ABSTRACT The Edwards aquifers are typically faulted, karstified, and transmissive. Water quality is generally excellent; the hydrochemical facies is mostly a calcium bicarbonate water with total dissolved solids (TDS) <500–1000 mg/L. Exceptions to this result from both natural and anthropogenic factors. In the Edwards Plateau, mixing of the formation water with underlying water from the Trinity aquifers or Permian rocks increases salinity to the west. Along the Balcones fault zone, the southern and eastern borders of the Edwards (Balcones Fault Zone) Aquifer are demarcated by a bad-water line where salinity rises to over 1000 mg/L. Detailed studies show that this line is a band, because salinities in the aquifer are not uniform with depth. The bad-water (or saline-water) zone is relatively stable over time, and six hydrochemical facies were identified, which are created by different combinations of dissolution of evaporite and other minerals, mixing with basinal brines, dedolomitization, and cross-formational flow from underlying formations. Flow in this zone is restricted, the waters are reducing, and recent studies suggest that microbes play important chemical and physical roles. The bad-water zone has sufficient water in storage and sufficient permeability so that desalination could be a future water-source option.

ABSTRACT The western boundary of the San Antonio segment of the Edwards (Balcones Fault Zone) Aquifer has been historically mapped to extend to a groundwater divide thought to be near Brackettville in Kinney County, Texas. A revised conceptualization is developed here that contends the Edwards Aquifer forms a separate pool in Kinney County, referred to as the Kinney Pool, which extends from a groundwater divide located between Mud Spring and Pinto Spring on the west to an effective structural, hydraulic barrier near the Kinney-Uvalde County line. The barrier is a result of dewatering of the permeable portion of the Edwards Aquifer in eastern Kinney County. No groundwater flow in the Edwards Aquifer from Kinney County to Uvalde County is expected during periods of low to average groundwater elevation, but limited flow from west to east could be possible during periods when groundwater elevations are high. Natural discharge from the Kinney Pool occurs as spring discharge and underflow through floodplains at the southern (downdip) boundary of the segment.

ABSTRACT The Edwards (Balcones Fault Zone) Aquifer in central Texas is typically defined as having three segments: the San Antonio, the Barton Springs, and the Northern segment, which are separated by groundwater divides or points of discharge. The San Antonio segment of the Edwards Aquifer is defined as extending from east of Brackettville in the west to Hays County in the east. The San Antonio segment has been further delineated into two pools, the San Antonio Pool and the Uvalde Pool, for water management purposes. The San Antonio Pool is the larger of the two pools and is recharged by the Dry Frio, Frio, Sabinal, Medina, Cibolo, Guadalupe, and Blanco River watersheds, in addition to direct recharge and flow from the Uvalde Pool via the Knippa Gap. To a lesser extent, interformational flow between units stratigraphically above and below the Edwards Formation limestone also occurs. The most prominent points of discharge from the San Antonio Pool are Comal, San Marcos, and Hueco Springs. San Pedro and San Antonio Springs in Bexar County discharge during periods of high stage in the aquifer. There are limited numbers of additional springs in the Frio River watershed with limited discharge. Significant water is discharged from the Medina Lake and Diversion Lake (downstream from Medina Lake dam) system via conduits and surface flow to recharge paleo-alluvial deposits (Leona Gravel) in the Medina River floodplain. This discharge had previously been assumed to recharge the Edwards Aquifer, but it continues downgradient in the Leona Gravel and is lost to the aquifer.

ABSTRACT The Uvalde Pool comprises the western portion of the San Antonio segment of the Edwards (Balcones Fault Zone) Aquifer. Assessment of available data on the hydrogeology of Uvalde County confirms the conceptualization that the Edwards Aquifer in Uvalde County to the west of the city of Knippa acts as a partially separate groundwater basin. This portion of the Edwards Aquifer is referred to as the Uvalde Pool. The Edwards Aquifer to the east of the Uvalde Pool is referred to as the San Antonio Pool. A constriction in groundwater flow between the two pools, referred to as the Knippa Gap, and marked differences in groundwater elevations on either side of the Knippa Gap are the motivation to treat the Uvalde and San Antonio Pools as separate hydrogeological features. The Uvalde Pool is unique because it is the only place where the Edwards Aquifer is in hydraulic communication with the overlying and younger Buda Limestone and the Austin Chalk Aquifers. Given the karstic nature of the Edwards Aquifer in the Uvalde Pool and its relatively limited spatial extent, the Uvalde Pool is characterized as a highly transmissive aquifer, but with relatively limited storage capacity.

ABSTRACT The San Antonio segment of the Edwards (Balcones Fault Zone) Aquifer of south-central Texas is one of the most important and prolific karst aquifers in the United States. Extending from Kinney County (west) to Hays County (northeast), it is the primary source of water for the municipal and agricultural communities surrounding the greater San Antonio area. Deposited in Early Cretaceous time, rocks of the Edwards Group 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 pathways 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 numerous processes, including infiltration of chemically aggressive surface water, hydraulic head, mixing corrosion, and biogenic acids. Well 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.