Environmental-isotope data from the Eagle Flat and Red Light Basins of southern Hudspeth County, Texas indicate that the ground-water flow systems of this region are recharged principally by infiltration through fractured bedrock in the mountains and uplands, and not by diffusive flux through the broad alluvial fans or the thick, fine-grained sediments of the basin floors. In the higher elevations, where bedrock of Precambrian, Cretaceous or Tertiary age is exposed, carbon-14 (¹⁴C) ranges from 100 to 55 percent modern carbon (pmc), and tritium (³H) is from 8 to 1 tritium units (TU). Beneath the proximal to medial alluvial fans, however, ¹⁴C decreases to between 20 and 4 pmc, and ³H is below the limits of detection. Beneath the basin floors, where the depth to ground water is as much as 1,000 ft in the Eagle Flat Basin and more than 400 ft in the Red Light Basin, ¹⁴C is as low as 4 to 2 pmc, and ³H is not detected. Well-developed and low-permeable calcrete horizons occur throughout the mountain fans, minimizing the potential for infiltration and explaining the low ¹⁴C and ³H values of ground water beneath the fans.

Uncorrected radiocarbon ages indicate residence times ranging from recent in ground waters with ¹⁴C approaching 100 pmc to between 30,000 and 25,000 years in the deeper ground waters with ¹⁴C concentrations of 4 to 2 pmc. Different factors designed to adjust for the influence of isotopic dilution by dead carbon consistently yield corrected ages of 15,000 to 10,000 years for the apparently oldest ground waters of the basins. Many of these factors, however, may not be appropriate for these flow systems, and may yield corrected ages that are thousands of years older or perhaps younger than the actual age of a sample.

The relationship between stable hydrogen and oxygen isotope ratios (d²H and d¹⁸O) and ¹⁴C may provide a nonquantitative method of deriving a conservative estimate of the residence time of ground water. Both d²H and d¹⁸O of 78 samples of ground water plot parallel to the Global Meteoric Water Line, indicating no evidence of enrichment attributable to thermally driven exchange with isotopically heavier carbonates or siliciclastics, or to evaporative concentration at the surface or in the unsaturated zone. The values of both d²H and d¹⁸O decrease by approximately 3 ‰ (SMOW) between areas where ground waters yield demonstrably modern ¹⁴C and ³H signatures and areas where ¹⁴C is consistently less than 20 pmc and ³H is not detectable. This relationship suggests that ground waters beneath the fans and the basin floors are mostly likely traceable to recharge during the late Pleistocene. The lower d²H and d¹⁸O values of the apparently oldest waters imply a time when ambient temperatures were from 5° to 8° C lower than modern temperatures. These lower temperatures are consistent with estimates derived from several studies of the climate of the southwest during the last glacial maximum, and we infer that residence times of from 20,000 to 10,000 years are highly probable for ground waters of these basins. Residence times greater than 10,000 years are consistent with estimates of confined ground wates in other areas of west Texas and southeastern New Mexico.

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