Browsing by Subject "Hydrogeology--Trans-Pecos (Tex. and N.M.)"
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Item Delineation of the ground-water flow systems of the Eagle Flat and Red Light Basins of Trans-Pecos Texas(1997) Darling, Bruce Kelley, 1951-; Sharp, John Malcolm, Jr., 1944-The hydrogeologic systems of the Eagle Flat and Red Light basins of Trans-Pecos are characterized by three-dimensional flow components, fracture and double-porosity flow, hydrochemically zoned water in alternating permeable and low-permeability rocks and poorly consolidated strata, and areal transitions between unconfined, confined, and leaky-confined aquifers. Ground-water depths greater than 900 ft (275 m) beneath the topographically lowest areas of the Eagle Flat Basin, along with closed potentiometric contours suggest drainage to a regional flow system that transports ground water beyond basin boundaries, probably beneath the Devil Ridge ground-water divide and the thick Tertiary basin fill deposits of the Red Light Basin, where hydraulic head along the Rio Grande is as much as 400 ft (122 m) below the lowest point of the potentiometric surface in the Eagle Flat Basin. Radiocarbon is unreliable as estimator of absolute age. This is attributable to contributions of dead carbon by the dissolution of carbonate rocks and/or to isotope exchange. Mixing with younger (tritiated) ground waters adds to the complexity. δ¹⁸O values of the apparently oldest ground waters, however, are about 3%₀ (SMOW) lower than those of demonstrably modem ground waters. This pattern is thought to be consistent with the influence of precipitation under an older and probably cooler climatic regime. This suggests that the oldest ground waters may have late Pleistocene recharge dates (e.g., 10,000 to 20,000 years). In addition, the temperature-fractionation gradient for δ¹⁸O suggests precipitation under climatic conditions that were from 5.5 to 7.3°C below those of average Holocene values. Isotopic data indicate that major recharge areas are focused in the upper elevations of the mountains and along mountain fronts. Recharge attributable to precipitation does not occur in the flats and draws, except where shallow water-table conditions exist in alluvial deposits adjacent to the Rio Grande. Low-permeability late-stage calcic soils limit the potential for precipitation recharge across the broad alluvial fans of the Eagle Mountains and the Indio Mountains. Estimates of precipitation recharge derived from a numerical flow model are about 0.07 in per year, or about 14% of estimates by the U.S. Geological SurveyItem The hydrogeology of the Davis Mountains, Trans-Pecos, Texas(1992) Hart, Margaret A.; Sharp, John Malcolm, Jr., 1944-The Davis Mountains, located in the Trans-Pecos region of Texas, are comprised of Tertiary volcanics and associated volcaniclastic facies of the McCutcheon Series. The study area covers approximately 3200 square miles and is bounded on the south by the surface drainage divide between Limpia Creek and Musquiz Creek, on the west roughly by State Routes 166 and 118, on the north by Interstate Highway 10, and on the east by the eastern boundary of the Barrilla Mountains. Volcanic rocks were deposited in the area which makes up the present day Davis Mountains during the Tertiary period from approximately 40 to 20 million years ago. The lithologic units of the mountains include rhyolite, tuft, basalt, and sedimentary interflow deposits. Structural features of the Davis Mountains and eastern Trans-Pecos play an important role in controlling the areal extent of permeable units in the McCutcheon Aquifer. In addition, they provide pathways for interaquifer flow between the Tertiary and underlying aquifers. The McCutcheon Series constitutes a single hydrostratigraphic unit, herein referred to as the McCutcheon aquifer. Water from the McCutcheon aquifer provides recharge to both local and regional aquifers of the eastern Trans-Pecos and Permian Basin of west Texas. Ground water flows out of the McCutcheon aquifer into the Tertiary and Quaternary alluvial outwash plains adjacent to the mountains; the Cenozoic Toyah aquifer; and the Cretaceous Edwards aquifer. The bolson aquifers of the Salt Basin and the Permian aquifers north of the mountains receive some McCutcheon discharge, and a very small portion of the discharge flows into the Marta Basin, south of the Davis Mountains. The geochemical signature of the McCutcheon aquifer is that of a calcium-bicarbonate, very low TDS water (less than 400 mg/I), indicative of flow along a relatively short path through the igneous rock mass. Mixing of ground water from the McCutcheon, Permian, and lower Cretaceous Edwards aquifers produces the sodium-calcium-chloride-sulfate waters issuing from such natural orifices as San Solomon, Giffin, and Phantom Lake springs. Significant recharge events in the Davis Mountains lower the TDS content of these springs from approximately 2200 mg/I TDS to less than 1000 mg/I TDS over a period of a few days, with a return to high TDS levels in the springs over a period of weeks to months. This pattern indicates rapid recharge to the Edwards along the eastern edge of the mountains accompanied by considerable storage in the lower permeable units of the McCutcheon aquifer and the adjoining alluvial aprons, which then discharge slowly into the Edwards aquifer. Hydrogeochemical zones are delineated adjacent to the Davis Mountains, primarily to the west and north, where low TDS water of the McCutcheon aquifer mixes with more saline waters of the upper Cretaceous and Permian aquifers. These zones produce water with a mixed anion chemical signature indicative of mixing of ground water from the McCutcheon and regional aquifers of the central Trans-Pecos.Item Hydrogeology of the Lobo and Ryan Flats area, Trans-Pecos Texas(1993) Black, Jeffrey W., 1955-; Sharp, John Malcolm, Jr., 1944-Lobo Flat and Ryan Flat are the southern extent of the Salt Basin of Trans-Pecos Texas. The Salt Basin is a fault-bounded, northwest trending alluvial basin resulting from Basin-and-Range extension approximately 24 m.y.a. The study area covers approximately 700 square miles (1813 km²), is about 75 miles (120 km) long and ranges from 5 to 25 miles (8 to 40 km) wide. An unconfined aquifer occurs in the Cenozoic alluvial fill. The basin fill is absent at the margins of the basin, and thickens to over 1000 feet (305 m) near the center of the basin. It consists of sediments derived from the surrounding Wylie and Davis Mountains to the east, the Van Horn and Sierra Vieja Mountains to the west and the Oak Hills to the south. The alluvium consists of poorly-sorted coarse gravels and sands adjacent to the mountains, and grades into and is interlayered with silts and clays of the basin interior. The presence of interlayered finer-grained deposits and clay lenses results in localized semiconfined and perched water-table conditions. The groundwater is generally low in total dissolved solids, ranging from 128 mg/l to 1240 mg/l and is classified into three hydrochemical facies based on dominant ions. A Ca-HCO₃ water is found in a few wells near the margins of the basin and is similar to water found in the volcanic aquifers of the Davis Mountains. The dominant water type is a Na-HCO₃ water which occurs over the entire area. Several wells yield Na-SO₄ water high in total dissolved solids. The groundwater chemistry is controlled by the dissolution of calcium carbonate, gypsum, and by ion exchange processes as the groundwater moves along the flow path. The basin is through-flowing, with natural discharge occuring only by lateral flow to the north. Agricultural development of the Lobo area in the north part of the basin resulted in drawdown of the water table of up to 150 feet (45 m) between 1950 and 1980. Since about 1980, agricultural pumpage has decreased resulting in about 50 feet (15 m) of water table rebound. Recharge to the aquifer occurs primarily adjacent to the mountains in the alluvial fans. Results of 2-D computer modeling using MODFLOW and MODINV indicate a complex flow system influenced by the presence of clay lenses. Four hydraulic conductivity fields with a range of hydraulic conductivities from 1 ft/day (3.2 x 10⁻⁵ m/s) to 28 ft/day (9.9 x 10⁻⁵ m/s) were required to adequately simulate groundwater flow in the area