Cover-collapse sinkhole development in the Cretaceous Edwards limestone, Central Texas

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Cover-collapse sinkhole development in the Cretaceous Edwards limestone, Central Texas

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Cover-collapse sinkhole development in the Cretaceous Edwards limestone, Central Texas
Series Title:
13th Sinkhole Conference
B. Hunt, Brian
A. Smith, Brian
T. Adams, Mark
E. Hiers, Scott
Brown, P.E.Nick
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Subjects / Keywords:
Central Texas ( local )
Sinkhole ( local )
Stormwater ( local )
Karst Conduit System ( local )
serial ( sobekcm )


Sudden cover-collapse sinkhole (doline) development is uncommon in the karstic Cretaceous-age Edwards limestone of central Texas. This paper presents a casestudy of a sinkhole that formed within a stormwater retention pond (SWRP) in southwest Austin. Results presented include hydrogeologic characterizations, fate of stormwater, and mitigation of the sinkhole. On January 24, 2012, a 11 cm (4.5 in) rainfall filled the SWRP with about 3 m (10 ft) of stormwater. Subsequently, a sinkhole formed within the floor of a SWRP measuring about 9 m (30 ft) in diameter and 4 m (12 ft) deep. About 26.5 million liters (7 million gallons) of stormwater drained into the aquifer through this opening. To determine the path, velocity, and destination of stormwater entering the sinkhole a dye trace was conducted. Phloxine B was injected into the sinkhole on February 3, 2012. The dye was detected at one well and arrived at Barton Springs in less than 4 days for a minimum velocity of 2 km/day (1.3 mi/day). Review of pre-development 2-foot topographic contour and geologic maps reveals that the SWRP was built within a broad (5,200 m2 ; 6 acre), shallow depression bounded by two inferred NE-trending fault zones. Photographs taken during SWRP construction showed steep west-dipping bedrock in the northern SWRP wall. Following collapse of the sinkhole, additional hydrogeologic characterization included excavation to a depth of 6.4 m (21 ft), surface geophysics (resistivity), and rock coring. Geologic materials consisted mostly of friable, highly altered, clayey limestone consistent with epikarst in-filled with terra rosa providing a cover of the feature. Dipping beds, and fractured bedrock support proximity to the mapped fault zone. Geophysics and surface observations suggested a lateral pathway for stormwater flow at the junction between the wet pond’s impermeable geomembrane and compacted clay liner for the retention pond. The collapse appears to have been caused by stormwater down-washing poorly consolidated sediments from beneath the SWR

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 89 of friable, highly altered, clayey limestone consistent of the feature. Dipping beds, and fractured bedrock and surface observations suggested a lateral pathway liner for the retention pond. The collapse appears to consolidated sediments from beneath the SWRP and into boulders to gravel, a geomembrane cover, and reinforced concrete cap. Additional improvements to the SWRP included a new compacted clay liner overlain by a geomembrane liner on the side slopes of the retention pond. Introduction Karst is a terrain with distinctive hydrology resulting developed solution channel porosity underground (Ford, 2004). Karst terrains and aquifers are characterized by sinking streams, sinkholes, caves, springs, and an transport groundwater from recharge features to springs (White, 1988; Todd and Mays, 2005). Sinkholes (also known as dolines) have long been characteristic of many karstic terrains in many areas of the world (White, age limestones of Texas in the Edwards Plateau and Balcones Fault Zone (Kastning, 1987). The purpose of Abstract study of a sinkhole that formed within a stormwater retention pond (SWRP) in southwest Austin. Results presented include hydrogeologic characterizations, fate of stormwater, and mitigation of the sinkhole. 26.5 million liters (7 million gallons) of stormwater drained into the aquifer through this opening. To determine the path, velocity, and destination of stormwater entering the sinkhole a dye trace was and arrived at Barton Springs in less than 4 days for a and geologic maps reveals that the SWRP was built within a broad (5,200 m 2 ; 6 acre), shallow depression Photographs taken during SWRP construction showed wall. Following collapse of the sinkhole, additional hydrogeologic characterization included excavation to a depth of 6.4 m (21 ft), surface geophysics (resistivity), COVER-COLLAPSE SINKHOLE DEVELOPMENT IN THE Brian B. Hunt, P.G., Brian A. Smith, Ph.D., P.G. Barton Springs/Edwards Aquifer Conservation District, 1124 Regal Row, Austin, Texas 78748 USA, Mark T. Adams, P.G. ACI Consulting, 1001 MoPac Cir., Ste. 100, Austin, Texas 78746 USA, Scott E. Hiers, P.G. City of Austin-Watershed Protection Dept., 505 Barton Springs Rd., Austin, Texas 78704 USA, Nick Brown, P.E. Bury+Partners, 221 W. 6th St. #600, Austin, Texas 78701 USA,


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 90 The collapse of sinkholes is clearly a natural phenomenon. humans can accelerate the process and “activate” or “induce” a collapse sinkhole. This occurs by increasing unconsolidated materials, creating a large void and caves in the soil or regolith, resulting in collapse. Sinkhole development in the karstic areas of Texas is a common occurrence and is documented in Kastning Many studies of the eastern United States document environmental problems (Newton and Tanner, 1987). damage in the karstic Edwards, although examples may exist in areas with thick soils. Instead, the Edwards has many relatively stable sinkholes that do not cause collapse sinkholes are more accurately described as the land surface due to erosion of the overlying strata. Other stable sinkholes are formed by more recent vadose dissolution (often with a combination of collapse) and are directly linked to the current surface hydrology. cover throughout central Texas as the karst bedrock is often exposed directly at the surface. Other factors include Setting and consists of an area of about 10,800 km 2 (4,200 mi 2 ). source of water for about two million people, plus numerous industrial, commercial, and irrigation users. The Edwards Aquifer system also supports 11 threatened or endangered species, aquatic habitats in rivers of three segments. North of the Colorado River is the Northern segment, and south of the southern hydrologic divide near the City of Kyle is the San Antonio segment (Figure 1). The Barton Springs segment is located between this paper is to document the development and mitigation limestones. This sinkhole occurred in the Arbor Trails retail development stormwater pond and is referred to as the Arbor Trails Sinkhole (ATS). This case study will lead to insights into how to avoid activating or inducing sinkhole collapse in the future. Sinkholes A broad discussion of sinkholes is beyond the scope as “a natural enclosed depression found in karst landscapes” (Williams, 2004). The mechanisms of washing), and regional subsidence. These mechanisms produce sinkholes described broadly as either a solution sinkhole, or a collapse sinkhole. A typical limestone sinkhole develops as a depression formed by the slow process of dissolution forming a broad bowl with a gentle slope. Solution sinkholes collapse rapidly due to gravitational forces following upward stoping of the cavern (void) ceiling. Sudden collapse, due to mechanically weakened unconsolidated sediments have a different morphology and can form and not collapse of mappable geologic units (Veni, 2012, also called dropout dolines, or simply collapse dolines (Williams, 2004; White, 1988). Development of sinkholes is related to the ability of (Williams, 2004). Recharge water dissolves the rock behind a highly corroded and permeable zone termed epikarst (Williams, 2004).


13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 91 into recharge features in the upland areas of the recharge zone (Slade et al., 1986). More recent water balance estimates of the Barton Springs segment suggest that more water could be recharged in the upland or intervening The Edwards Aquifer is inherently heterogeneous and et al., 2005). The Edwards Aquifer can be described as a triple porosity and permeability system consisting structural history, and hydrologic evolution (Lindgren et al., 2004). In the Barton Springs segment groundwater north toward Barton Springs. Numerous tracer tests have been performed on portions of the Edwards Aquifer demonstrating that rapid groundwater Johnson et al., 2012). In the Barton Springs segment these (secondary) fault and fracture trends presented on geologic al., 2002). Arbor Trails Pre-Development Site Characterization and Planning 2 Edwards Rules). These requirements include geologic and environmental assessments, and reduction of pollution in stormwater leaving the site. The City of Austin has the most stringent requirements (so called “SOS Ordinance”) that limit impervious cover and set nondegradation standards for the treatment of stormwater these two larger segments. The Shops at Arbor Trails is the is located within the recharge zone of the Barton Springs segment of the Edwards Aquifer (Figure 1). Development of the aquifer is also thought to have speleogenesis (Klimchouk, 2007; Schindel et al., 2008). zone, located upgradient and primarily west of the sinks into numerous caves, sinkholes, and fractures along numerous (ephemeral to intermittent) losing streams. For the Barton Springs segment, Slade et al. (1986) estimated that as much as 85% of recharge to the aquifer is from Figure 1. Location map of the study area. Indicated are the Brush Country well (BC well) and a USGS stream gage station on Williamson Creek.


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 92 karst recharge features, springs, and wetlands. From 1994 to 2006, several development permit applications were submitted for the study property resulting in numerous environmental and geologic assessments. Beside the assessments provided in 1994 and 2004, respectively, at least two phase one environmental assessments were prepared to address hazardous material and general environmental concerns (Kleinfelder, 2005). In 2004 a karst survey and geologic assessment was property (Figure 2). The fault zone and the geologic units are consistent with the geologic map of Small et al., 1996. The on the Edwards Aquifer recharge zone. To achieve this standard, a variety of water quality measures, including construction of Storm Water Retention Ponds (SWRP) are required for development sites. Within the Edwards Aquifer recharge zone SWRPs are a type of permanent runoff and sediment so that sediments and other contaminants are not carried further downstream or into the Edwards Aquifer. The failure of a SWRP permits sediment and contaminated stormwater to leave a site and likely enter the aquifer. Both the State and the City permitting processes stipulate that a karst survey be completed to identify and evaluate all karst recharge features. In addition to the State permitting, the City requires an environmental assessment Figure 2. Predevelopment topographic map. Basemap is USGS Oak Hill Quadrangle (10-ft contours in brown). Geologic information from HBC/Terracon (2004). Geologic units and faults are consistent with Small et al., 1996. Black lines are City of Austin 2-ft topographic contours dated 1981, prior to major highway (MoPac). Contours create a depression centered around the SWRP (shown as dashed lines).


13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 93 SWRP is to capture storm runoff from impervious areas (buildings and parking lots) and then irrigate vegetative areas with the stormwater throughout the property. The SWRP consists of two water quality controls; a and main permanent pool area that are separated by a berm. The wet pond was constructed for aesthetics within the retention basin. The retention pond has its capture volume above permanent pool elevation for the wet pond. The capture volume for the retention pond extends up 1.8 m (6 ft) onto the slope areas of the quality control structure for the surrounding shopping center. During a rain event stormwater captured by the retention basin is held and then irrigated on vegetated Hydrologic Conditions and Sinkhole Collapse Prior to collapse of the ATS, central Texas had been experiencing a severe drought. Beginning in late January, rainfall and subsequent recharge brought the aquifer out of drought conditions. On January 24, 2012, an 11 cm (4.5 in) rainfall event occurred in the area of the Arbor Trails development 4). On January 25, 2012, maintenance crews noticed the pond was draining, and that a sinkhole had developed ft) in diameter and 4 m (12 ft) deep. About 26.5 million liters (7 million gallons) of storm water drained into the aquifer through this opening. associated with the late January (and March) rainfall. These types of increases are relatively common in Conservation District (District) staff observed the runoff and recharge into swallets (Brodie Cave) within nearby tributaries of Slaughter Creek from the same rainfall event that created the ATS. It was noted that the stormwater entering those features was very turbid. to the failure of the SWRP. Sinkhole Characterization Studies Following the collapse, the sinkhole was further characterized by excavation, surface geophysics, and three features were evaluated and scored as sensitive (i.e., they could be pathways for contamination) in the report, they had a small surface catchment area. The fault zone had no surface expression observed and was located based upon published maps (Small et al., 1996). The fault was not from the map alone. The geologic assessment concluded Edwards Aquifer beneath the site is considered very low” As part of the site permitting processes, City staff evaluated described in the reports. This resulted in an additional karst recharge features on the study site, or a large depression in subsurface voids encountered during construction. Review of topographic contours from the City of Austin 1) reveals a very shallow and large (5,200 m 2 ; 6 acre) depression centered on the SWRP (Figure 2). The contours agree with an even more subtle depression appears well drained from the aerial as no ponded features the subdued nature of the feature and the subsequent disturbance from the highway that bisected the eastern portion of the depression would make detection of the As part of the site engineering studies, geotechnical cores and borings were conducted throughout the site. quality designation (RQD) of very poor to incompetent The location of the SWRP for the Arbor Trails


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 94 Figure 3. Detailed site map with key elements of the stormwater retention pond (SWRP), sinkhole location, and 2012 geophysical lines and boreholes. Figure 4. Photograph of sinkhole, all photos facing north. A) photo taken the day the sinkhole was observed (credit Heather Beatty, TCEQ). B) Photo taken two days after collapse and prior to excavation. Note the limestone beds are dipping to the west.


13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 95 borehole (core) drilling by ACI Consulting (Austin, Texas). Prior to those studies the District and City of Austin (CoA) conducted the dye tracing studies. The ATS was excavated to a total depth of 6.4 m (21 ft) (Figure 6). Most of the excavated geologic material in the sinkhole consisted of friable, highly altered (weathered), clayey limestone fragments consistent Figure 5. Photograph locations indicated in Figure 4. A) Photo during construction of SWRP showing west-dipping beds in the northern wall of the forebay (photo credit Andrew Backus, 4/2/2006); B) Photo of the northern wall of the sinkhole taken two days after collapse and prior to excavation. Figure 6. Sketch of sinkhole after excavation (by Mike Warton of ACI Consulting). Very little competent bedrock was encountered in the excavations. Solution fractures striking to the north, the northern retaining wall, were observed (Figure 5). steeply dipping bedrock suggesting the ATS developed proximal to a fault zone. Geophysics The nature of collapse suggested the possible existence unstable material to further collapse into a void of unknown dimensions. To assess the void and assure structural stability for equipment and workers safety, a mechanism for subsurface evaluation was needed. Based on an initial review of the collapse, ACI proposed a geophysical approach. ACI uses geophysics on by geotechnical borings and subsequent construction authorities, a geophysical electrical resistivity array was Inc. (Round Rock, Texas) to evaluate the shallow surface for anomalies and take a deeper look at the subsurface.


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 96 spacing on lines 1 and 2 was 1.5 m (5 ft), which allowed for moderate penetration depth (18 m, 60 Other survey lines had spacing on the order of 2.1 m (7 ft), reducing resolution, but increasing the depth to over 24 m (80 ft). Each probe is connected to an Each probe alternated acting as an electrical source and receiver. The electrical pulses were recorded and the electrical energy loss recorded and the results are illustrated in Figure 7. conditions near the void and assess the surrounding area. The second bay (permanent pool) of the pond was not accessible as it was being used as a backup water quality control for development. For the array, metal spikes were driven into the ground to a depth determined based on desired resolution and survey depth. As this investigation was designed to evaluate the subsurface for the collapse geometry and to assure worker safety, a moderate spacing was chosen. Probe Figure 7. Resistivity profile from lines 1 and 2 (shown on Figure 3). The sinkhole was located between these two lines. Note the interpretation of water infiltration. This is based upon the resistivity data and the voids observed in the compacted clay material of the retention pond.


13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 97 so many karstic features in the area, the results indicate system of the aquifer. Sinkhole Mitigation and SWRP Improvements An engineered closure design by Bury + Partners (Austin, Texas) was reviewed and approved by the City and State to mitigate the sinkhole. The plan consisted vapor barrier lined the top of the gravel and a reinforced concrete slab was poured on the top and anchored into the splitter box. A compacted clay liner was installed over the concrete followed by a geomembrane liner, both of which covered the entire SWRP (Figure 11). In addition to the closure of the sink, the owners of the Since “resistivity” is a relative measure, two geotechnical to physically evaluate the subsurface and calibrate the geophysical model. Based on the borings, warmer (red) colors representing higher resistivity were determined to be relatively competent (crystalline) limestone. Cooler blue colors representing lower resistivity (high conductivity) were determined from Boring 1 to be wet recovery also suggesting highly fractured rock. Activation of Collapse Small voids observed in the compacted clay liner of the side of the SWRP, suggest the most likely pathway for observations along with the geophysics and other data suggest that water from the SWRP was bypassing the liner. Other interpretations of pathways beneath the liner stormwater, resulted in a collapse of the relatively weak cover material and development of the sinkhole. Sinkhole Recharge and Groundwater Flow path, velocity, and destination of groundwater in a karst setting. A dye trace was performed to better understand therefore springshed, the ATS was developed within. The results will help scientists understand the fate of the stormwater in the ATS, and also how future contaminant study site, will move. 2012 (Figure 8). The dye was detected at one well and within the Sunset Valley groundwater basin as previously Figure 8. Phloxine B dye injection at Arbor Trails sinkhole. Dye was injected on February 3, 2012. A mass of about 7 kg (16 lbs) was mixed in a trash can and then gravity injected via a hose and polyvinyl chloride (PVC) pipe using water from an adjacent wet pond.


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 98 Figure 9. Map of results from the Arbor Trails dye trace. Pink circles indicate positive detections (very high confidence, both labs) of Phloxine B. White circles are wells with tentative detections (single detections from EAA lab), and solid black circles are locations with non-detects (both labs). Dashed pink line represents estimated flow route and is coincident with the “Sunset Valley Flow Route” defined by Hauwert et al., 2004. Small gray circles are existing water-supply wells. Light gray potentiometric lines are from February 2002 high flow conditions (10-ft contour intervals). Groundwater basins are defined in Hauwert et al., 2004. to prevent future leakage and sinkhole development (Figure 11). Existing geomembrane liner was replaced level of the retention pond (previously the liner only existed for the wet pond). The subgrade underneath the geomembrane liner within the retention pond was replaced with new high quality compacted clay liner and geomembrane line. All masonry walls in the SWRP were grouted and sealed to prevent leakage. Discussion Figure 12 illustrates a conceptual hydrogeologic model of depression is indicative of a solution sinkhole (Figures 2 and 12A). Evidence of a fault zone include fractures and borings revealed highly fractured and altered epikarst rock within the SWRP. The SWRP removed about 6 likely acted as a mantle of poorly consolidated material


13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 99 covered sinkhole. In addition, geotechnical studies occur without the input from geologists surveying for karst features. Finally, geologists are not required to inspect the SWRP excavation during its construction. Despite these problems inherent in the development process, the studies and site remediation were a model of communication, transparency, and cooperation among the various regulators, scientists, engineers, and owners. All of these parties have a goal to understand the problem and provide the best solution. Conclusions can develop in the central Texas Cretaceous karst system. In this case the cover is a thick horizon of terra over a fractured and dissolved karstic fault zone (Figure the geomembrane liner and through the epikarst zone pipes (Figure 5), and upwards stoping of the void ceiling at depth. Sudden failure occurred as mechanically weak the bedrock (Figures 5 and 12C). Dye tracing established system (Figure 9). The sinkhole was mitigated with Improvements to the SWRP included extending the liner Under the current development process it is unlikely that the regulators or developers of the area in which the sinkhole occurred would have recognized the risk associated with the location of the SWRP, or predicted the failure. Only after compiling all the information does it become clear that human activities (placement of the SWRP on the sinkhole) activated the sinkhole collapse. Part of the challenge is that the land development process in the karstic Edwards Aquifer has inherent problems of communication between geologists, engineers, consultants, and owners over the life of engineered, and then the geologic assessment occurs, SWRP was located precisely in the lowest portion of the property, which makes sense from a engineering standpoint. But in this case the low elevation was a Figure 10. Photographs during sinkhole mitigation. A) Boulders and coarse fill and filter fabric; 5/2/12, B) graded cobble to gravel fill; 5/7/12, C) Gravelfilled sinkhole and filter fabric; 5/9/12, D) Reinforced concrete cap and blue vapor barrier; 5/10/12. Figure 11. A) Looking east from the splitter box showing new compacted clay liner overlain by new geomembrane. B) Looking south at the stone splitter box and the finished SWRP after significant rainfall event. New soil and vegetation cover in place over geomembrane in SWRP; 7/11/12. Note the sinkhole was located in front of the splitter box.


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 100 Acknowledgments This study would not have been possible without the full support of the property owner of the Arbor Trails Christopher Commercial Inc. (CCI). We would also like to thank the CCI executive team of Vice President Maintenance Manager Dan Manchiela, and SWRP We would like to extend our thanks to the Edwards and Steve Johnson for their support of the dye tracing portion of this study. The EAA provided Phloxine B the dye tracing portion of this study. Justin Camp (CoA) helped sample the springs as part of the dye trace study. study confirms how human activities, superimposed upon natural karst features, can activate a connected these features can be with the aquifer be avoided if geologists and engineers are aware of the potential risks associated with SWRPs initiating sinkhole collapse. To reduce the risk of future SWRP failures, studies should be performed beyond current standards for areas impounding water, such as an SWRP. Additional studies could include detailed mapping, topographic surveys, traditional karst surveys, geophysics, and additional geotechnical borings (extending below the final grade) focused around a potential location of an SWRP. Excavations should be inspected periodically by geoscientists and engineers during construction looking for features that could contribute to sinkhole initiation. Figure 12. Conceptual hydrogeologic model of sinkhole in four stages: A) Pre-SWRP development, B) SWRP and sinkhole activation, C) covercollapse, and D) mitigation.


13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 101 The Shops at Arbor Trails, MoPac Expressway at William Cannon Drive, Austin, Texas. Report by at Arbor Trails, MoPac at William Cannon Drive, distribution of permeability in the Edwards Aquifer. controls on porosity development in platform carbonates, South Texas. The University of Texas structure of the Edwards Aquifer, South Texas— Implications for aquifer management. The University of Texas at Austin, Bureau of Economic evidenced by groundwater dye tracing in the Barton Springs segment of the Edwards Aquifer, Central Texas: Implications for Modeling San Antonio, Texas. April 1. central Texas: Ordovician to Quaternary. In: Beck Engineering and Environmental Applications, Proceedings 2nd Multidisciplinary Conference on Sinkholes and the Environmental Impacts of Karst. Kleinfelder. 2005. Phase I environmental assessment: National Cave and Karst Research Institute Special Paper 1. Carlsbad, New Mexico. Painter S. 2004. Conceptualization and simulation of the Edwards Aquifer, San Antonio region, Texas. We would like to further thank the careful reviews and suggestions that improved this paper by Robert K. Denton, Jr., John M. Caccese, and Tony L. Cooley. References Beck B, Sinclair WC. 1986. Sinkholes in Florida: An introduction. Florida Sinkhole Research Institute studies (1996), Barton Springs segment of the Aquifer Conservation District unnumbered publication, Austin, Texas. caves and karst science. New York (NY): Fitzroy Dearborn, p. 902. New York (NY): Fitzroy Dearborn. Antonio segment of the Edwards Aquifer: matrix, fractures, or conduits? In: Wicks CM, Sasowsky transport in carbonate aquifers. Rotterdam Edwards Aquifer, southern Travis and northern and the City of Austin Watershed Protection and Development Review Department. within the Barton Springs segment of the Edwards to the Barton Springs segment of the Edwards Aquifer. World Lake Conference, Austin, Texas. channel and river contributions. City of Austin short report in preparation.


NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 102 Maclay RW, Small TA. 1986. Carbonate geology and hydrogeology of the Edwards Aquifer in the San Antonio Area, Texas. Texas Water Development Board Report 296. sinkholes in the eastern United States. In: Beck Engineering and Environmental Applications, Proceedings 2nd Multidisciplinary Conference on Sinkholes and the Environmental Impacts of Karst. new conceptual model to explain aquifer dynamics. Sharp JM Jr. 1990. Stratigraphic, geomorphic and structural controls of the Edwards Aquifer, Texas: U.S.A.. In: Simpson ES, Sharp, JM Jr, editors. framework and hydrogeologic characteristics of the Edwards Aquifer outcrop (Barton Springs water quality of the Edwards Aquifer associated with Barton Springs in the Austin area, Texas. US Slade R Jr, Ruiz L, Slagle D. 1985. Simulation of the Edwards Aquifer in the Austin Area, Texas. US ed. New York (NY): John Wiley & Sons, Inc. terrains. Oxford (England): Oxford University Press. Encyclopedia of caves and karst science. New York


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