ABSTRACT The Edwards (Balcones Fault Zone) Aquifer is structurally controlled by the system of normal faults following the Balcones Escarpment, with major domains, including contributing, recharge (unconfined), and artesian (confined) zones, dictated by the large-displacement (50 m to >250 m throw) normal faults and depth of erosion. Faults and extension fractures, in many cases enhanced by dissolution, localize recharge and flow within the Balcones fault zone and into the subsurface of the artesian zone. Juxtaposition of the Edwards with other aquifers provides avenues for interaquifer communication, while juxtaposition against impermeable layers and concentration of clay and mineralization along faults locally produce fault seals for compartmentalization and confinement. Fault block deformation, including small faults and extension fractures, leads to aquifer permeability anisotropy. Faults also locally provide natural pathways for groundwater discharge through springs above the confined (artesian) zone. Although the importance of joints and faults in the Edwards (Balcones Fault Zone) Aquifer system is recognized, there has not been a systematic analysis of the meter-scale structures in the Edwards and associated confining units and their influence on groundwater flow. Here, we review evidence from several key areas showing that an analysis of faults and fractures in the Edwards (Balcones Fault Zone) Aquifer and associated aquifers and confining units is needed to characterize structural fabrics and assess the permeability architecture critical for the next generation of groundwater modeling of the aquifer.

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 Tracer testing is established as one of the best techniques for determining groundwater velocities and identifying groundwater flow directions in karstic systems. It has been employed in the Edwards (Balcones Fault Zone) Aquifer since the mid-1980s. Nontoxic, fluorescent organic dyes are most commonly used because they are comparatively inexpensive, relatively easily accessible, detectable at low concentrations, and not harmful to organisms that use or dwell in the aquifer or its springs. Tracer tests provide empirical evidence that is difficult to obtain any other way. Tracer tests have shown rapid groundwater velocities in the contributing, recharge, and artesian zones. Groundwater velocities were found to range from 915 to 9150 m/d in the Barton Springs segment of the aquifer; 1–3600 m/d in the San Marcos Springs area; 300–640 m/d near Comal Springs; 13 to >5300 m/d in San Antonio/northern Bexar County; and 1–1367 m/d in Kinney County, Texas. Tracer testing has shown: (1) preferential flow paths are conduit-dominated; (2) in places, there is a hydraulic connection with the underlying Glen Rose Formation; (3) large offsets on faults are not barriers to flow; and (4) portions of the aquifer act as separate pools.