HYDROCHEMICAL AND 14C CONSTRAINTS ON GROUNDWATER RECHARGE AND INTERBASIN FLOW IN A DESERT WATERSHED - A CASE STUDY FROM SOUTH-EASTERN NEVADA

Historical estimates of groundwater recharge and interbasin flow rates in the Deep Regional Carbonate Aquifer (DRCA) province of the southwestern US relied mainly on classic soil water budget and Darcy’s Law applications. Because these require difficult-to-constrain input parameters, they provide uncertain results for remote watersheds with limited calibration data. In this study, groundwater geochemical data were used to characterize the flow system of an uninhabited watershed in south-eastern Nevada that is currently explored for its sustainable yield. Major ion, δD/δ¹⁸O and ¹⁴C data indicate that wells located closer to the western mountainous margin produce mainly from locally derived mountain-system recharge, whereas wells closer to the eastern mountain front are more influenced by older, regionally derived paleowater. Groundwater ¹⁴C age gradients deduced from nested well clusters indicate variable and generally low recharge rates of <10 mm/yr, and significant components of interbasin flow at depths >400 m below the water table. Rates of interbasin flow derived from watersheds to the north were estimated based on the deviation of the groundwater δ¹⁸O time-series pattern for the Pleistocene-Holocene transition from that of a local vein calcite (which is considered a proxy for local climate change). The calculated rates (75 - 120 m/yr) translate into hydraulic conductivity (K) values strikingly similar to those obtained from pump tests conducted in the region. These K values, together with information on hydraulic gradients may be used as critical input parameters for predicting watershed-wide groundwater recharge and sustainable yield via calibration of groundwater flow models.