Abstract

Environmental-isotope data from the Eagle Flat and Red Light Draw basins indicate that the oldest ground waters of the basins are of late Pleistocene age. 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 less than 4 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 ages that are thousands of years older or perhaps younger than the actual age of a sample.

The relationship between ¹⁸O and ¹⁴C may provide a non-quantitative method of deriving a baseline estimate of the residence time of ground water. The ³H and ¹⁸O values of 78 samples of ground water from the Eagle Flat and Red Light Basins plot parallel to the Global Meteoric Water Line. The δ¹⁸O values, however, decrease by approximately 3 per mil 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. The lower δ¹⁸O values of the oldest waters are interpreted to indicate precipitation in a climate that was from 5° to 8° C lower than the Holocene average for the region. These lower temperatures are consistent with estimates derived from studies of the climate of the southwest during the Wisconsinan glacial maximum, and we infer that residence times of 10,000 to 20,000 years are highly probable for ground waters of these basins.