which includes the Balcones Fault Zone and outcrops of intrusive igneous rocks that Stiff (Figure 7) diagrams from sites designated as Knippa m o C. QW Site 8, the least mineralized well sampled, is the only exception to the temperature range listed, with a value of 25 o from the Uvalde Pool to the Leona gravels. Stiff diagrams for QW Site 2 is thought to lie near the bad boundary the freshwater portions of the Edwards aquifer. Increased mineralization is a result in increased contact with gypsum and has more limited development of 349 Figure 6. Piper diagram of groundwater in the study area showing water quality types ranging from waters within the Knippa Gap (within black circle) to waters derived from mixing of high sulfate and chloride waters associated with residual evaporites in less dynamic flow zones (see wells 2, 5, and 7 in Table 1).

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE study area. with the Uvalde salient (star on Figure 5). Tracer testing is one of the most effective ways of quantifying groundwater movement in karst aquifers, and will provide empirical data that will aid in the determination of the groundwater components for this region (Schindel et al., 2008). A synoptic potentiometric map of the study area conditions during the summer of 2012. This effort will the Edwards Aquifer Authority (EAA), and the results will be used to evaluate potential boundaries, assess variability of aquifer hydraulic properties, and indicate A hydrostratigraphic analysis, incorporating a conceptual boundaries), aid in determining physical constraints 350 Figure 7. Conceptual model of the Edwards aquifer in the study area, showing Stiff diagrams that reflect major element concentrations dissolved in groundwater (in green), approximate locations of boundaries of flow through the Knippa Gap (curved blue lines), major flow directions through the Knippa Gap constriction (blue arrows), subsurface overflow from the Uvalde Pool to the Leona gravels (black arrow), and exposures of igneous intrusives associated with the Devils River Trend of the Uvalde salient (in red). Sampling sites of wells for which chemical analyses are reported are shown by black dots; the numbers refer to the sampled wells discussed in Table1.

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 Klar R.V., Morris, A.P., 2006, Evaluation of the Edwards Aquifer in Kinney and Uvalde Counties, Texas. Southwest Research Institute. Conducted for the Edwards Aquifer Authority. Analysis of the Water Resources of the Area Centered Within and Near Uvalde and Zavala Counties and Review of a Report on a Potential Well Field and Pipeline in Uvalde County, Texas Final Report Prepared for City of Uvalde and Wintergarden Investigating the Secondary Aquifers of the Uvalde County Underground water Conservation District Division Southwest Research Institute. Seco Creek and Medina River where They Overlie Engineering Division Southwest Research Institute. Data Report for 2010. geologic controls on porosity development in platform carbonates, South Texas: The University karst, fractures, and permeability in the Cretaceous Edwards aquifer: analogs for fractured carbonate reservoirs: Society of Petroleum Engineers Annual Permeability structure of the Edwards Aquifer, south Texasimplications for aquifer management: The University of Texas at Austin, Bureau of Economic Acknowledgments general manager for the Uvalde County Underground Water Conservation District, for all the help he gave his knowledge about Uvalde County groundwater was outstanding, and he shared his knowledge with me freely. I am truly grateful to him for his assistance. of the Southwest Research Institute for providing me with an internship, for guidance, and for encouragement throughout the summer. Their insight was invaluable. Edwards Aquifer Authority, not only provided me with an internship and invaluable geochemical and hydrogeologic insight, but he made resources and personnel available to assist in this endeavor. The staff of the Edwards Aquifer Authority, especially Rob Esquilin, incredibly generous in sharing their knowledge and data from the Edwards aquifer. Finally, I would like to thank all the well owners in Uvalde County who allowed me access to their property and facilitated sampling and measuring hydrogeologic parameters of their wells. I know their water is precious to them, and I thank them for trusting me and facilitating my study. References hydrogeologic characteristics of the Edwards Survey, Water Resources Investigations Report J., 2010, Late Triassic Texas uplift preceding Society of America Ferrill, David A., Sims, Darrell W., Franklin Nathan, Morris, Alan P., Waiting, Deborah J.,2004, Science, University of Texas at San Antonio. 351

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE New York. Young, S.C., Doherty, J., Budge, R., Deeds, N., 2009, Application of PEST to recalibrate the groundwater (Plateau) and Pecos Valley Aquifers DRAFT. Contract report for the Texas Water Development Board by URS Corporation, Watermark Numerical Pty, Ltd, and Intera Inc. Flow in the Edwards AquiferCharacterizing the Role of Fractures and Conduits in the Balcones Fault Zone Segment: Final Contract, Bureau of Special Paper no. 1, National Cave and Karst Research Institute, Carlsbad, NM, 106 pp. in the Edwards aquifer, San Antonio region, Texas, Painter, Scott., Jiang, Yefang, Woodbury, Allan., 2002, report: Southwest Research Institute, variably paginated. Palmer, A. N., 1991, Origin and morphology of limestone subsurface, central Texas. Bureau of Economic Alexander, E. C., Jr., Alexander, changes during a recharge event in the karstic Society of America Abstracts with Program. Available from: to explain squifer dynamics: Adapted from oral Schultz, A. L., 1994, Review and update of the position interface from Uvalde to Kyle, Texas: Edwards Trace Results. Northwestern Arkansas: unpublished M.S. thesis, University of Arkansas, Fayetteville. and Palmer, M.V., eds., Caves and Karst of the USA: 352


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