Quantifying mountain system recharge in the upper San Pedro basin, Arizona, Using geochemical tracers

Abstract

Use of geochemical tracers has improved our conceptual and quantitative understanding of mountain system recharge (MSR) in the Upper San Pedro Basin, Arizona. These isotopic, major anion, and noble gas tracers resolved the location, rate and seasonality of recharge as well as groundwater flowpaths and residence times. Detectable anthropogenic 14C and tritium revealed less than 50 years residence times for the fast fraction of flow on the mountain block and in riparian areas. Maximum 14C residence times of over 10,000 years occurred for basin groundwater entering the San Pedro River riparian area. Groundwater fluxes determined from 14C imply MSR rates between 2 x 10^6 m^3 /yr and 9 x 10^6 m^3 /yr. Stable isotopic signatures indicate that MSR has a 40-90% contribution from winter precipitation and a 10-60% contribution from summer precipitation. Geochemical data is recommended to help calibrate groundwater flow models and substantially reduce uncertainty in the estimated MSR rate.