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 exceptional groundwater community inhabiting the karstic Edwards-Trinity Aquifer system in central Texas has inspired generations of biologists seeking to understand diversification in an extreme environment. Since the late 1990s, molecular genetic tools have increasingly been used to uncover hidden diversity and infer the evolutionary history of groundwater species inhabiting the Edwards-Trinity system. The field of phylogeography—the study of the spatial distribution of genealogical lineages within and among intraspecific populations and closely related species—has provided unparalleled insight into patterns of Edwards-Trinity groundwater biodiversity. Similar to other global groundwater biodiversity hotspots, phylogeographic studies in the Edwards-Trinity Aquifer system have documented exceptionally high levels of endemism and strong population structure due to isolation across naturally fragmented habitat. Cryptic species (two or more morphologically similar but genetically distinct species) have been discovered in a number of phylogeographic investigations, including Eurycea salamanders, Dionda minnows, and Stygobromus amphipods. A number of these species are threatened or endangered with extinction due to habitat loss and degradation resulting from urbanization. Accurately delimiting species boundaries has had significant implications for biodiversity and groundwater conservation in the Edwards-Trinity region because the Endangered Species Act has been used to regulate unrestricted groundwater withdrawal in the eastern Edwards Aquifer where listed species are found. New developments in deoxyribonucleic acid (DNA) sequencing technology coupled with advancements in model-based inference will provide powerful tools for furthering our understanding of Edwards-Trinity biodiversity and predicting its response to a rapidly changing environment.

ABSTRACT The Washita Prairie segment of the Edwards (Balcones Fault Zone) Aquifer is a shallow unconfined aquifer that supports several historical springs, perennial streamflow to Lake Waco, and water for rural households and livestock. Secondary porosity in the aquifer is from neotectonic fractures and epikarst in the Georgetown and Edwards Formations. The fractures produce an “effective” porosity of ~1%. Thin soils allow rapid recharge, as indicated by water-level responses in wells within 24 h of rainfall events. Discharge is generally along second-order streams; topography is the dominant influence on groundwater flow direction. The interbedded clays in the Georgetown Formation create a preferred horizontal to vertical anisotropy. The fractured nature of the aquifer produces local heterogeneity, but regionally, the aquifer acts as a diffuse rather than conduit flow system. Weathering results in a layered flow system with greater effective porosity and permeability in an upper zone compared to the deeper zone. Washita Prairie springs are perennial, with discharges generally <0.05 m 3 /s. The groundwater is calcium bicarbonate facies with total dissolved solids (TDS) <500 mg/L in most springs and shallow-zone wells. Water quality in deeper wells is more variable, as these encounter the deeper flow system with slower circulation and higher TDS. The shallow water table and rapid recharge through fractures allow surface activities to impact water quality, and nitrate levels appear to be elevated above average background values in places. The Washita Prairie segment of the Edwards (Balcones Fault Zone) Aquifer may be able to supply over 50,000,000 m 3 of sustainable water on an annual basis with continued study and proper management.

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 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 In Texas, the investigation and implementation of desalination began in the 1960s. The earliest operating desalination plants in Texas were in Port Mansfield (south of Corpus Christi) in 1965 and Dell City (far West Texas) in 1968. Since 1999, the number and capacity of desalination plants operating in Texas have steadily increased. In 2016, there were 49 municipal desalination plants in the state, and the total municipal desalination capacity was ~142 million gallons per day (537 million liters per day). The predominant desalination technology used today in municipal desalination plants is reverse osmosis, a membrane filtration process in which dissolved solids (salts) are removed from saline water by applying pressure and forcing the water through a semipermeable membrane. Three desalination plants are currently in operation within the Edwards-Trinity (Plateau) Aquifer boundaries, and additional desalination of brackish groundwater from the Edwards-Trinity (Plateau) and Edwards (Balcones Fault Zone) Aquifers can alleviate stress on water resources from projected population growth and lessen potential water scarcity in central Texas.

ABSTRACT The northern segment of the Edwards (Balcones Fault Zone) Aquifer is an important source of water for municipalities, industry, and landowners in central Texas. Rapid population growth in this part of Texas has increased interest in the north segment of the aquifer and heightened concerns about groundwater availability. The aquifer consists of Cretaceous limestone stratigraphic units that crop out along its western margin and dip toward the east. Groundwater primarily flows from the aquifer outcrop recharge zones toward discharge zones along perennial rivers and streams in the outcrop area and to a lesser extent toward deeper parts of the aquifer, eventually discharging by cross-formational flow to overlying stratigraphic units, such as the Del Rio Clay, Buda Limestone, and Austin Chalk. Groundwater isotope compositions in the aquifer indicate that groundwater flow is most active in the unconfined parts of the aquifer and that most recharge occurs during late fall and winter months, even though highest monthly precipitation occurs during the spring. Pumping from the northern segment of the Edwards (Balcones Fault Zone) Aquifer is ~6.8 × 10 7 L/d, having peaked at ~1.0 × 10 8 L/d in 2004, but still up from ~3.4 × 10 7 L/d in the 1980s. Most of this pumping (~90%) is for municipal uses. However, in the rural northern and heavily urbanized southern parts of the aquifer, domestic and manufacturing uses, respectively, account for a significant portion of total pumping.

ABSTRACT The Edwards Aquifer along the Balcones fault zone is in a rapidly growing, urbanizing area. Urbanization creates major hydrogeological impacts, generally increasing impervious cover and flooding intensity, water demands, groundwater recharge, and temperatures both above and below the land surface; covering springs and small streams; altering the porosity and permeability fields; and contaminating groundwater, surface water, and soils. Urbanization also alters topography, natural flora, and the local climate. Several of these effects have either been documented or predicted for the Edwards Aquifer. Groundwater recharge from leaky utility systems and irrigation return flow is significant, particularly during times of low rainfall. The hydraulic properties of the epikarst, particularly the permeability field, can be highly modified. Aquifer water quality remains excellent, but increased anthropogenic chemical nitrate and chloride concentrations, and occasional bacteriological contamination have been observed. The eventual effects of these changes on the aquifers’ unique ecosystems is not known. Urbanization and urban sprawl are projected to increase, which will continue to alter the Edwards Aquifer system physically, chemically, and biologically. Understanding of these changes, their causes, and their effects is necessary to addressing the critical and growing environmental and water-resources issues of urban areas in the coming century.

ABSTRACT Geophysical methods have been an important component of effective hydrogeologic investigations over the Edwards and Trinity Aquifers in central Texas. Various electrical and electromagnetic methods have been used to map stratigraphy and geologic structure and to locate buried karst features. Geophysical methods can also characterize faults and fractures in the Balcones fault zone. Six case studies across three segments (San Antonio, Barton Springs, and Northern segments) of the Edwards Aquifer show that the locations of buried caves and sinkholes, on all three segments, are best defined using a combination of two- and three-dimensional resistivity imaging and natural potential (self-potential) methods. Localization and characterization of the Haby Crossing and Mount Bonnell faults, which are known to be the most significant faults in the Balcones fault zone, are best accomplished by integrating multiple geophysical methods (e.g., electrical resistivity, natural potential, magnetic, ground-penetrating radar, conductivity, and seismic refraction tomography). It is noted, however, that other karstic regions could respond differently to different geophysical methods and require different primary geophysical methods.

ABSTRACT Both people and the environment require water. The environment in Texas depends upon water discharged from the Edwards Aquifers, including ~1.5 million megaliters per year (ML/yr) from the Edwards-Trinity (Plateau) Aquifer and ~1 million ML/yr from the Edwards (Balcones Fault Zone) Aquifer. The first people in the area used the aquifer springs as drinking water and subsequently irrigated with the springs’ flow and then began drilling and pumping wells. Well production in the Edwards (Balcones Fault Zone) Aquifer was ~120,000 ML/yr in the 1930s and steadily increased over the next three decades to ~500,000 ML/yr, which is the average use from 1970 to 2015. Production from the Edwards-Trinity (Plateau) Aquifer was ~250,000 ML/yr from 1984 through 2016, while production from the Edwards-Trinity (High Plains) Aquifer was ~25,000 ML/yr from 1984 through 2016. The Interstate 35 growth corridor, extending from Bexar County (San Antonio) through New Braunfels, San Marcos, and Austin, Texas, and up to Bell County, is expected to grow from 4.6 million people in 2020 to 8.7 million in 2070. Despite the needs of this growing population, groundwater availability and regional water planning information suggests that pumping from the Edwards (Balcones Fault Zone) Aquifer over the next 50 yr will be limited. Groundwater availability numbers suggest that pumping in the Edwards-Trinity (Plateau) Aquifer could double from current levels, although planning information currently projects a more modest increase. Unsettled groundwater law and climate change could also affect future levels of pumping.

ABSTRACT The Edwards (Balcones Fault Zone) Aquifer is a high-yield aquifer that provides water for municipal, military, irrigation, domestic, and livestock uses in south-central Texas, and it discharges to several springs that support groundwater ecosystems. Natural water cycling in the Edwards (Balcones Fault Zone) Aquifer is driven by recharge, which depends on precipitation and runoff over the catchment area and recharge zone of the aquifer. This chapter analyzes the water fluxes in the Edwards (Balcones Fault Zone) Aquifer and how they vary with climatic variability and might vary with modern-age climatic change. This work also evaluates the safe yield of the Edwards (Balcones Fault Zone) Aquifer under historic climatic conditions, which is ~400 thousand acre · feet, or 493 × 10 6 m 3 , annually. These results have implications for aquifer groundwater extraction and human and environmental water requirements, such that future groundwater extraction must be adaptive to precipitation and recharge fluctuations to preserve groundwater ecosystems.

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 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.