Browsing by Subject "Aquifers--Texas"

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Correlation of structural lineaments and fracture traces to water-well yields in the Edwards Aquifer, Central Texas
(1990) Alexander, Kenneth Bower, 1961-; Bennett, Philip C. (Philip Charles), 1959-
Lineaments are "straight lines visible from afar on the surface of the earth". In the Austin, Texas area, lineaments reflect the structural grain of the Balcones-Ouachita fault zone and may indicate subsurface geologic phenomena such as faults, fractures, and joints. These structural features often represent discrete zones of high permeability, and thus, areas of enhanced flow of groundwater capable of transmitting greater quantities of water than surrounding, non-fractured, rock. For this study more than 900 lineaments and fracture traces, identified in aerial photographs during a previous study, were detected in the Barton Springs section of the Edwards Aquifer. The endpoints of each linear feature were digitized and tagged with a unique identification label. Rose plots, Cartesian histograms, and a series of statistical operations were utilized to illustrate regional trends in the orientation of lineaments. As an indicator of well productivity, specific capacities of 27 wells in the area were obtained. Sixty-one water samples were collected and analyzed to test for possible chemical evidence of lineament-well interactions. The orientations of lineaments and fracture traces in the study area clearly display a bimodal distribution with a primary trend of N 40 E and a secondary peak of N 50 W. A general correlation exists between increased well productivity and decreased distances to the nearest lineament, particularly within 200 feet of lineaments. Also, 10 of the 13 largest specific-capacity values are from wells located southeast of southwest-northeast trending lineaments. Nonparametric statistical methods show that direction from lineaments is a significant factor in predicting water-well yields. Lineaments provide a tool for predicting possible sites of environmental sensitivity with respect to groundwater resources. Examples include the siting of groundwater monitoring wells for point sources of pollution, predicting the likely underground flow paths of a pollution plume or locating dam sites for recharge enhancement. Awareness of the location, orientation, and density of structural lineaments will allow the water-resource manager to identify discrete groundwater flow paths, and, thus, predict contaminant plume migration.
The secondary permeability of "impervious" cover in Austin, Texas
(2007-12) Wiles, Thomas Jefferson, 1970-; Sharp, John Malcolm, 1944-
The term "impervious" is commonly used in urban settings to describe the permeability of buildings, roads, and parking lots. When estimating recharge to an aquifer underlying an urbanized area, impervious cover becomes a prime consideration. It is commonly assumed that an increase in impervious cover leads to a decrease in precipitation recharge. However, even a cursory glance at most roads, sidewalks, or parking lots reveals that, far from being impervious, there are abundant fractures that may provide avenues of infiltration. For this study, method was developed to determine the secondary permeability of pavements using a double ring infiltrometer to measure the infiltration rate of water into fractured pavements. Linear extrapolation is employed to determine the infiltration rate as the water depth approaches zero, which is used as a proxy for hydraulic conductivity by assuming that the gradient is unity. Data were collected on concrete and asphalt pavements located in Austin, Texas, at each point a fracture or expansion joint intersected along 30-meter scanlines. By dividing the sum of the discharges for each fracture by the area represented by the scanline we are able to determine the equivalent-porous-media hydraulic conductivity. The equivalent hydraulic conductivities for discrete fractures were found to range at least three orders of magnitude, from >10⁻² to 10⁻⁵ cm/sec; scanline hydraulic conductivities range two orders of magnitude from >10⁻⁴ to 10⁻⁶ cm/sec; permeability along the scanlines tends to be dominated by one or two highly conductive fractures; and the hydraulic conductivity of the entire paved surface is 5.9·10⁻⁵ cm/s. Both apertures and point hydraulic conductivities were found to have logarithmic distributions but cross plots demonstrated no correlation, which indicated that a combination of the fill material and sub grade, not the fractures and expansion joints themselves, limit infiltration. By multiplying the paved surface hydraulic conductivity by the time the surface can be expected to be saturated, we find that 170 mm or 21 percent of mean annual rainfall is available as potential recharge. When coupled with an enhanced subsurface permeability structure resulting from the installation of utilities and the reduction of evapotranspiration from the reduction of vegetation, the net effect of roads and parking lots could be an increase in precipitation recharge.
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.