ABSTRACT Multiport monitor wells have been used by the Barton Springs/Edwards Aquifer Conservation District (BSEACD) to study complex, multilayer, and stacked aquifers in central Texas. Much of the data from water wells that are used for hydrogeological studies are of limited use owing to the thickness of the aquifers, vertical variation in hydraulic properties, and the often-uncertain completion of the wells. To address these concerns, hydrogeologists and engineers have employed various methods, such as installation of nested wells, multilevel completions in a single borehole, and multiport wells. The BSEACD has used multiport wells to determine vertical variations in an aquifer and the hydraulic relationships between stacked aquifers. With multiport wells, properties such as hydraulic head, temperature, hydraulic conductivity, and water quality of discrete units within an aquifer can be determined. The use of multiport wells has shown how portions of the Upper Trinity lithologic units are hydraulically connected to the overlying Edwards lithologic units, and how the Edwards Aquifer is hydraulically isolated from the Middle and Lower Trinity Aquifers.

ABSTRACT The Edwards Aquifer Habitat Conservation Plan (EAHCP) protects the federally listed species in the Comal and San Marcos Springs. The plan was developed through the Edwards Aquifer Recovery Implementation Program, a consensus-based process involving a diverse body of stakeholders including industries, agricultural users, municipalities, water purveyors, river authorities, environmental organizations, four state agencies, and downstream interests. Since 2013, the partners of the EAHCP have implemented a multifaceted program mainly focused on spring-flow protection and habitat improvements to ensure the persistence of the covered species and to create more certainty in the region’s water supplies.

ABSTRACT Karst aquifers have hydrogeologic characteristics that render them uniquely vulnerable to contamination from emerging contaminants (ECs). ECs comprise numerous chemical groups, including pharmaceuticals, personal-care products, flame retardants, perfluorinated and polyfluorinated compounds, nanoparticles, and microplastics. Many ECs have sources, transport pathways, and chemical characteristics that facilitate their infiltration into groundwater, either indirectly from surface water or directly from sources such as landfill leachate and septic systems. What little is known about the occurrence, fate, and transport of ECs in the Edwards (Balcones Fault Zone) Aquifer indicates that the aquifer might be increasingly vulnerable to this type of contamination. The natural physical characteristics of this karst aquifer and anthropogenic sources of ECs associated with increased urbanization in central Texas contribute to this vulnerability. In this chapter, we review groups of ECs and their sources, occurrence of ECs in groundwater and karst, and current knowledge about the occurrence of ECs in the Edwards Aquifer. We conclude by discussing specific factors, such as rapid flow and contaminant sources, that contribute to the vulnerability of the Edwards Aquifer to contamination by ECs.

ABSTRACT Aquifer storage and recovery (ASR) is a proven water-supply strategy that uses an aquifer to store surplus water that will be available for later use when that stored water is needed. Although only three ASR systems are currently operating in Texas, recent incentives from the state, along with changes in regulatory framework, have helped to encourage consideration of ASR as a viable water-supply strategy. The changes in Texas law primarily reduced the power of groundwater conservation districts to regulate ASR, and they put the majority of the role of project authorization in the hands of the Texas Commission on Environmental Quality. Two Edwards-named aquifers in Texas were considered in this work: the Edwards-Trinity (Plateau) and the Edwards (Balcones Fault Zone) Aquifers. Both of the aquifers have areas that appear to be suitable, from a hydrogeologic standpoint, for ASR. The Edwards hydrostratigraphic unit of the Edwards-Trinity (Plateau) Aquifer has generally good productivity and water quality. However, in some locations, the high natural gradient combined with low porosity may increase the design challenge due to bubble drift. These same characteristics exist in many areas of the freshwater portion of the Edwards (Balcones Fault Zones) Aquifer, although its high productivity makes for very attractive per-well recharge and recover rates. The lower natural gradient (and thus smaller potential for bubble drift) in the brackish portion of the Edwards Aquifer may make it a good candidate in areas where productivity is sufficient.

ABSTRACT The Barton Springs segment of the Edwards (Balcones Fault Zone) Aquifer is a prolific karst aquifer system containing the fourth largest spring in Texas, Barton Springs. The Barton Springs segment of the Edwards Aquifer supplies drinking water for ~60,000 people, provides habitat for federally listed endangered salamanders, and sustains the iconic recreational Barton Springs pool. The aquifer is composed of Lower Cretaceous carbonate strata with porosity and permeability controlled by depositional facies, diagenesis, structure, and karstification creating a triple permeability system (matrix, fractures, and conduits). Groundwater flow is rapid within an integrated network of conduits discharging at the springs. Upgradient watersheds provide runoff to the recharge zone, and the majority of recharge occurs in the streams crossing the recharge zone. The remainder is direct recharge from precipitation and other minor sources (inflows from Trinity Group aquifers, the San Antonio segment, the bad-water zone, and anthropogenic sources). The long-term estimated mean water budget is 68 ft 3 /s (1.93 m 3 /s). The Barton Springs/Edwards Aquifer Conservation District developed rules to preserve groundwater supplies and maximize spring flow rates by preserving at least 6.5 ft 3 /s (0.18 m 3 /s) of spring flow during extreme drought. A paradox of the Barton Springs segment of the Edwards Aquifer is that rapid recharge allows the Barton Springs segment of the aquifer to be sustainable long term, but the aquifer is vulnerable and limited in droughts. The karstic nature of the aquifer makes the Barton Springs segment vulnerable to a variety of natural and anthropogenic contaminants. Future challenges will include maintaining the sustainability of the aquifer, considering climate change, population growth, and related land-use changes.

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 Numerical models have been an integral component in management of the Edwards Aquifer for over four decades. The scale and complexity of the models have varied considerably during this time, with the changes attributed to improvements in both numerical software and the conceptual models on which the models are predicated. Resolution of early models was coarse, which rendered them useful only in large-scale, groundwater-resource assessments. Increased resolution and improved refinement in the conceptualization of the complex hydrostratigraphic framework of the Edwards Aquifer have led to expanded applicability of the ensuing models in terms of accommodating local-scale hydraulic features such as pumping scenarios and changes in recharge/discharge mechanisms (i.e., urbanization and climate change). As part of these improvements and advancements, regional-scale groundwater availability models augmented by local-scale models based upon improved conceptualization provide the ability to: (1) replicate more extreme conditions, such as the drought-of-record; (2) improve boundary conditions by extending models to include natural hydraulic boundaries; and (3) couple groundwater flow models with surface-water flow models so that the entire terrestrial water cycle can be accommodated during water-resource management scenario assessment.

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.

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.