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 groundwater flow system composed of the Edwards-Trinity (Plateau) Aquifer and the Hill Country portion of the Trinity Aquifer together occupy an area of ~100,000 km 2 of west-central Texas. In addition to the common groundwater flow system, these aquifers also share a common, contiguous hydrostratigraphy—the Trinity Group hydrostratigraphic unit. The aquifers provide the primary source of water for the Edwards Plateau and Texas Hill Country and also sustain numerous springs and streams in the region. The sensitivity of the aquifers to drought and well discharge has raised concerns over the availability of water from these aquifers. Groundwater discharge takes the form of (1) discharge to streams and springs; (2) evapotranspiration; (3) pumpage from wells; and (4) cross-formational flow across the Balcones fault zone boundary to the Edwards (Balcones Fault Zone) Aquifer and underlying parts of the Trinity Aquifer. Groundwater inflow to these aquifers occurs by diffuse and discrete infiltration through the aquifer outcrops. Due to regional variability of lithologic compositions, hydraulic conductivity and storativity vary both vertically and laterally throughout the aquifer, with hydraulic conductivity decreasing with depth and from north to south.

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 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 The Edwards Aquifer supports an important ecosystem with rarely seen faunas that have unique adaptations to a dark and thermally stable environment. We tallied over 60 species of aquifer-adapted (stygobitic) species in the Edwards Aquifer, and 30 more in other Texas aquifers, including snails, flatworms, worms, crustaceans, mites, and beetles. Exploration and research continue, with nine new species described in the last two years. Vertebrate species include Eurycea salamanders and ictalurid catfish, including a blind species ( Prietella phreatophila ) recorded for the first time in the United States from the Edwards–Trinity Plateau Aquifer in 2016. Contributing to the stygobite diversity are ten state or federally listed species, including the Texas blind salamander ( Eurycea rathbuni ), which was one of the first species to be listed as endangered by the U.S. Fish and Wildlife Service (USFWS) in 1970. Major springs of the Edwards (Balcones fault zone), Edwards–Trinity Plateau, Trinity, and other aquifers are under constant threat of drying due to aquifer overdraft and climate change. These springs provide habitat for 26 state or federally listed spring-adapted species. Aquifer species in general are known to provide ecosystem services, including water purification, nutrient cycling, and biological indication; however, the function and biology of these species in central Texas have not been studied. Considering the Edwards Aquifer ranks among the top aquifers in the world for number of species, the gaps in understanding remain enormous.

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, groundwater and surface water are managed differently. Surface water is state-owned and governed by the doctrine of prior appropriation. Groundwater law is evolving and based upon a combination of court cases and legislative actions. Like soil, groundwater is property of the surface landowner. The 1904 East case confirmed the absolute ownership of water or the “rule of capture” for beneficial use, limited only in that malicious use or willful waste and, later, subsidence of adjoining properties were not permitted. The Texas Supreme Court later indicated that rule of capture might not be appropriate doctrine, but it declined to replace it with an alternative management doctrine. In the 1990s, there were two major legal changes to the existing legal regime. First, groundwater conservation districts were designated as the preferred method for groundwater management, further emphasizing local control. Second, protection of endangered species in Edwards Aquifer springs necessitated pumping restrictions. More recently, the Day case determined that landowners had a vested property right in water below their land prior to its capture, and the Bragg case determined that, in the facts of that case, the restricting of pumping of groundwater was a taking requiring compensation. Legal constraints will be an important factor in determining the future development of the Edwards Aquifer and associated water resources.

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 Edwards Aquifer Authority (EAA) was established in 1996 after lawsuits that were filed for the protection of endangered species in Comal and San Marcos Springs revealed the need for aquifer management. Several events set the stage for water management in Texas and the creation of the EAA. The EAA succeeded the Edwards Underground Water District, which was formed after the 1951–1956 drought of record. The EAA was eventually granted regulatory responsibilities, including issuing permits for wells, setting limits on groundwater pumpage, and development of a habitat conservation plan to protect endangered species.

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.