Browsing by Subject "Hydrogeology--Texas--Edwards Aquifer"

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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
Groundwater flow and recharge within the Barton Springs segment of the Edwards Aquifer, southern Travis and northern Hays Counties, Texas
(2009-05) Hauwert, Nico Mark; Sharp, John Malcolm, 1944-
The Barton Springs Segment, part of the karstic Edwards aquifer in Central Texas, is a Sole Source aquifer, is habitat to rare karst species, and provides water to a well-loved municipal swimming pool, yet its hydrogeologic properties remain insufficiently understood. For this study, the hydrogeologic characteristics of the Barton Springs Segment were investigated using several approaches, including mapping of hydrostratigraphic units and faults, measurement of upland infiltration, groundwater traces, and aquifer tests. The depositional environment, diagenesis, fracturing, down-dropped and dipping faulted blocks, and subsequent dissolution were determined to play important roles in controlling groundwater flow-path development within the Barton Springs Segment. In particular, downdropped fault blocks create groundwater gradients to the southeast that influence flow in the Edwards outcrop area. Upland internal drainage basins were found to be extremely efficient at conveying recharge to the underlying aquifer. The maturity of natural internal drainage sinkholes can be measured by its bowl volume, which grows in proportion to the catchment area it captures. A 19-hectare internal drainage basin, HQ Flat sinkhole, was monitored for rainfall, evapotranspiration, soil moisture, and discrete runoff to the cave drain. During a 505-day period, 5.5% of measured rainfall entered the cave drain as discrete recharge, 26% of measured rainfall infiltrated through soils on the slopes, and the remaining 68% was lost through evapotranspiration. This amount of upland infiltration is consistent with infiltration measurements in other karst areas and is much larger than the 1% upland recharge of rainfall that was previously estimated. A chloride mass balance indicates that at the adjacent Tabor research site, about 50% of rainfall infiltrates to a 6-meter depth. Dye-tracing and pump tests demonstrated that primary and secondary groundwater flow paths are the major influence on transmissivity within the Barton Springs Segment. Groundwater tracing breakthroughs reveal very high advection and relatively low dispersion. Drawdown response to pump tests indicates a very high degree of anisotropy, controlled by location of groundwater flow paths. Overall the Barton Springs Segment is a mature karst aquifer with highly developed rapid, discrete network for both recharge and groundwater-flow.
Permeability structure in fractured aquifers
(2000) Halihan, Todd, 1970-; Sharp, John Malcolm, Jr., 1944-
Understanding water movement through fractured and karstic aquifers is difficult, but it is important for those managing these resources. Determining which features contribute to flow in these aquifers is important because accurate predictions of flow and transport are most sensitive to variations in the permeability field. A continuum approach to these aquifers has led to two problems that are approached with two new techniques that quantify the permeability. First, continuum hydraulics does not allow an understanding of which individual features in an aquifer provide flow. This permeability structure problem is manifested in scale dependent permeability (larger permeability values for larger scale tests). Interpretations of these aquifers are limited by ignoring small-scale data in addressing larger scale problems. The Edwards aquifer of central Texas was used to determine if data sets of matrix permeability, fracture aperture, and conduit size from cores and outcrops can be effective utilized to interpret permeabilities measured at the small-, well-, and regional-scales. The results demonstrate that by quantifying permeability on the small-scale, larger scale interpretations of the aquifer are possible and have a stronger quantitative basis by utilizing geologic information from the aquifer. The second problem is that standard hydraulic measurement techniques are optimized for porous media. This approach does not allow individual features and their connections with other features to be easily evaluated in these aquifers. Fractured carbonate aquifers in Wisconsin and Australia were evaluated using asymmetric dipole-flow tests to determine if the structure of permeability could be determined more effectively. Dipole-flow testing, analogous to resistivity dipole testing, is a relatively new technique that was developed from the use of recirculation wells in contaminant remediation. This asymmetric technique may overcome many of the problems inherent in other testing strategies. Asymmetric dipole-flow tests provided rapid testing and demonstrated the ability to quantify heterogeneities. The results demonstrate that boreholes can be connected in complex geometries with drawdown occurring above and below areas of pressure buildup.