Estimate of recharge from radiocarbon dating of groundwater and numerical flow and transport modeling

Abstract. This paper combines radiocarbon age and hydraulic data to estimate recharge to a regional groundwater aquifer. The 14C ages of groundwater are first corrected for the effects of chemical reactions through geochemical modeling. Recharge rates to the aquifer are then calibrated to observed 14C ages using a linked numerical 14C transport and flow model while hydraulic conductivity values are proportionally adjusted to match observed heads. The methodology is general but has particular applications to semiarid and arid regions where the need for reliable recharge estimates is greatest but the task is the most difficult. The ability of this method for estimating recharge on the scale of thousands of years makes it a valuable tool for studies of global environmental changes and performance assessment of radioactive waste repositories. A case study of a regional aquifer in northeastern Arizona shows the recharge estimates are within the bounds determined by chloride mass balance but are significantly higher than previous estimates derived from the Maxey-Eakin method. 1