Estimation of Groundwater Recharge at Pahute Mesa using the Chloride Mass-Balance Method

Abstract

Groundwater recharge on Pahute Mesa was estimated using the chloride mass-balance (CMB) method. This method relies on the conservative properties of chloride to trace its movement from the atmosphere as dry-and wet-deposition through the soil zone and ultimately to the saturated zone. Typically, the CMB method assumes no mixing of groundwater with different chloride concentrations; however, because groundwater is thought to flow into Pahute Mesa from valleys north of Pahute Mesa, groundwater flow rates (i.e., underflow) and chloride concentrations from Kawich Valley and Gold Flat were carefully considered. Precipitation was measured with bulk and tipping-bucket precipitation gauges installed for this study at six sites on Pahute Mesa. These data, along with historical precipitation amounts from gauges on Pahute Mesa and estimates from the PRISM model, were evaluated to estimate mean annual precipitation. Chloride deposition from the atmosphere was estimated by analyzing quarterly samples of wet-and dry-deposition for chloride in the bulk gauges and evaluating chloride wetdeposition amounts measured at other locations by the National Atmospheric Deposition Program. Mean chloride concentrations in groundwater were estimated using data from the UGTA Geochemistry Database, data from other reports, and data from samples collected from emplacement boreholes for this study. Calculations were conducted assuming both no underflow and underflow from Kawich Valley and Gold Flat. Model results estimate recharge to be 30 mm/yr with a standard deviation of 18 mm/yr on Pahute Mesa, for elevations >1800 m amsl. These estimates assume Pahute Mesa recharge mixes completely with underflow from Kawich Valley and Gold Flat. The model assumes that precipitation, chloride concentration in bulk deposition, underflow and its chloride concentration, have been constant over the length of time of recharge.iv ACKNOWLEDGMENTS

Evaluating the Importance of Barometric Pumping for Subsurface Gas Transport Near an Underground Nuclear Test Site

“…All surface nodes were assigned a transient barometric pressure boundary condition to mimic the measured atmospheric conditions and a constant infiltration flux to simulate a low rate of infiltration on Pahute Mesa (5 mm/yr). The infiltration rate is based on an estimate of the Pahute Mesa recharge rate of 7 mm/yr for elevations between 1800 and 2000 m, used for the USGS and Pahute Mesa Phase I Flow models (Cooper et al, 2013). The bottom has a fixed, fully saturated boundary to represent the water table, which provides a no-flow gas boundary.…”