Climate Change Effects on Groundwater Recharge East of Yucca Mountain

In order to better understand the general flow system and climate-induced changes in recharge around Fortymile Wash, near Yucca Mountain, groundwater geochemical data from the Amargosa Desert region were analyzed. Also, chloride mass balance (CMB) was applied to drill cuttings from borehole NC-EWDP-22S near Fortymile Wash. Stable isotopic hydrogen-2 and oxygen-18 data indicate that less depleted groundwater is found under the flowpath of Fortymile Wash compared with groundwater perpendicular to the wash, and in the lower end of the wash compared with the source in the canyon. In addition, water isotope data under Fortymile Wash plot below the global meteoric water line (GMWL), suggesting low evaporation before infiltration. Total dissolved solids (TDS) and chloride (Cl) concentrations are lowest in the groundwater along the wash indicating less rock/water interaction and low evaporation prior to infiltration. In consequence, stable isotope, TDS and Cl data are most consistent with a pattern of infiltration and recharge of surface runoff subsequent to runoff-generating storms. Carbon-14 data corrected with carbon-13 data presents ages between 8,000 years before present (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. In contrast, groundwater adjacent to Fortymile Wash appears to be older than that beneath the wash. Furthermore, the trend of groundwater age increase and further stable isotope depletion beneath Fortymile Wash with increasing distance from the canyon suggests that the average reach of recharge and runoff events diminished over time as the climate became warmer and dryer. CMB results present two different pore velocities, the slower one nearer to the surface and with the transition occurring between 6 and 26 meters in depth corresponding to 8,5000 and 11,000 BP. Considered together, these facts suggest that ground water under Fortymile Wash is not derived primarily from migration of adjacent ground water, as indicated by coarse contoured water levels, but instead from past focused infiltration that diminished due to a changing climate.