Delineation of the ground-water flow systems of the Eagle Flat and Red Light Basins of Trans-Pecos Texas

Abstract

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 Survey