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Groundwater residence time estimates obscured by anthropogenic carbonate

April 21, 2021

Groundwater is an important source of drinking and irrigation water. Dating groundwater informs its vulnerability to contamination and aids in calibrating flow models. Here, we report measurements of multiple age tracers (14C, 3H, 39Ar, and 85Kr) and parameters relevant to dissolved inorganic carbon (DIC) from 17 wells in California’s San Joaquin Valley (SJV), an agricultural region that is heavily reliant on groundwater. We find evidence for a major mid-20th century shift in groundwater DIC input from mostly closed- to mostly open-system carbonate dissolution, which we suggest is driven by input of anthropogenic carbonate soil amendments. Crucially, enhanced open-system dissolution, in which DIC equilibrates with soil CO2, fundamentally affects the initial 14C activity of recently recharged groundwater. Conventional 14C dating of deeper SJV groundwater, assuming an open system, substantially overestimates residence time and thereby underestimates susceptibility to modern contamination. Because carbonate soil amendments are ubiquitous, other groundwater-reliant agricultural regions may be similarly affected.

Publication Year 2021
Title Groundwater residence time estimates obscured by anthropogenic carbonate
DOI 10.1126/sciadv.abf3503
Authors Alan Seltzer, David Bekaert, Peter H. Barry, Kathryn Durkin, Emily Mace, Craig E. Aaselth, Jake Zappala, Peter Mueller, Bryant Jurgens, Justin T. Kulongoski
Publication Type Article
Publication Subtype Journal Article
Series Title Science Advances
Index ID 70227154
Record Source USGS Publications Warehouse
USGS Organization California Water Science Center