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 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 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 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 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 Devils River in south-central Texas is recognized as one of the remaining pristine rivers in the state. Adding to its importance, the Devils River is a key tributary to the Rio Grande, providing essential freshwater flows to south Texas and the Rio Grande Valley. An efficient conveyance system for groundwater is shown to have formed in the karst carbonate watershed, located in a semiarid environment with modest distributed recharge, oftentimes less than 1–2 cm/yr. This conveyance system comprises preferential flow pathways that developed coincident with river channels. A strong correlation between wells with high well yield and proximity to higher-order river channels (i.e., within 2.5 km) was used as evidence of the presence of preferential flow pathways. An important observation is that groundwater flow in the Devils River watershed appears to be controlled by the morphology of the area more than the bulk hydraulic properties of the rocks. Flow measurements in the Devils River measured under relatively high- and low-flow conditions support the hypothesis that the river is gaining in downstream reaches. This characteristic leads to perennial river flow being restricted to only the lower reach of the river. Last, essentially all of the recharge to Amistad Reservoir that is derived from the Devils River watershed is contributed as surface flow from the river, and there is minimal underflow or cross-formational flow from the watershed at the point where the watershed abuts Amistad Reservoir.