Groundwater Sources in a Desert Watershed (Tule Basin, Nevada) - Estimates of Recharge and Interbasin Flow from Groundwater 14C Patterns and Deviations from a Local Paleoclimate Archive

Abstract:

Estimating maximum sustainable groundwater extraction rates in the hydrologically interconnected watersheds of the Deep Regional Carbonate Aquifer (DRCA) of the southwestern United States is difficult as water enters fractured aquifers via a combination of local recharge and interbasin flow. As a result, over-extraction may impact water availability on local and regional scales. In this study, a combination of geochemical data from groundwater and vein calcite samples enabled rate of local recharge and interbasin flow in the remote Tule Desert watershed in southeastern Nevada to be quantified. Groundwater ¹⁴C age gradients below the water table in 3 multi-level well clusters indicate recharge rates of 1 mm/yr to 2 mm/yr which correspond to a maximum sustainable yield of 5 x 10^-4 km³/yr to 1 x 10^-3 km³/yr. This is less than previous estimates derived from different methods (e.g., Cl mass balance, water budget modeling) and probably reflects uncertainties in the applied effective porosity values and increasing horizontal interbasin flow components at greater depths below the water table. The deviation of the groundwater 𝛿¹⁸O time-series pattern for the Pleistocene-Holocene transition from that of the Devils Hole vein calcite (which is considered a reliable proxy for local climate change) combined with the regional groundwater 𝛿¹⁸O gradient allows interbasin flow rates of northerly derived groundwater to be estimated. The reported rates (61.2 m/yr – 101 m/yr) are slightly higher than those derived upon Darcy’s Law, but indicate hydraulic conductivity values strikingly similar to those obtained from pump tests conducted elsewhere in the DRCA province. These data may therefore provide the unique opportunity to predict basin-wide groundwater recharge (and sustainable yield) via groundwater flow model calibration based on set hydraulic gradient and conductivity parameters.