ISOTOPIC EVIDENCE OF PAST FOCUSED INFILTRATION AT FORTYMILE WASH

Groundwater isotopic and chemical data from sampling location in the Amargosa Desert region are analyzed to better understand recharge at Fortymile Wash, near Yucca Mountain. Geochemical data suggests that groundwater beneath Fortymile Wash follows the surface of the wash until it appears to merge and mix with groundwater beneath the Amargosa River. Furthermore, corrected and uncorrected carbon-14 values from groundwater beneath Fortymile Wash are younger and fresher (i.e., with lower TDS and chloride values) than those on adjacent highlands, which indicates that the highlands are not the source for the groundwater below the wash. Beginning at the source of Fortymile Wash in the north and following south along the wash, groundwater becomes sequentially older, with ages between 8,000 years BP in the upper canyon region and 14,000 years BP in the lower region near the Amargosa Desert, This range in ages corresponds to the end of the Pleistocene and early Holocene epochs, marking the end of Wisconsin glaciation and the start of the current interglacial period, and suggests that the average reach of runoff events and recharge have diminished over time. Regional hydrogen-2 and oxygen-18 values suggest a humid-climate precipitation, with little surface evaporation before infiltration. Stable isotope values beneath the wash fall parallel to the Global Meteoric Water Line (GMWL), with successive depletion of hydrogen-2 and oxygen-18 values suggesting not an evaporation curve but evidence of a past climate change from a cold to warmer climate, although still colder than the present. The spatial distributions of hydrogen-2 and oxygen-18 are similar to those of carbon-14, and together they indicate changes to the groundwater system as the climate became warmer and dryer during the past 14,000 years. The signature from Fortymile Wash is believed to represent the relic of focused infiltration of surface runoff along the course of the wash during past pluvial periods, when the climate was colder and wetter than the present and the amount of runoff in the wash was significantly greater.