EVIDENCE OF PAST FOCUSED INFILTRATION AT FORTYMILE WASH

An analysis of geochemical groundwater data surrounding Fortymile Wash was conducted to better understand the recharge patterns in the wash and the groundwater flow in south-central Nevada. Regional groundwater ²H versus ¹⁸O isotope data lie mostly below the global meteoric water line (GMWL), demonstrate a humid-climate type of precipitation with some evaporation before infiltration, and correspond to a climate range colder than contemporary. A regional digital elevation map (DEM) with overlain contours of ²H and ¹⁸O isotope data, present patterns that surround and follow the general shape of the wash with the less depleted isotope values at the upper part of the wash. Corrected and uncorrected ¹⁴C values from groundwater beneath Fortymile Wash have a similar pattern as the stable isotope data. Furthermore, groundwater surrounding Fortymile Wash, dated with ¹⁴C and corrected with ¹³C isotopes, presents ages between 8,000 years in the upper region and 14,000 years in the lower region near the Amargosa River. Groundwater directly beneath Fortymile Wash is younger and fresher (i.e. lower TDS) than those under the adjacent highlands. This contrast indicates that the source of the groundwater under Fortymile Wash is not the groundwaters under the adjacent highlands, but rather that the true source is focused infiltration of surface runoff along the course of the wash, occurring in the past when the amount of runoff in the wash was significantly greater than present. Commencing at the beginning of the Fortymile Wash in the north and following it south along its path, groundwater ²H and ¹⁸O isotope values beneath the wash register parallel to the GMWL with successive further depletion of ²H and ¹⁸O. This depletion of groundwater isotopes down the course of the wash accompanied by gradual groundwater age increase, suggest that the average reach of runoff events and recharge diminished over time as the climate became warmer and dryer. Supporting evidence is provided by a principal component analysis of the major ion data and a cluster analysis of these results that group wells with a similar genesis into different hydrochemical facies. The resulting hydrochemical facies overlain on a DEM of the region demonstrate patterns similar to those presented by the isotopic data.