Geochemical Quantification of Semiarid Mountain Recharge

Analysis of a typical semiarid mountain system recharge (MSR) setting demonstrates that geochemical tracers help resolve the location, rate, and seasonality of recharge as well as ground water flowpaths and residence times. MSR is defined as the recharge at the mountain front that dominates many semiarid basins plus the oftenoverlooked recharge through the mountain block that may be a significant ground water resource; thus, geochemical measurements that integrate signals from all flowpaths are advantageous. Ground water fluxes determined from carbon-14 ( 14 C) age gradients imply MSR rates between 2 3 10 6 and 9 3 10 6 m 3 /year in the Upper San Pedro Basin, Arizona, USA. This estimated range is within an order of magnitude of, but lower than, prior independent estimates. Stable isotopic signatures indicate that MSR has a 65% 6 25% contribution from winter precipitation and a 35% 6 25% contribution from summer precipitation. Chloride and stable isotope results confirm that transpiration is the dominant component of evapotranspiration (ET) in the basin with typical loss of more than 90% of precipitation minus runoff to ET. Such geochemical constraints can be used to further refine hydrogeologic models in similar basins with high relief and can provide practical first estimates of MSR rates for basins lacking extensive prior hydrogeologic measurements.