PNNL

Abstract

Determining the time scale of groundwater recharge is critical for sustainable management and often relies on measurements of radiocarbon (14C) in dissolved inorganic carbon (DIC). In agricultural regions, however, contaminated irrigation runoff may alter DIC chemistry and thereby substantially obfuscate 14C-derived residence times. Here we investigate groundwater residence time distributions in the San Joaquin Valley (California) using 14C, 3H and inert age tracers (85Kr and 39Ar). We find evidence for a mid-20th century shift in the scaling factor between 14C activity in groundwater DIC and atmospheric CO2 at the time of recharge, which is a critical parameter for groundwater dating. Based on insight from an idealized DIC model, we suggest that modern surface runoff laden with particulate agricultural lime–a ubiquitous soil amendment–has promoted rapid carbonate dissolution and isotopic exchange with soil CO2 in the vadose zone. Because this anthropogenic shift has masked the natural pre-development controls on groundwater 14C, apparent 14C-based age estimates in deeper wells may overestimate true residence times by thousands of years, rendering these wells more susceptible than previously assumed to modern contamination.