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The Geological Society of America Memoir 215 OPEN ACCESS

Genesis of the Edwards ( Zone)

Geary M. Schindel Authority, 900 E. Quincy Street, , 78215, USA

ABSTRACT

The San Antonio segment of the Edwards (Balcones Fault Zone) Aquifer of south- is one of the most important and prolific karst in the . Extending from Kinney County (west) to Hays County (northeast), it is the primary source of water for the municipal and agricultural communities sur- rounding the greater San Antonio area. Deposited in Early Cretaceous time, rocks of the vary from 150 to 300 m thick and include eight members with highly variable hydraulic attributes and solubility. Its complex tectonic, weathering, and geologic history has allowed dissolution of the highly soluble members to form a highly transmissive karst aquifer. Regionally, the Balcones fault zone provides path- ways that allow captured streams to flow into the aquifer in the contributing and recharge zones. Karstification of the aquifer has occurred by multiple processes, both epigenic and hypogenic, with visual documentation obvious in numerous caves of the area. Currently, overprinting of hypogenic systems by epigenic systems is common. The en echelon down-to-the-south faulting of the Balcones fault zone has resulted in deep burial of the aquifer in the artesian zone, with dissolution at depth driven by numer- ous processes, including infiltration of chemically aggressive surface water, hydraulic head, mixing corrosion, and biogenic acids. production in the artesian zone is commonly limited only by the discharge rate of the pump. The Edwards Aquifer is also noted for its diverse and widespread aquifer-adapted fauna, implying that the aquifer has a well-integrated karst conduit system.

INTRODUCTION in Kinney County, eastward to San Antonio, and then northeast to the groundwater divide near in Hays County (Fig. 1; The San Antonio segment of the Edwards (Balcones Fault Sharp et al., this volume). The aquifer varies from 10 to 60 km zone) Aquifer is one of the most important and prolific karst wide and contains freshwater to depths greater than 1200 m aquifers in the United States. Rocks of the Edwards Group crop below the surface (Edwards Aquifer Authority, 2010). The aqui- out from the Rio Grande River along the Mexican border near fer is the source of water for more than two million people in Del Rio, Texas, eastward to San Antonio, and then northeastward the greater San Antonio area (Schindel et al., 2004). It is used through the cities of New Braunfels, San Marcos, and Austin, and for agricultural, industrial, and municipal purposes, and it creates into Bell County for a distance of 450 km. The San Antonio seg- critical habitat for endangered species that depend on continuous ment of the Edwards Aquifer extends from east of Brackettville spring flows at Comal and (Edwards Aquifer

Schindel, G.M., 2019, Genesis of the Edwards (Balcones Fault Zone) Aquifer, in Sharp, J.M., Jr., Green, R.T., and Schindel, G.M., eds., The Edwards Aquifer: The Past, Present, and Future of a Vital Water Resource: Geological Society of America Memoir 215, p. 9–18, https://doi.org/10.1130/2019.1215(02). © 2019 The Authors. Gold Open Access: This chapter is published under the terms of the CC-BY license and is available open access on www.gsapubs.org.

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Figure 1. Depositional provinces of the Ed- wards Aquifer showing Maverick Basin, Dev- ils River Trend, and San Marcos Platform (modified from Maclay, 1995).

Authority, 2012b). The aquifer also provides water for ecosystem Limestone, which forms the lower confining unit. The artesian maintenance; a recreation industry on the spring-fed Comal and zone encompasses ~14,000 km2. Within the artesian zone, the San Marcos Rivers; and municipal, agricultural, and industrial aquifer is fully saturated and contains most of the large produc- use along the rivers that flow to the Gulf of Mexico. tion . The Edwards Aquifer is in the 150- to 300-m-thick Edwards Initial deposition of limestones of the Edwards Group was as Group rocks, which were deposited in late Early Cretaceous time. a biochemical precipitate in both shallow- and deep-water marine The Edwards Aquifer system has been divided into three major environments, as well as interspersed evaporite deposits. Since hydrogeologic zones—the contributing zone, the recharge zone, deposition, these limestones have undergone subaerial exposure and the artesian zone. The Edwards Aquifer is located south of in the Early Cretaceous; burial in the middle to Late Cretaceous; and at a lower elevation than the Edwards-Trinity Aquifer, which uplift with secondary subaerial exposure beginning in the late occurs in the . Mesozoic to early Cenozoic; igneous intrusion, faulting, and The contributing zone encompasses the largest area and is uplift in the Miocene; and weathering since the latest subaerial composed of higher-elevation outcrops of the Edwards Group exposure. The faulting is complex and is an important factor in and the underlying Glen Rose Limestone. The contributing zone controlling aquifer hydrology; it is related to uplift in central makes up part of the Edwards Plateau and covers more than Texas and subsidence related to formation of the Gulf of Mexico, 14,000 km2. The contributing zone collects rainwater and spring both of which are related to the regional tectonics. flow and forms the headwaters of major surface streams in the The Edwards Group rocks have been subjected to disso- region. These include the Nueces, Frio, Sabinal, Medina, Cibolo, lution over geologic time and have resulted in a subdued karst Guadalupe, and Blanco Rivers. landscape where exposed at the surface. Within the subsurface, Faulting along the Balcones fault zone (Ferrill et al., this the rocks are noted for highly developed secondary and tertiary volume) has resulted in rocks of the Edwards Group being down- permeabilities, with water withdrawal only limited by the size of thrown toward the Gulf of Mexico to the south. These down-to- the pump (Maclay and Small, 1984). the-south faults juxtapose Lower Cretaceous rocks against Upper Cretaceous rocks. The northernmost area where the Edwards STRATIGRAPHY AND LITHOLOGY Group is exposed at the surface is defined as the recharge zone of the aquifer. This covers 3200 km2 and identifies the area of Physiographically, the Balcones fault zone escarpment is the capture of surface water from streams flowing southward and terminus for the southern and eastern edge of the Edwards Plateau eastward from the contributing zone. The recharge zone approxi- where it intersects the Gulf Coastal Plain. The Hill County rep- mately overlies the Balcones fault zone, which, along with the resents the dissected edge of the Edwards Plateau. The landscape contributing zone to a lesser degree, allows vertical downward is covered with oak and juniper, separating the nearly flat-lying movement of surface water into the Edwards Aquifer. rocks of the Edwards Plateau from the gently coastward-dipping The artesian zone occurs downgradient where the Edwards sediments of the subsiding Gulf of Mexico (Maclay and Small, Aquifer is confined between the , which forms the 1984). Annual rainfall averages from 55 cm in Uvalde County to upper confining unit, and marl units within the upper Glen Rose 80 cm in Hays County (Maclay and Small, 1984).

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The Glen Rose Limestone is a marine fossiliferous lime- 1 to 7 m thick. The was deposited on stone and dolostone interbedded with shale, clay, and marl with the eroded surface of the Person Formation in deeper water than occasional layers of gypsum and anhydrite. Total thickness was characteristic for most of the Edwards Group deposition of the Glen Rose Limestone may be up to 400 m. It is divided (Small and Hanson, 1994). into two informal members, upper and lower, based on lithol- The Del Rio Clay is a calcareous shale that overlies the ogy and reef structures. The lower member consists of relatively Georgetown Formation and is distinctively recognizable in out- massive beds of limestone, dolostone, and dolomitic limestone crop, in cuttings, in core, and on geophysical logs. Hydrogeo- with mollusk fossils and local rudist reefs. In Bexar County, the logically, the Del Rio Clay acts as a confining unit separating the rudist biostrome ranges from 10 to 17 m thick (Clark, 2003b). Edwards Group, including the Georgetown Formation, from over- Reef structures mostly occur in the southeastern part of the Hill lying units, and it is 20–40 m thick. The base of the Del Rio Clay Country within the uppermost intervals of the lower member of is used to identify the top of the Edwards Aquifer. The Del Rio the Glen Rose Limestone (Barker and Ardis, 1996). They can Clay is noted for megafossils, most notably Ilymatogyra arietina. provide relatively high yield to wells in Bexar and Comal Coun- Located above the Del Rio Clay, there are the Buda Lime- ties. The upper member consists of thin- to-medium-bedded soft stone, , Austin Group, Anacacho Limestone, limestones and marls with resistant beds of dolostone, mudstone, and Escondido formations (Small and Clark, 2000). These and limestone. No reef structures are present in the upper mem- units range from shales (Eagle Ford Group) to dense mudstone ber. Alternating resistant and nonresistant beds give the upper () to chalky to marly limestone and marl (Aus- Glen Rose Limestone a stairstep appearance in the Hill Coun- tin Group). Many of the units in the Austin Group are karstic try. Most wells located in the Glen Rose Limestone produce with well-developed secondary and tertiary permeability struc- sufficient volumes for domestic use. The Glen Rose Limestone tures. Regional faulting can provide interconnection between the typically develops into a well-developed karst surface. The lower Edwards Group and younger units. San Antonio and San Pedro Glen Rose Limestone contains Honey Creek Cave, the longest Springs are located in the Austin Group in San Antonio but dis- mapped cave in Texas at almost 30 km. The marl units in the charge water from the Edwards Aquifer. upper Glen Rose Limestone act as the lower confining units for the Edwards Aquifer. STRUCTURAL GEOLOGY The Edwards Group overlies the Glen Rose Limestone and contains three depositional provinces in the San Antonio segment The Balcones fault zone is the dominant structural feature as defined by Maclay (1995) that are correlated in time: From controlling the hydrogeology of the Edwards Aquifer. It is an west to east, they are the Maverick Basin, the Devils River Trend, arc-shaped series of en echelon faults that spans much of central and the San Marcos Platform (Fig. 1). Texas. It ranges between 10 and 60 km wide and trends to the The Maverick Basin is found in southern and western Uvalde east in Uvalde County to Bexar County and San Antonio, and and Kinney Counties where the Edwards Group has been subdi- from there, it trends toward the northeast and north toward Aus- vided, from oldest to youngest, as the West Nueces, McKnight, tin. Ferrill et al. (this volume) discusses the geologic structure of and Salmon Peak Formations. The Maverick Basin was a site of the Edwards Aquifer (Ferrill et al., 2003). deposition occurring below the wave base. The estimated thick- Faults appear to play an important role in controlling ground- ness of the Maverick Basin is as much as 300 m (Clark, 2003a). water flow in the Edwards Aquifer. In some locations, they may The Devils River Trend was deposited between the Maverick restrict water flow where the Edwards Group is completely Basin and the San Marcos Platform and represents a shoal area displaced and juxtaposed against other less permeable units formed under largely open, shallow-marine conditions. The Dev- (Maclay, 1995), or they may also provide pathways for water ils River Trend is composed of the Devils River Formation (Lozo flow between karstified units within the aquifer, and interforma- and Smith, 1964), which was deposited on the western margin of tional flow between the Glen Rose Limestone and the Edwards the San Marcos Platform and occupies most of Medina County Aquifer (Johnson and Schindel, 2015). and eastern and northern Uvalde County. The Devils River Trend can be as much as 230 m thick. HYDROGEOLOGY The San Marcos Platform was an area of tidal flats, sab- khas, and subaerial erosion. Rose (1972) differentiated the The Edwards Aquifer is one of the most productive ground- San Marcos Platform into the Person and Kainer Formations water systems in the United States, characterized by extremely in Hays, Comal, and Bexar Counties. These formations have productive water wells (300 L/s), and high spring discharges been further subdivided into eight informal units on the basis of (9 m3/s). The Edwards Aquifer is a triple permeability sys- regionally correlated cyclical depositional patterns (Hovorka et tem composed of the rock matrix, fractures and bedding plane al., 1995). The estimated thickness of the combined Person and partings, and conduits (apertures greater than 2 cm) and caves Kainer Formations is 100–130 m. The overlying Georgetown (apertures generally greater than 25 cm). The aquifer exhibits Formation is typically lumped with the Edwards Group as part extremely high (cavernous) porosity and permeability throughout of the Edwards Group. The Georgetown Formation ranges from much of the recharge and artesian zones (Lindgren et al., 2004).

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Hydraulic heads in the freshwater portion of the aquifer are total of 3.7 × 108 m3 (302,348 acre-feet) was discharged from highly variable, depending upon natural and human-induced springs. Industrial users accounted for 2.8 × 107 m3 (22,600 acre- stresses on the aquifer. During a large storm event in October feet) of water use, irrigation was 1.1 × 108 m3 (90,600 acre-feet), 1998, for example, water levels in some wells in the recharge and municipal use was 3.181 × 108 m3 (257,900 acre-feet) of the zone rose more than 40 m within a few days (Johnson et al., discharge by pumping. 2002). Yields to wells in the recharge zone are generally much less than in the artesian zone but are still generally capable of pro- Contributing Zone viding for domestic and stock needs (Maclay and Land, 1988). The contributing zone is composed of less-soluble rocks of Groundwater Recharge and Discharge the Glen Rose Limestone, with some outcrop of Edwards Group limestones located on hilltops and the southern and eastern edges Recharge and discharge to the Edwards Aquifer are moni- of the Edwards Plateau. Surface water and groundwater flow to tored and recorded by the Edwards Aquifer Authority (EAA). surface streams and continue to flow downgradient as both sur- The EAA measures water quantity in acre-feet (1 acre-foot = face water and groundwater to the point where they cross the 1233 m3). Between 1934 and 2016, the median recharge to the recharge zone. Edwards Aquifer across nine major recharge basins was deter- mined to be 6.8 × 108 m3/yr (557,800 acre-feet/yr), and the mean Recharge Zone recharge was determined to be 8.7 × 108 m3/yr (706,500 acre- feet/yr; EAA, 2018). The wide range between median and mean The recharge zone is composed of the exposed, easily recharge reflects the extreme variation in rainfall conditions soluble rocks of the Edwards Group, and it covers more than found in south-central Texas, with the lowest recorded recharge of 3200 km2. Allogenic water from surface streams that originated 5.4 × 107 m3/yr (43,700 acre-feet/yr) occurring in 1956 (drought on the contributing zone crosses the Edward Group limestone of record) and the highest recorded recharge of 2.96 × 109 m3/yr and sinks into the ground through fractures, faults, and caves. (2.4 million acre-feet/yr) occurring in 1992 (EAA, 2018). During heavy rainfall, the infiltration capacity of streambeds may The volume of water recharging the aquifer from stream loss be exceeded, and runoff will exit the lower (downstream) end is such that water must be entering fractures, conduits (1 cm to of the recharge zone to continue flowing to the Gulf of Mexico. 25 cm), and caves (>25 cm) within the Edwards Aquifer. How- Autogenic recharge from precipitation on the recharge zone also ever, during flood flow, streams carry a tremendous bed load of contributes to recharge of the aquifer. material that effectively buries most open conduits. Water can The average depth to water in the recharge zone is typically infiltrate through the alluvial material, but there are few open greater than 70 m. However, during large storm events, water lev- cave entrances associated with streambeds in the recharge zone. els in the Edwards Aquifer (recharge zone) can rise more than Between 1934 and 2016, the median discharge from wells 40 m in less than 24 h (Johnson et al., 2002). Water entering the (municipal, agricultural, and industrial) located in the San Anto- aquifer from the recharge zone moves south and east to enter the nio segment of the Edwards Aquifer was determined to be 4.04 × fully saturated artesian zone. 108 m3/yr (327,800 acre-feet/yr), with a mean discharge of 3.9 × 108 m3/yr (315,500 acre-feet/yr; EAA, 2018). Artesian Zone Natural discharge from the San Antonio segment of the aqui- fer occurs at a series of springs. From largest to smallest dis- Faulting along the Balcones fault zone has resulted in the charge, these are Comal, San Marcos, Hueco, Leona, San Pedro, formation of the artesian zone. The upper boundary of the arte- and . Between 1934 and 2016, the median sian zone is formed by the Del Rio Clay, and the lower boundary spring discharge was determined to be 4.735 × 108 m3/yr (383,900 is the low-permeability marl beds located in the upper Glen Rose acre-feet/yr; EAA, 2018). During periods of drought, Leona, San Limestone. Faulting associated with the Balcones fault zone has Antonio, San Pedro, and Hueco Springs may stop flowing. Dur- dropped the aquifer to hundreds of meters below the surface ing the drought of record, also stopped flowing, along the aquifer’s southern boundary. Hydrostatic pressure from and San Marcos Springs, the lowest spring in the San Antonio the upgradient artesian and recharge zones results in a potentio- segment in terms of elevation, continued to flow, but at greatly metric surface above the bottom of the Del Rio Clay. In places, diminished levels. the potentiometric surface is located above the land surface and During 2016, ~26% of discharge occurred as municipal pump- results in large flowing artesian wells in the downdip section of ing, 6% was irrigation pumping, and 3% was industrial pumping, the aquifer (Maclay, 1995). In southern Medina County, fresh­ with 63% occurring as spring flow. During dry years, agricul- water from the South Medina saline transect well can be found tural pumping makes up a larger proportion of discharge from the as deep as 1038 m below ground surface (EAA, 2010, 2012a). aquifer, sometimes amounting to as much as half of all discharge Generally, groundwater in the artesian zone flows from the (EAA, 2018). For example, in 2012, a total of 4.745 × 108 m3 west to the east to discharge at large springs located in Comal (384,700 acre-feet) of water was discharged from wells, and a and Hays Counties. Groundwater gradients are ~0.5 m/km. Flow

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in portions of the aquifer is turbulent in nature in some loca- More than 60 endemic species utilize the aquifer and include both tions, including the recharge zone and at Comal and San Marcos vertebrate and invertebrate species (Longley, 1981; Hutchins, Springs (Schindel et al., 2002). 2017). Some have been found across wide areas of the aquifer Water circulates through the Edwards Aquifer as part of the and provide insight into the interconnectedness of conduits. Sev- hydrologic cycle from recharge areas to discharge points (springs eral species, including two nonpigmented, blind catfish species, and wells). As water flows south and east from the contributing are found along the saline water–freshwater interface (Longley zone, most surface streams lose all or most of their water as they and Karnei, 1978). These areas at the interface are far from rapid pass over the recharge zone. During dry conditions, streams may influx of storm water during rain events and their entrained detri- completely lose their flow into the cavernous zone of the upper tus. Ecological communities at the interface appear to be based Glen Rose Limestone before crossing onto the recharge zone on microbial organisms that have evolutionally adapted to this (Veni, 1994; Clark, 2003b). Groundwater flow has been traced restricted environment, specifically, bacteria that are utilizing the using fluorescent dyes from the upper Glen Rose Limestone chemical gradient along the saline water–freshwater interface to the Edwards Aquifer with groundwater velocities as great as a source of nutrients (Rye et al., 1981; Hutchins et al., 2016; as 2 km/d (Johnson et al., 2010). Few streams flow beyond the Hutchins, 2017; Sharp and Smith, this volume). recharge zone to reach the Gulf of Mexico during droughts. AQUIFER FLOW PATHS Interformational Flow The EAA initiated a synoptic water-level program to col- Water also enters the Edwards Aquifer through interforma- lect water levels from wells across the entire region of the San tional flow between the lower and upper Glen Rose Limestone Antonio segment (EAA, 2012a). Water-level elevations recorded and the Edwards Group though deeper and longer flow paths. from wells with precise land-surface elevations were used to pre- Interformational flow has been demonstrated through tracer test- pare a series of synoptic water-level maps. The data supported ing, and the EAA has a research initiative to quantify this vol- the development of groundwater-flow models for the Edwards ume. The underlying geology of the contributing zone includes Aquifer (Green et al., this volume, Chapter 3). Water levels were the Edwards Limestone and less permeable lower and upper Glen collected: (1) before initiation of irrigation, in January/February; Rose Limestone. The contributing zone covers ~14,000 km2. (2) postirrigation in late July; and (3) in October/November, when limited agricultural pumping is experienced. Water levels Bad-Water Line were generally collected within a 1 wk time period. The synoptic water-level program results indicated that The downgradient extent of the Edwards Aquifer is commonly regional groundwater flow in the artesian zone moves from west defined by the bad-water line (Sharp and Smith, this volume), to east across Uvalde, Medina, and Bexar Counties (Fig. 2). In where the recommended drinking water standard of 1000 mg/L central and eastern Bexar County, groundwater flows in a north- total dissolved solids (TDS) is exceeded. Fresh and saline zones easterly direction through Comal and Hays Counties to discharge may vertically interfinger with depth. The bad-water portion at Comal and San Marcos Springs. (EAA, 2012a). The ground-

of the aquifer has high concentrations of biogenic-derived H2S water divide for the western portion of the San Antonio segment resulting from dissolution of evaporite facies compared to lime- is in Kinney County, generally to the east of Brackettville (Green stone facies (Maclay, 1995). et al., 2006; Green et al., this volume, Chapter 6). Groundwater outflow from south-central Uvalde County Groundwater Age includes discharge to the Leona Formation (gravel) and also an eastern component of flow to Medina County (Green et al., this The age of groundwater within the Edwards Aquifer is volume, Chapter 5). The Leona Formation was deposited during extremely variable. Data from carbon-14 and hydrogen-3 (tri- the Quaternary and most likely buried active springs discharging tium) age dating indicate that some water in the aquifer may be the Edwards Aquifer. Aquifer discharge was insufficient to keep decades old (Hunt et al., 2015), whereas fluorescent-dye trace the spring orifice(s) open, but the gravel has sufficient perme- testing and other direct observations document that ground­ ability to allow discharge from the aquifer. Some of this water in water along short flow paths may be only days old (Johnson the Leona Formation discharges from Leona Springs, but most of et al., 2010). the water exits as subsurface flow in the gravel and is lost to the Edwards Aquifer (Green et al., 2004, 2012). Groundwater Ecology In eastern Uvalde County, series of large igneous intrusions occur in the artesian zone near the community of Knippa (Clark, The Edwards Aquifer has one of the most diverse and wide- 2003a; Adkins, 2013). This and related tectonic features have spread faunal assemblages of any groundwater system in the resulted in groundwater from Uvalde County flowing northeast United States (Krejca and Reddell, this volume). This attests to around the intrusions, through a constriction called the Knippa the interconnected, cavernous void size of parts of the aquifer. Gap, into the unconfined Edwards Group limestone and then into

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/4842023/mwr215-02.pdf by guest on 01 October 2021 14 G.M. Schindel Figure 2. Potentiometric surface of San Antonio segment of the Edwards Aquifer. Source: Esquilin et al. (2012). MSL—above mean sea level. Source: Esquilin et al. (2012). MSL—above Aquifer. of the Edwards Antonio segment of San Figure 2. Potentiometric surface

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the San Antonio Pool, and then southeast back into the artesian lating meteoric waters along structural and paleokarst features. zone (Maclay, 1995; Green et al., this volume, Chapter 5). Epigenetic karst theory assumes karst features are produced from The Edwards Aquifer (Green et al., 2004) also appears to shallow downward or near-horizontal groundwater movement be recharging the Leona Formation in Medina County where with aggressive groundwater derived from meteoric processes. the Haby Crossing fault forces groundwater upward from the However, Klimchouk (2007) concluded that rising waters (hypo- Edwards Group and into the high-permeability gravels (Green et genetic) from depth are also important agents of karst develop- al., this volume, Chapter 4). This water is derived from recharge ment. Worthington (2001) has shown that deep circulation sys- occurring in the area of Diversion Lake on the Medina River. tems that are warmed by the geothermal gradient have decreased Flow paths around Comal Springs are complex and appear viscosity and are more efficient in forming regional flow systems. to be controlled by faulting, allowing compartmentalization of Hypogene processes in the Edwards Aquifer were little known, flow along fault blocks between the artesian zone and recharge nor understood, until recently. zone (Green et al., this volume, Chapter 4). Discharge from both The current conceptualization of regional karst flow sys- zones occurs at Comal and San Marcos Springs. Comal Springs tems in the Edwards Aquifer involves a combination of different is located on the Comal Springs fault at the contact between the processes. Epigenic processes are occurring in the contributing, recharge and artesian zones, separated by the Comal Springs recharge, and shallow artesian zone and have helped create the fault. This fault has as much as 275 m of displacement and is current topography. The contributing and recharge zones are thought to integrate mixing of deep and shallow waters from characterized by small shallow sinkholes, losing streams, and the aquifer. Water from the artesian fault block around Comal caves. Hypogenic processes, which involve upward flow in the Springs rises upward from a depth of ~200 m to discharge into artesian zone, occur along the artesian flow path and are best Landa Lake, forming the headwaters of the . The exhibited at Comal and San Marcos Springs. Dissolution in the groundwater gradient between the City of San Antonio and deep artesian portion of the aquifer is driven by biogenic acids

Comal Springs is ~0.5 m/km. However, the head loss in the distal (H2S) and mixing corrosion between different water chemistries, 500 m of flow in the artesian zone at Comal can be as much as as defined by Bögli (1964). 7 m, which suggests a gradient of 14 m/km. This is interpreted to The morphology of numerous caves in the recharge zone be a result of the filling of spring conduits from bed load and sed- indicates that hypogene processes were important in their forma- iment from the high-flow-gradient Blieders and Panther Creeks, tion and most likely occurred during or before the upper confin- which lie upstream and flow into Landa Lake. Groundwater from ing units were removed by erosion. Many caves in the recharge the recharge zone (Comal Springs fault block) also discharges zone have a ramiform pattern, cupolas in their ceilings and walls, at Comal Springs (Johnson et al., 2012). Potentiometric surface entrances not related to the surface topography, and other fea- maps around Comal Springs indicate that groundwater flow as tures that are indicative of formation by ascending water. Most of little as 2 km to the northwest of the spring mixes with deep these caves are now relicts of the former hypogenic flow system groundwater flow to discharge at San Marcos Springs (Johnson and are now being overprinted by current epigenic processes. et al., 2012). Average flow at Comal Springs is estimated to be Some of the best examples of hypogene processes are pre- 8 m3/s (288 cfs; EAA, 2018). served in four caves located in northeastern Uvalde County, north Flow in the area around San Marcos Springs also originates of the town of Knippa. Before groundwater from the Uvalde Pool from both the artesian and recharge zones, with flow emerg- was captured by the San Antonio Pool, groundwater from the ing from the Comal Springs fault block at a depth of more than Uvalde Pool appears to have discharged from a series of large 100 m. Water from the recharge zone within the Hueco Springs paleosprings (caves). These caves (Frio Bat Cave, Big Easy fault block also discharges from San Marcos Springs (Green et Cave, Dripstone Cave, and Frio Queen Cave) contain strong evi- al., this volume, Chapter 4). dence of upward flow in the form of cupolas in the walls and ceilings and large wide ascending passages. All four entrances SPELEOGENESIS are located on topographic highs and do not appear to be related to the surface topography (Fig. 3). Recent discoveries in one of Evaporites and limestones of the Edwards Group have been the caves found that water-filled conduits are still present in the highly karstified, resulting in a triple-permeability aquifer con- deepest areas of exploration. It is not clear whether these repre- taining matrix, fracture, and conduit permeability (Halihan et al., sent the subsurface Frio River, Dry Frio River, or groundwater 2000; Hovorka et al., 1995, 2004). Karstification occurred soon from the Uvalde Pool. Surface processes are now overprinting after deposition when the rocks were first subaerially exposed the hypogene processes in these four caves, with minor dissolu- during depositional highstands. After subsidence and burial, tion of the limestone surface around the entrances and deposition paleokarst features may have been inception horizons for later of speleothems in the cave. As the Uvalde Pool was integrated karstification, which may have occurred prior to downfaulting into the San Antonio Pool, water levels in the aquifer declined, and formation of the artesian zone. Early researchers (Maclay and the Knippa springs became paleospring caves. and Small, 1984) proposed epigenetic (near-surface) karst pro- The evolution of the Edwards Aquifer was initiated by down- cesses for formation of the Edwards Aquifer, driven by circu- cutting of the system, exhuming the ­structurally

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Figure 3. Area of Knippa Gap showing location of paleospring caves, modified from Adkins (2013). UTM Zone 14 NAD 83—Universal Transverse Mercator Zone 14, North American Datum 1983; msl—above mean sea level.

higher Edwards Group limestones in the western basins (Wood- San Antonio Springs are ephemeral and will commonly stop dis- ruff and Abbott, 1979). The dissolution process increased as flow charging during prolonged droughts when aquifer levels drop. of aggressive surface water flushed saturated water from the Comal Springs, located at an elevation of 189 m amsl, is aquifer. The discharge of water from the initial development of the largest of the spring complexes draining the Edwards Aquifer the Edwards Aquifer beneath Uvalde County probably occurred and forms the headwaters of the Comal River. Karst dissolution at the paleosprings located north of the town of Knippa, as noted processes, driven by hydraulic head, created a positive feedback above, and possibly at Leona Springs. As the Edwards Group loop and resulted in the increase in discharge at Comal Springs limestones were exposed at lower elevations to the east, hydrau- over time. The increased efficiency of the flow system at Comal lic head forced the integration of the Uvalde Pool with the San Springs resulted in a lowering of the regional potentiometric sur- Antonio Pool. This resulted in a decline in water levels and the face and diminished flow at San Antonio and . abandonment of the Knippa spring caves, leaving them as high San Marcos Springs, located at an elevation of 174 m amsl, relics of a former aquifer flow system. is the second largest spring complex draining the Edwards The sequence and timing of the opening of the San Antonio Aquifer. San Marcos was the last spring to form in the Edwards and San Pedro Springs (Bexar County), Comal Springs (Comal Aquifer. Currently, San Marcos Springs is an underflow spring County), and San Marcos Springs (Hays County) are difficult to to Comal Springs. However, the regional hydraulic head and determine but most likely occurred in the Neogene (Woodruff positive feedback associated with karst dissolution may result and Abbott, 1979). Faults in central Bexar County allowed the in San Marcos Springs pirating flow from Comal Springs as its formation of San Antonio Springs and San Pedro Springs. Water discharge, becoming more efficient over geologic time. This may rose through the faults to create the springs within the Austin result in Comal Springs becoming an overflow spring to San Group limestones. The elevation of these springs ranges from Marcos Springs, which would become the dominant spring in the 199 to 202 m above mean sea level (amsl). Both San Pedro and Edwards Aquifer.

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Both Comal and San Marcos Springs have significant upward Clark, A.K., 2003a, Geologic Framework and Hydrogeologic Features of the groundwater gradients with water discharging along major faults Edwards Aquifer, Uvalde County, Texas: U.S. Geological Survey Water- Resources Investigations Report 03-4010, 17 p. and are examples of hypogene processes at the discharge end of Clark, A.K., 2003b, Geologic Framework and Hydrogeologic Features of large regional flow paths. the Glen Rose Limestone, Camp Bullis Training Site, Bexar County, The ultimate fate of the Edwards Aquifer may be that Barton Texas: U.S. Geological Survey Water-Resources Investigations Report 03-4081, 9 p. Springs, located at an elevation of 130 m amsl near the Colorado Edwards Aquifer Authority (EAA), 2010, Hydrologic Data Report for 2010 River in south Austin, may become the ultimate discharge point (Hamilton, J.M., Johnson, S., Esquilin, R., Burgoon, C., Luevano, G., for the San Antonio segment of the Edwards Aquifer. Support- Gregory, D., Mireles, J., Gloyd, R., and Schindel, G.M., contributors): Edwards Aquifer Authority Report 11-01, 340 p. ing this hypothesis is the observation that during droughts, the Edwards Aquifer Authority (EAA), 2012a, Edwards Aquifer Authority Syn- groundwater divide between the San Antonio segment and the optic Water Level Program 2005–2009 Water Level Data (Esquilin, R., segment of the aquifer disappears, and water sink- Hamilton, J.M., and Schindel, G.M., contributors): Edwards Aquifer Authority Report 12-02, 82 p. ing in the bed of the Blanco River discharges at Barton Springs, Edwards Aquifer Authority (EAA), 2012b, Recovery Implementation Pro- indicating that this process may have already begun. gram—Habitat Conservation Plan (November): San Antonio, Texas, Edwards Aquifer Authority, 414 p. Edwards Aquifer Authority (EAA), 2018, Edwards Aquifer Authority Recharge CONCLUSION 2017 Groundwater Recharge Report: San Antonio, Texas, Edwards Aqui- fer Authority, 7 p. The Edwards Aquifer is one of the largest and most prolific Esquilin, R., Hamilton, J.M., and Schindel, G.M., 2012, Edwards Aquifer Authority Synoptic Water Level Program 2005-2012 Water Level Data: aquifer systems in the United States. For the San Antonio seg- Report Number 12-02 (August), 82 p. ment of the Edwards Aquifer, surface water from the contribut- Ferrill, D.A., Sims, D.W., Morris, A.P., Waiting, D.J., and Franklin, N., 2003, ing zone directly recharges the aquifer through interformational Structural Controls on the Edwards Aquifer/Trinity Aquifer Interface in the Camp Bullis Quadrangle, Texas: San Antonio, Texas, Center for flow or as surface water when it crosses onto the recharge zone. Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Flow in the artesian zone is from west to east and emerges in a Institute, 126 p. series of large springs located in the urban areas of San Antonio, Ferrill, D.A., Morris, A.P., and McGinnis, R.N., 2019, this volume, Geologic structure of the Edwards Aquifer, in Sharp, J.M., Jr., Green, R.T., and New Braunfels, and San Marcos. Flow paths in the San Anto- Schindel, G.M., eds., The Edwards Aquifer: The Past, Present, and Future nio segment exceed 200 km with groundwater velocities in the of a Vital Water Resource: Geological Society of America Memoir 215, recharge zone typically exceeding 2 km/d. The highly developed https://doi.org/10.1130/2019.1215(14). Green, R.T., Franklin, N., and Prikryl, J., 2004, Constraining models of ground- tertiary permeability (matrix, fractures/faults, and caverns and water flow using results from an electrical resistivity survey, in Proceed- conduits) from karst processes results in very high well yields, ings of the 2004 Symposium on the Application of Geophysics to Engi- neering and Environmental Problems (SAGEEP) Conference: Colorado a low hydraulic gradient, rapid response to recharge and drought Springs, Colorado. Published by Environmental and Engineering Geo- conditions, and an extensive aquifer fauna with more than 60 physical Society; https://doi.org/10.4133/1.2923274. obligate species. Green, R.T., Bertetti, F.P., Franklin, N.M., Morris, A.P., Ferrill, D.A., and Klar, R.V., 2006, Evaluation of the Edwards Aquifer in Kinney and Uvalde Both epigenic and hypogenic processes occur in the aqui- Counties, Texas: report prepared by San Antonio, Texas, Southwest fer and have resulted in the high tertiary permeabilities. The Research Institute, for Edwards Aquifer Authority, June, 53 p. faults associated with the Balcones fault zone play an important Green, R.T., Bertetti, F.P., McGinnis, R.N., and Prikryl, J., 2012, Measur- ing Floodplain Hydraulics of Seco Creek and the Medina River Where role in controlling groundwater movement as well as interfor- It Overlies the Edwards Aquifer: contract project conducted by San mational flow. Antonio, Texas, Southwest Research Institute, for the Edwards Aquifer Evolution of the aquifer was initiated in the western portion Authority, 36 p. + attachments. Green, R.T., Winterle, J., and Fratesi, B., 2019, this volume, Chapter 3, Numeri- of the aquifer. Lowering of the land surface in the east resulted cal groundwater models for Edwards Aquifer systems, in Sharp, J.M., Jr., first in formation of springs at San Antonio and San Pedro and Green, R.T., and Schindel, G.M., eds., The Edwards Aquifer: The Past, then at Comal and San Marcos as dissolution processes created a Present, and Future of a Vital Water Resource: Geological Society of America Memoir 215, https://doi.org/10.1130/2019.1215(03). positive feedback loop and improved spring efficiency. The ulti- Green, R.T., Bertetti, F.P., Fratesi, B., and Schindel, G.M., 2019, this volume, mate fate of the Edwards Aquifer discharge is that Barton Springs Chapter 4, San Antonio Pool of the Edwards (Balcones Fault Zone) will become the primary spring discharge for the Edwards Aqui- Aquifer, in Sharp, J.M., Jr., Green, R.T., and Schindel, G.M., eds., The Edwards Aquifer: The Past, Present, and Future of a Vital Water Resource: fer over geologic time. Geological Society of America Memoir 215, https://doi.org/10.1130/ 2019.1215(04). 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