Browsing by Subject "Hydrogeology--Texas"

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Detailed hydrogeologic maps of the Comal and San Marcos Rivers for endangered species habitat definition, Texas
(1994) Crowe, Joanna Catherine, 1970-; Sharp, John Malcolm, Jr., 1944-
The Edwards aquifer of Texas is a regionally-extensive carbonate aquifer, and Comal and San Marcos springs are its largest natural discharge points. They issue from faults in the confined portion of the aquifer to create Comal and San Marcos Rivers, which create habitat for a number of endangered species including the fountain darter (etheostoma fonticola), Texas wild rice (zinzania texana), the San Marcos gambusia (gambusia georgei), and the San Marcos salamander (eurycea nana). Protection of the endangered species living in the spring systems provided the impetus for this study. Pumping from wells in the Edwards has been increasing over the past few decades, and if this trend continues, natural springs in the aquifer will eventually cease to flow. The species in the rivers are not only endangered because of low populations, but also by the possibility of a loss of spring discharge. There are many different endangered species habitats present in the Comal and San Marcos systems, and this thesis presents maps and data on their characteristics. Hydrochemical, substrate, soil, vegetation, and flow information are included in these characteristics. Substrates have been separated into size classes to create maps of the river beds. Hydrochemical parameters in the rivers were examined showing remarkable uniformity in their measurements along the entire lengths of both rivers, including a year-round nearly constant temperature. The soils surrounding each river were mapped and their hydrologic characteristics were examined. Vegetation in the rivers was also mapped, because of its importance to the habitats. The plants provide a structure that protects the endangered species from high flow velocities in the rivers, and they help stabilize the substrate. Flow characteristics, locations of seeps and springs, and velocity measurements, have been mapped for each river. Habitats are defined through the combination of vegetation, substrate, soil, hydrochemistry, and flow type for different areas of both rivers. The flow velocities in the rivers are the most important characteristic, because it is the flow rate that controls the morphology of the entire river and the substrate that is present on the river bed. Through control of the substrate, the flow rate indirectly controls vegetation and habitat distributions in the rivers. Ranges of flow velocities necessary to maintain habitats in both rivers are estimated, because a change in the flow rate would alter the distribution of substrate on the river beds, and the current habitats would no longer exist. The Comal and San Marcos Rivers are unique because of their combinations of habitats, which is due primarily to the flow velocity variations in the rivers
Ground-water flow and solute transport in a fractured chalk outcrop, North-Central Texas
(1998) Mace, Robert E. (Robert Earl), 1967-; Sharp, John Malcolm, Jr., 1944-; Dutton, Alan R.
It is important to understand hydrogeologic controls on ground-water flow and solute transport in weathered and faulted zones because these zones are common in outcrops all over the world, relied on for ground water in many locales, and susceptible to contamination due to their proximity to the land surface. The weathered and faulted zones of the Austin Chalk in Texas are especially susceptible to contaminants from municipal, industrial, and agricultural sources because it underlies the Interstate 35 growth corridor that extends from Dallas through Austin to San Antonio. This study shows that the distribution and properties of fractures affect ground-water flow and solute transport in weathered, unweathered, and faulted chalk. Characterization data reveal that the weathered zone has: (1) a limited thickness (about 2.5 m); (2) a fracture intensity 300 times greater and a permeability 150 times greater than unweathered and unfaulted VII bedrock; and (3) water levels that respond rapidly to rainfall. Characterization data also show that faulted zones have: (1) permeability as much as 160 to 200,000 times greater than permeability of the weathered zone and unfractured, unweathered chalk, respectively; (2) connectivities that extend over vertical and lateral distances at least as great as 90 and 1,300 m; and (3) different hydraulic behavior depending on their connection with the land surface and topography. Numerical models based on conceptual models developed from the characterization data suggest that: (1) over 99 percent of ground-water flow occurs in the weathered zone; (2) faulted zones can greatly increase the depth and rate of transport in the chalk; and (3) ground-water travel times in the weathered zone depend on vertical variations in hydraulic conductivity, water-table position, and point of entry into the flow system and can be 10 times faster at higher than at lower water table positions. Geometric mean residence time for water in the weathered zone under wet conditions is about 16 days with a geometric mean velocity of 48.2 m d⁻¹. Small-scale numerical studies indicate that matrix heterogeneity may lead to longer back-diffusion times when attempting to remediate contaminated sites.
The source of the water along the Balcones fault escarpment
(1924-06) Tyson, Alfred Knox
Stygobite phylogenetics as a tool for determining aquifer evolution
(2005) Krejca, Jean Kathleen; Hillis, David M.; Hendrickson, Dean.
Abstract: The use of aquifer-dwelling organisms (stygobites) for learning about past and present subterranean hydrologic connections was evaluated in the Edwards (Balcones Fault Zone), Trinity, and EdwardsTrinity (Plateau) aquifers of Texas and adjacent areas in north Mexico, an area with complex karst groundwater flow and sociopolitical problems stemming from overuse and contamination. A priori predictions of subterranean hydrogeologic history were made based on a literature review, and these predictions were compared to phylogenies of two aquifer dwelling isopods created based on mitochondrial gene sequences (16S ribosomal RNA and cytochrome c oxidase subunit I). Using likelihood and parsimony-based comparisons, Cirolanides (Isopoda: Cirolanidae) was found to have a phylogenetic history congruent with a priori predictions of subterranean hydrogeologic history in its terminal nodes. Conversely, basal branches of the phylogenetic tree had placement that was not predicted by this history, a phenomenon that may be indicative of a lack of understanding of subterranean hydrogeology of the area. Lirceolus (Isopoda: Asellidae) had a phylogenetic history congruent with an alternative hypothesis of water flow, namely the patterns of surface drainages. This difference of patterns for two species that both live in the aquifer is probably related to their ecology and evolutionary history, with Cirolanides having invaded the cave habitat as a single marine population and Lirceolus invading the cave habitat as a freshwater migrant with possible pre-existing genetic structure determined by surface drainages. This study pioneers testing of a priori biogeographic hypotheses using phylogenies of aquifer organisms and the creation of hydrogeologic histories in a karst setting, and supports the use of these methods to aid in understanding biogeography and aquifer evolution.