Stygobiont Diversity in the San Marcos Artesian Well and Edwards Aquifer Groundwater Ecosystem, Texas, USA

The Edwards Aquifer and related Edwards-Trinity Aquifer of Central Texas, USA, is a global hotspot of stygobiont biodiversity. We summarize 125 years of biological investigation at the San Marcos Artesian Well (SMAW), the best studied and most biodiverse groundwater site (55 stygobiont taxa: 39 described and 16 undescribed) within the Edwards Aquifer Groundwater Ecosystem. Cluster analysis and redundancy analysis (RDA) incorporating temporally derived, distance-based Moran’s Eigenvector Mapping (dbMem) illustrate temporal dynamics in community composition in 85 high-frequency samples from the SMAW. Although hydraulic variability related to precipitation and discharge partially explained changes in community composition at the SMAW, a large amount of temporal autocorrelation between samples remains unexplained. We summarize potential mechanisms by which hydraulic changes can affect community structure in deep, phreatic karst aquifers. We also compile information on 12 other Edwards and Edwards-Trinity Aquifer sites with 10 or more documented stygobionts and used distance-based RDA to assess the relative influences of distance and site type on three measures of β-diversity. Distance between sites was the most important predictor of total dissimilarity and replacement, although site type was also important. Species richness difference was not predicted by either distance or site type.

Isotopic tracers of sources of water for springs from the Edwards Aquifer, Central Cexas, USA

The Edwards Aquifer (EA) in Central Texas provides water supply for over two million people and contains springs that are hydrologically and ecologically important to the region. The residence time of groundwater in the EA ranges from a few days to many thousands of years, since water in the aquifer is contained and transported within both matrix porosity and large conduits. In this study, stable isotopes of water from five springs are investigated for tracing the origin of water and hydrological processes in the EA system during 2017–2019. There is a quick response of the isotopic signals measured at these springs to changes in the isotopic compositions of precipitation. By utilizing an isotope mixing model, we have identified sources of water for these springs with a bi-modal distribution of groundwater supply in the EA: water supplied from deep groundwater with a longer residence time (an average of 67%) and supplemental epikarst interflow with a shorter residence time (an average of 33%). The evolution of hydrochemical water types from HCO3–Ca to HCO3·Cl–Ca·Mg along the EA flowpaths indicates that inputs from epikarst interflow are greater in springs within the artesian zone than the springs within the contributing zone.

Groundwater Recharge for Drought and Endangered Species Protection

San Antonio Water Systems (SAWS) developed its H2Oaks aquifer storage and recovery project in response to pumping restrictions set on its primary source of water supply, the Edwards Aquifer. The H2Oaks project pumps water from the Edwards Aquifer during wet years and transports it to the H2Oaks project site, where it is injected into the Carrizo-Wilcox aquifer for storage. Stored water is withdrawn to meet municipal demand when restrictions on Edwards Aquifer pumping are in place. Although created for the purpose of securing supplies for SAWS, the H2Oaks project became a centerpiece for regional water management. Storage is used during drought to mitigate impacts on pumping while ensuring minimum springflows needed to protect endangered species in the Edwards aquifer. Currently, the project stores over 176,000 acre-feet of water. This case study traces the development of the H2Oaks project from the passage of the Edwards Aquifer Act to the project’s current implementation. The H2Oaks project demonstrates the potential for groundwater recharge projects to store water as protection against drought conditions. It also demonstrates how storage by one entity can support water management needs across the broader community of water users.

Rayleigh wave and well head response to calculate porosity in the Edwards aquifer of south-central Texas, USA

We use the magnitude and centroid period of Rayleigh wave along with the amplitude of fluctuations of water level in a well to calculate effective porosity of a karst aquifer at the site scale. The radial and vertical displacements of Rayleigh wave are first related to the confining pressure of rock, which is then related to fluid pressure via the Gassmann equation. Three seismograms recorded at station 633A of the USARRAY and the induced responses of Well J-17 in the Edwards Aquifer (Texas) allow the calculation of an effective porosity between 17.0 and 24.4 percent, the average of which is close to the total porosity of core samples determined by geophysical well logs. This paper provides an innovative method to measure effective porosity in aquifers. Because of the long wavelengths of Rayleigh wave, the interdisciplinary approach is advantageous in that the resulting effective porosity is at the site scale which includes large conduits or voids.

Phenotypic Variation in Endangered Texas Salamanders: Application of Model-Based Clustering for Identifying Species and Hybrids

The endangered Barton Springs and Austin blind salamanders (Eurycea sosorum and E. waterlooensis, respectively) are micro-endemics to the Barton Springs segment of the Edwards Aquifer and its contributing zone in Central Texas. Although vertically segregated within the aquifer system, both species are known from the same spring outflows and occasionally hybridize. We used geometric morphometrics and model-based clustering applied to a large sample of standardized salamander photographs to evaluate the potential for objective phenotypic assignment to either species, as well as putative hybrids. In addition to characterizing variation in head shape, our analyses inferred sets of clusters corresponding to ontogenetic series in both species but did not infer any distinct hybrid clusters. Eurycea sosorum and E. waterlooensis have distinctive head size to trunk length allometries, which contributed to the effective clustering of species, even at small body sizes. We also observed subtle, but significant, microgeographic variation in E. sosorum, suggesting the possibility of population substructuring, phenotypic plasticity, or undetected hybridization.