Genesis of the Edwards (Balcones Fault Zone) Aquifer Open Access

The San Antonio segment of the Edwards (Balcones Fault zone) Aquifer is one of the most important and prolific karst aquifers in the United States. Rocks of the Edwards Group crop out from the Rio Grande River along the Mexican border near Del Rio, Texas, eastward to San Antonio, and then northeastward through the cities of New Braunfels, San Marcos, and Austin, and into Bell County for a distance of 450 km. The San Antonio segment of the Edwards Aquifer extends from east of Brackettville in Kinney County, eastward to San Antonio, and then northeast to the groundwater divide near Onion Creek in Hays County (Fig. 1; Sharp et al., this volume). The aquifer varies from 10 to 60 km wide and contains freshwater to depths greater than 1200 m below the surface (Edwards Aquifer Authority, 2010). The aquifer is the source of water for more than two million people in the greater San Antonio area (Schindel et al., 2004). It is used for agricultural, industrial, and municipal purposes, and it creates critical habitat for endangered species that depend on continuous spring flows at Comal and San Marcos Springs (Edwards Aquifer Authority, 2012b). The aquifer also provides water for ecosystem maintenance; a recreation industry on the spring-fed Comal and San Marcos Rivers; and municipal, agricultural, and industrial use along the rivers that flow to the Gulf of Mexico.

The Edwards Aquifer is in the 150- to 300-m-thick Edwards Group rocks, which were deposited in late Early Cretaceous time. The Edwards Aquifer system has been divided into three major hydrogeologic zones—the contributing zone, the recharge zone, and the artesian zone. The Edwards Aquifer is located south of and at a lower elevation than the Edwards-Trinity Aquifer, which occurs in the Edwards Plateau.

The contributing zone encompasses the largest area and is composed of higher-elevation outcrops of the Edwards Group and the underlying Glen Rose Limestone. The contributing zone makes up part of the Edwards Plateau and covers more than 14,000 km². The contributing zone collects rainwater and spring flow and forms the headwaters of major surface streams in the region. These include the Nueces, Frio, Sabinal, Medina, Cibolo, Guadalupe, and Blanco Rivers.

Faulting along the Balcones fault zone (Ferrill et al., this volume) has resulted in rocks of the Edwards Group being downthrown toward the Gulf of Mexico to the south. These down-to-the-south faults juxtapose Lower Cretaceous rocks against Upper Cretaceous rocks. The northernmost area where the Edwards Group is exposed at the surface is defined as the recharge zone of the aquifer. This covers 3200 km² and identifies the area of capture of surface water from streams flowing southward and eastward from the contributing zone. The recharge zone approximately overlies the Balcones fault zone, which, along with the contributing zone to a lesser degree, allows vertical downward movement of surface water into the Edwards Aquifer.

The artesian zone occurs downgradient where the Edwards Aquifer is confined between the Del Rio Clay, which forms the upper confining unit, and marl units within the upper Glen Rose Limestone, which forms the lower confining unit. The artesian zone encompasses ~14,000 km². Within the artesian zone, the aquifer is fully saturated and contains most of the large production wells.

Initial deposition of limestones of the Edwards Group was as a biochemical precipitate in both shallow- and deep-water marine environments, as well as interspersed evaporite deposits. Since deposition, these limestones have undergone subaerial exposure in the Early Cretaceous; burial in the middle to Late Cretaceous; uplift with secondary subaerial exposure beginning in the late Mesozoic to early Cenozoic; igneous intrusion, faulting, and uplift in the Miocene; and weathering since the latest subaerial exposure. The faulting is complex and is an important factor in controlling aquifer hydrology; it is related to uplift in central Texas and subsidence related to formation of the Gulf of Mexico, both of which are related to the regional tectonics.

The Edwards Group rocks have been subjected to dissolution over geologic time and have resulted in a subdued karst landscape where exposed at the surface. Within the subsurface, the rocks are noted for highly developed secondary and tertiary permeabilities, with water withdrawal only limited by the size of the pump (Maclay and Small, 1984).

Physiographically, the Balcones fault zone escarpment is the terminus for the southern and eastern edge of the Edwards Plateau where it intersects the Gulf Coastal Plain. The Hill County represents the dissected edge of the Edwards Plateau. The landscape is covered with oak and juniper, separating the nearly flat-lying rocks of the Edwards Plateau from the gently coastward-dipping sediments of the subsiding Gulf of Mexico (Maclay and Small, 1984). Annual rainfall averages from 55 cm in Uvalde County to 80 cm in Hays County (Maclay and Small, 1984).

The Glen Rose Limestone is a marine fossiliferous limestone and dolostone interbedded with shale, clay, and marl with occasional layers of gypsum and anhydrite. Total thickness of the Glen Rose Limestone may be up to 400 m. It is divided into two informal members, upper and lower, based on lithology and reef structures. The lower member consists of relatively massive beds of limestone, dolostone, and dolomitic limestone with mollusk fossils and local rudist reefs. In Bexar County, the rudist biostrome ranges from 10 to 17 m thick (Clark, 2003b). Reef structures mostly occur in the southeastern part of the Hill Country within the uppermost intervals of the lower member of the Glen Rose Limestone (Barker and Ardis, 1996). They can provide relatively high yield to wells in Bexar and Comal Counties. The upper member consists of thin- to-medium-bedded soft limestones and marls with resistant beds of dolostone, mudstone, and limestone. No reef structures are present in the upper member. Alternating resistant and nonresistant beds give the upper Glen Rose Limestone a stairstep appearance in the Hill Country. Most wells located in the Glen Rose Limestone produce sufficient volumes for domestic use. The Glen Rose Limestone typically develops into a well-developed karst surface. The lower Glen Rose Limestone contains Honey Creek Cave, the longest mapped cave in Texas at almost 30 km. The marl units in the upper Glen Rose Limestone act as the lower confining units for the Edwards Aquifer.

The Edwards Group overlies the Glen Rose Limestone and contains three depositional provinces in the San Antonio segment as defined by Maclay (1995) that are correlated in time: From west to east, they are the Maverick Basin, the Devils River Trend, and the San Marcos Platform (Fig. 1).

The Maverick Basin is found in southern and western Uvalde and Kinney Counties where the Edwards Group has been subdivided, from oldest to youngest, as the West Nueces, McKnight, and Salmon Peak Formations. The Maverick Basin was a site of deposition occurring below the wave base. The estimated thickness of the Maverick Basin is as much as 300 m (Clark, 2003a).

The Devils River Trend was deposited between the Maverick Basin and the San Marcos Platform and represents a shoal area formed under largely open, shallow-marine conditions. The Devils River Trend is composed of the Devils River Formation (Lozo and Smith, 1964), which was deposited on the western margin of the San Marcos Platform and occupies most of Medina County and eastern and northern Uvalde County. The Devils River Trend can be as much as 230 m thick.

The San Marcos Platform was an area of tidal flats, sabkhas, and subaerial erosion. Rose (1972) differentiated the San Marcos Platform into the Person and Kainer Formations in Hays, Comal, and Bexar Counties. These formations have been further subdivided into eight informal units on the basis of regionally correlated cyclical depositional patterns (Hovorka et al., 1995). The estimated thickness of the combined Person and Kainer Formations is 100–130 m. The overlying Georgetown Formation is typically lumped with the Edwards Group as part of the Edwards Group. The Georgetown Formation ranges from 1 to 7 m thick. The Georgetown Formation was deposited on the eroded surface of the Person Formation in deeper water than was characteristic for most of the Edwards Group deposition (Small and Hanson, 1994).

The Del Rio Clay is a calcareous shale that overlies the Georgetown Formation and is distinctively recognizable in outcrop, in cuttings, in core, and on geophysical logs. Hydrogeologically, the Del Rio Clay acts as a confining unit separating the Edwards Group, including the Georgetown Formation, from overlying units, and it is 20–40 m thick. The base of the Del Rio Clay is used to identify the top of the Edwards Aquifer. The Del Rio Clay is noted for megafossils, most notably Ilymatogyra arietina.

Located above the Del Rio Clay, there are the Buda Limestone, Eagle Ford Group, Austin Group, Anacacho Limestone, and Escondido formations (Small and Clark, 2000). These units range from shales (Eagle Ford Group) to dense mudstone (Buda Limestone) to chalky to marly limestone and marl (Austin Group). Many of the units in the Austin Group are karstic with well-developed secondary and tertiary permeability structures. Regional faulting can provide interconnection between the Edwards Group and younger units. San Antonio and San Pedro Springs are located in the Austin Group in San Antonio but discharge water from the Edwards Aquifer.

The Balcones fault zone is the dominant structural feature controlling the hydrogeology of the Edwards Aquifer. It is an arc-shaped series of en echelon faults that spans much of central Texas. It ranges between 10 and 60 km wide and trends to the east in Uvalde County to Bexar County and San Antonio, and from there, it trends toward the northeast and north toward Austin. Ferrill et al. (this volume) discusses the geologic structure of the Edwards Aquifer (Ferrill et al., 2003).

Faults appear to play an important role in controlling groundwater flow in the Edwards Aquifer. In some locations, they may restrict water flow where the Edwards Group is completely displaced and juxtaposed against other less permeable units (Maclay, 1995), or they may also provide pathways for water flow between karstified units within the aquifer, and interformational flow between the Glen Rose Limestone and the Edwards Aquifer (Johnson and Schindel, 2015).

The Edwards Aquifer is one of the most productive groundwater systems in the United States, characterized by extremely productive water wells (300 L/s), and high spring discharges (9 m³/s). The Edwards Aquifer is a triple permeability system composed of the rock matrix, fractures and bedding plane partings, and conduits (apertures greater than 2 cm) and caves (apertures generally greater than 25 cm). The aquifer exhibits extremely high (cavernous) porosity and permeability throughout much of the recharge and artesian zones (Lindgren et al., 2004).

Hydraulic heads in the freshwater portion of the aquifer are highly variable, depending upon natural and human-induced stresses on the aquifer. During a large storm event in October 1998, for example, water levels in some wells in the recharge zone rose more than 40 m within a few days (Johnson et al., 2002). Yields to wells in the recharge zone are generally much less than in the artesian zone but are still generally capable of providing for domestic and stock needs (Maclay and Land, 1988).

Recharge and discharge to the Edwards Aquifer are monitored and recorded by the Edwards Aquifer Authority (EAA). The EAA measures water quantity in acre-feet (1 acre-foot = 1233 m³). Between 1934 and 2016, the median recharge to the Edwards Aquifer across nine major recharge basins was determined to be 6.8 × 10⁸ m³/yr (557,800 acre-feet/yr), and the mean recharge was determined to be 8.7 × 10⁸ m³/yr (706,500 acre-feet/yr; EAA, 2018). The wide range between median and mean recharge reflects the extreme variation in rainfall conditions found in south-central Texas, with the lowest recorded recharge of 5.4 × 10⁷ m³/yr (43,700 acre-feet/yr) occurring in 1956 (drought of record) and the highest recorded recharge of 2.96 × 10⁹ m³/yr (2.4 million acre-feet/yr) occurring in 1992 (EAA, 2018).

The volume of water recharging the aquifer from stream loss is such that water must be entering fractures, conduits (1 cm to 25 cm), and caves (>25 cm) within the Edwards Aquifer. However, during flood flow, streams carry a tremendous bed load of material that effectively buries most open conduits. Water can infiltrate through the alluvial material, but there are few open cave entrances associated with streambeds in the recharge zone.

Between 1934 and 2016, the median discharge from wells (municipal, agricultural, and industrial) located in the San Antonio segment of the Edwards Aquifer was determined to be 4.04 × 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).

Natural discharge from the San Antonio segment of the aquifer occurs at a series of springs. From largest to smallest discharge, these are Comal, San Marcos, Hueco, Leona, San Pedro, and San Antonio Springs. Between 1934 and 2016, the median spring discharge was determined to be 4.735 × 10⁸ m³/yr (383,900 acre-feet/yr; EAA, 2018). During periods of drought, Leona, San Antonio, San Pedro, and Hueco Springs may stop flowing. During the drought of record, Comal Springs also stopped flowing, and San Marcos Springs, the lowest spring in the San Antonio segment in terms of elevation, continued to flow, but at greatly diminished levels.

During 2016, ~26% of discharge occurred as municipal pumping, 6% was irrigation pumping, and 3% was industrial pumping, with 63% occurring as spring flow. During dry years, agricultural pumping makes up a larger proportion of discharge from the aquifer, sometimes amounting to as much as half of all discharge (EAA, 2018). For example, in 2012, a total of 4.745 × 10⁸ m³ (384,700 acre-feet) of water was discharged from wells, and a total of 3.7 × 10⁸ m³ (302,348 acre-feet) was discharged from springs. Industrial users accounted for 2.8 × 10⁷ m³ (22,600 acre-feet) of water use, irrigation was 1.1 × 10⁸ m³ (90,600 acre-feet), and municipal use was 3.181 × 10⁸ m³ (257,900 acre-feet) of the discharge by pumping.

The contributing zone is composed of less-soluble rocks of the Glen Rose Limestone, with some outcrop of Edwards Group limestones located on hilltops and the southern and eastern edges of the Edwards Plateau. Surface water and groundwater flow to surface streams and continue to flow downgradient as both surface water and groundwater to the point where they cross the recharge zone.

The recharge zone is composed of the exposed, easily soluble rocks of the Edwards Group, and it covers more than 3200 km². Allogenic water from surface streams that originated on the contributing zone crosses the Edward Group limestone and sinks into the ground through fractures, faults, and caves. During heavy rainfall, the infiltration capacity of streambeds may be exceeded, and runoff will exit the lower (downstream) end of the recharge zone to continue flowing to the Gulf of Mexico. Autogenic recharge from precipitation on the recharge zone also contributes to recharge of the aquifer.

The average depth to water in the recharge zone is typically greater than 70 m. However, during large storm events, water levels in the Edwards Aquifer (recharge zone) can rise more than 40 m in less than 24 h (Johnson et al., 2002). Water entering the aquifer from the recharge zone moves south and east to enter the fully saturated artesian zone.

Faulting along the Balcones fault zone has resulted in the formation of the artesian zone. The upper boundary of the artesian zone is formed by the Del Rio Clay, and the lower boundary is the low-permeability marl beds located in the upper Glen Rose Limestone. Faulting associated with the Balcones fault zone has dropped the aquifer to hundreds of meters below the surface along the aquifer’s southern boundary. Hydrostatic pressure from the upgradient artesian and recharge zones results in a potentiometric surface above the bottom of the Del Rio Clay. In places, the potentiometric surface is located above the land surface and results in large flowing artesian wells in the downdip section of the aquifer (Maclay, 1995). In southern Medina County, freshwater from the South Medina saline transect well can be found as deep as 1038 m below ground surface (EAA, 2010, 2012a).

Generally, groundwater in the artesian zone flows from the west to the east to discharge at large springs located in Comal and Hays Counties. Groundwater gradients are ~0.5 m/km. Flow in portions of the aquifer is turbulent in nature in some locations, including the recharge zone and at Comal and San Marcos Springs (Schindel et al., 2002).

Water circulates through the Edwards Aquifer as part of the hydrologic cycle from recharge areas to discharge points (springs and wells). As water flows south and east from the contributing zone, most surface streams lose all or most of their water as they pass over the recharge zone. During dry conditions, streams may completely lose their flow into the cavernous zone of the upper Glen Rose Limestone before crossing onto the recharge zone (Veni, 1994; Clark, 2003b). Groundwater flow has been traced using fluorescent dyes from the upper Glen Rose Limestone to the Edwards Aquifer with groundwater velocities as great as 2 km/d (Johnson et al., 2010). Few streams flow beyond the recharge zone to reach the Gulf of Mexico during droughts.

Water also enters the Edwards Aquifer through interformational flow between the lower and upper Glen Rose Limestone and the Edwards Group though deeper and longer flow paths. Interformational flow has been demonstrated through tracer testing, and the EAA has a research initiative to quantify this volume. The underlying geology of the contributing zone includes the Edwards Limestone and less permeable lower and upper Glen Rose Limestone. The contributing zone covers ~14,000 km².

The downgradient extent of the Edwards Aquifer is commonly defined by the bad-water line (Sharp and Smith, this volume), where the recommended drinking water standard of 1000 mg/L total dissolved solids (TDS) is exceeded. Fresh and saline zones may vertically interfinger with depth. The bad-water portion of the aquifer has high concentrations of biogenic-derived H₂S resulting from dissolution of evaporite facies compared to limestone facies (Maclay, 1995).

The age of groundwater within the Edwards Aquifer is extremely variable. Data from carbon-14 and hydrogen-3 (tritium) age dating indicate that some water in the aquifer may be decades old (Hunt et al., 2015), whereas fluorescent-dye trace testing and other direct observations document that groundwater along short flow paths may be only days old (Johnson et al., 2010).

The Edwards Aquifer has one of the most diverse and widespread faunal assemblages of any groundwater system in the United States (Krejca and Reddell, this volume). This attests to the interconnected, cavernous void size of parts of the aquifer. More than 60 endemic species utilize the aquifer and include both vertebrate and invertebrate species (Longley, 1981; Hutchins, 2017). Some have been found across wide areas of the aquifer and provide insight into the interconnectedness of conduits. Several species, including two nonpigmented, blind catfish species, are found along the saline water–freshwater interface (Longley and Karnei, 1978). These areas at the interface are far from rapid influx of storm water during rain events and their entrained detritus. Ecological communities at the interface appear to be based on microbial organisms that have evolutionally adapted to this restricted environment, specifically, bacteria that are utilizing the chemical gradient along the saline water–freshwater interface as a source of nutrients (Rye et al., 1981; Hutchins et al., 2016; Hutchins, 2017; Sharp and Smith, this volume).

The EAA initiated a synoptic water-level program to collect water levels from wells across the entire region of the San Antonio segment (EAA, 2012a). Water-level elevations recorded from wells with precise land-surface elevations were used to prepare a series of synoptic water-level maps. The data supported the development of groundwater-flow models for the Edwards Aquifer (Green et al., this volume, Chapter 3). Water levels were collected: (1) before initiation of irrigation, in January/February; (2) postirrigation in late July; and (3) in October/November, when limited agricultural pumping is experienced. Water levels were generally collected within a 1 wk time period.

The synoptic water-level program results indicated that regional groundwater flow in the artesian zone moves from west to east across Uvalde, Medina, and Bexar Counties (Fig. 2). In central and eastern Bexar County, groundwater flows in a northeasterly direction through Comal and Hays Counties to discharge at Comal and San Marcos Springs. (EAA, 2012a). The groundwater divide for the western portion of the San Antonio segment is in Kinney County, generally to the east of Brackettville (Green et al., 2006; Green et al., this volume, Chapter 6).

Groundwater outflow from south-central Uvalde County includes discharge to the Leona Formation (gravel) and also an eastern component of flow to Medina County (Green et al., this volume, Chapter 5). The Leona Formation was deposited during the Quaternary and most likely buried active springs discharging the Edwards Aquifer. Aquifer discharge was insufficient to keep the spring orifice(s) open, but the gravel has sufficient permeability to allow discharge from the aquifer. Some of this water in the Leona Formation discharges from Leona Springs, but most of the water exits as subsurface flow in the gravel and is lost to the Edwards Aquifer (Green et al., 2004, 2012).

In eastern Uvalde County, series of large igneous intrusions occur in the artesian zone near the community of Knippa (Clark, 2003a; Adkins, 2013). This and related tectonic features have resulted in groundwater from Uvalde County flowing northeast around the intrusions, through a constriction called the Knippa Gap, into the unconfined Edwards Group limestone and then into the San Antonio Pool, and then southeast back into the artesian zone (Maclay, 1995; Green et al., this volume, Chapter 5).

The Edwards Aquifer (Green et al., 2004) also appears to be recharging the Leona Formation in Medina County where the Haby Crossing fault forces groundwater upward from the Edwards Group and into the high-permeability gravels (Green et al., this volume, Chapter 4). This water is derived from recharge occurring in the area of Diversion Lake on the Medina River.

Flow paths around Comal Springs are complex and appear to be controlled by faulting, allowing compartmentalization of flow along fault blocks between the artesian zone and recharge zone (Green et al., this volume, Chapter 4). Discharge from both zones occurs at Comal and San Marcos Springs. Comal Springs is located on the Comal Springs fault at the contact between the recharge and artesian zones, separated by the Comal Springs fault. This fault has as much as 275 m of displacement and is thought to integrate mixing of deep and shallow waters from the aquifer. Water from the artesian fault block around Comal Springs rises upward from a depth of ~200 m to discharge into Landa Lake, forming the headwaters of the Comal River. The groundwater gradient between the City of San Antonio and Comal Springs is ~0.5 m/km. However, the head loss in the distal 500 m of flow in the artesian zone at Comal can be as much as 7 m, which suggests a gradient of 14 m/km. This is interpreted to be a result of the filling of spring conduits from bed load and sediment from the high-flow-gradient Blieders and Panther Creeks, which lie upstream and flow into Landa Lake. Groundwater from the recharge zone (Comal Springs fault block) also discharges at Comal Springs (Johnson et al., 2012). Potentiometric surface maps around Comal Springs indicate that groundwater flow as little as 2 km to the northwest of the spring mixes with deep groundwater flow to discharge at San Marcos Springs (Johnson et al., 2012). Average flow at Comal Springs is estimated to be 8 m³/s (288 cfs; EAA, 2018).

Flow in the area around San Marcos Springs also originates from both the artesian and recharge zones, with flow emerging from the Comal Springs fault block at a depth of more than 100 m. Water from the recharge zone within the Hueco Springs fault block also discharges from San Marcos Springs (Green et al., this volume, Chapter 4).

Evaporites and limestones of the Edwards Group have been highly karstified, resulting in a triple-permeability aquifer containing matrix, fracture, and conduit permeability (Halihan et al., 2000; Hovorka et al., 1995, 2004). Karstification occurred soon after deposition when the rocks were first subaerially exposed during depositional highstands. After subsidence and burial, paleokarst features may have been inception horizons for later karstification, which may have occurred prior to downfaulting and formation of the artesian zone. Early researchers (Maclay and Small, 1984) proposed epigenetic (near-surface) karst processes for formation of the Edwards Aquifer, driven by circulating meteoric waters along structural and paleokarst features. Epigenetic karst theory assumes karst features are produced from shallow downward or near-horizontal groundwater movement with aggressive groundwater derived from meteoric processes. However, Klimchouk (2007) concluded that rising waters (hypogenetic) from depth are also important agents of karst development. Worthington (2001) has shown that deep circulation systems that are warmed by the geothermal gradient have decreased viscosity and are more efficient in forming regional flow systems. Hypogene processes in the Edwards Aquifer were little known, nor understood, until recently.

The current conceptualization of regional karst flow systems in the Edwards Aquifer involves a combination of different processes. Epigenic processes are occurring in the contributing, recharge, and shallow artesian zone and have helped create the current topography. The contributing and recharge zones are characterized by small shallow sinkholes, losing streams, and caves. Hypogenic processes, which involve upward flow in the artesian zone, occur along the artesian flow path and are best exhibited at Comal and San Marcos Springs. Dissolution in the deep artesian portion of the aquifer is driven by biogenic acids (H₂S) and mixing corrosion between different water chemistries, as defined by Bögli (1964).

The morphology of numerous caves in the recharge zone indicates that hypogene processes were important in their formation and most likely occurred during or before the upper confining units were removed by erosion. Many caves in the recharge zone have a ramiform pattern, cupolas in their ceilings and walls, entrances not related to the surface topography, and other features that are indicative of formation by ascending water. Most of these caves are now relicts of the former hypogenic flow system and are now being overprinted by current epigenic processes.

Some of the best examples of hypogene processes are preserved in four caves located in northeastern Uvalde County, north of the town of Knippa. Before groundwater from the Uvalde Pool was captured by the San Antonio Pool, groundwater from the Uvalde Pool appears to have discharged from a series of large paleosprings (caves). These caves (Frio Bat Cave, Big Easy Cave, Dripstone Cave, and Frio Queen Cave) contain strong evidence of upward flow in the form of cupolas in the walls and ceilings and large wide ascending passages. All four entrances are located on topographic highs and do not appear to be related to the surface topography (Fig. 3). Recent discoveries in one of the caves found that water-filled conduits are still present in the deepest areas of exploration. It is not clear whether these represent the subsurface Frio River, Dry Frio River, or groundwater from the Uvalde Pool. Surface processes are now overprinting the hypogene processes in these four caves, with minor dissolution of the limestone surface around the entrances and deposition of speleothems in the cave. As the Uvalde Pool was integrated into the San Antonio Pool, water levels in the aquifer declined, and the Knippa springs became paleospring caves.

The evolution of the Edwards Aquifer was initiated by downcutting of the Nueces River system, exhuming the structurally higher Edwards Group limestones in the western basins (Woodruff and Abbott, 1979). The dissolution process increased as flow of aggressive surface water flushed saturated water from the aquifer. The discharge of water from the initial development of the Edwards Aquifer beneath Uvalde County probably occurred at the paleosprings located north of the town of Knippa, as noted above, and possibly at Leona Springs. As the Edwards Group limestones were exposed at lower elevations to the east, hydraulic head forced the integration of the Uvalde Pool with the San Antonio Pool. This resulted in a decline in water levels and the abandonment of the Knippa spring caves, leaving them as high relics of a former aquifer flow system.

The sequence and timing of the opening of the San Antonio and San Pedro Springs (Bexar County), Comal Springs (Comal County), and San Marcos Springs (Hays County) are difficult to determine but most likely occurred in the Neogene (Woodruff and Abbott, 1979). Faults in central Bexar County allowed the formation of San Antonio Springs and San Pedro Springs. Water rose through the faults to create the springs within the Austin Group limestones. The elevation of these springs ranges from 199 to 202 m above mean sea level (amsl). Both San Pedro and San Antonio Springs are ephemeral and will commonly stop discharging during prolonged droughts when aquifer levels drop.

Comal Springs, located at an elevation of 189 m amsl, is the largest of the spring complexes draining the Edwards Aquifer and forms the headwaters of the Comal River. Karst dissolution processes, driven by hydraulic head, created a positive feedback loop and resulted in the increase in discharge at Comal Springs over time. The increased efficiency of the flow system at Comal Springs resulted in a lowering of the regional potentiometric surface and diminished flow at San Antonio and San Pedro Springs.

San Marcos Springs, located at an elevation of 174 m amsl, is the second largest spring complex draining the Edwards Aquifer. San Marcos was the last spring to form in the Edwards Aquifer. Currently, San Marcos Springs is an underflow spring to Comal Springs. However, the regional hydraulic head and positive feedback associated with karst dissolution may result in San Marcos Springs pirating flow from Comal Springs as its discharge, becoming more efficient over geologic time. This may result in Comal Springs becoming an overflow spring to San Marcos Springs, which would become the dominant spring in the Edwards Aquifer.

Both Comal and San Marcos Springs have significant upward groundwater gradients with water discharging along major faults and are examples of hypogene processes at the discharge end of large regional flow paths.

The ultimate fate of the Edwards Aquifer may be that Barton Springs, located at an elevation of 130 m amsl near the Colorado River in south Austin, may become the ultimate discharge point for the San Antonio segment of the Edwards Aquifer. Supporting this hypothesis is the observation that during droughts, the groundwater divide between the San Antonio segment and the Barton Springs segment of the aquifer disappears, and water sinking in the bed of the Blanco River discharges at Barton Springs, indicating that this process may have already begun.

The Edwards Aquifer is one of the largest and most prolific aquifer systems in the United States. For the San Antonio segment of the Edwards Aquifer, surface water from the contributing zone directly recharges the aquifer through interformational flow or as surface water when it crosses onto the recharge zone. Flow in the artesian zone is from west to east and emerges in a series of large springs located in the urban areas of San Antonio, New Braunfels, and San Marcos. Flow paths in the San Antonio segment exceed 200 km with groundwater velocities in the recharge zone typically exceeding 2 km/d. The highly developed tertiary permeability (matrix, fractures/faults, and caverns and conduits) from karst processes results in very high well yields, a low hydraulic gradient, rapid response to recharge and drought conditions, and an extensive aquifer fauna with more than 60 obligate species.

Both epigenic and hypogenic processes occur in the aquifer and have resulted in the high tertiary permeabilities. The faults associated with the Balcones fault zone play an important role in controlling groundwater movement as well as interformational flow.

Evolution of the aquifer was initiated in the western portion of the aquifer. Lowering of the land surface in the east resulted first in formation of springs at San Antonio and San Pedro and then at Comal and San Marcos as dissolution processes created a positive feedback loop and improved spring efficiency. The ultimate fate of the Edwards Aquifer discharge is that Barton Springs will become the primary spring discharge for the Edwards Aquifer over geologic time.