Groundwater residence time estimates obscured by anthropogenic carbonate

Science Advances
By: , and 



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.

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Publication type Article
Publication Subtype Journal Article
Title Groundwater residence time estimates obscured by anthropogenic carbonate
Series title Science Advances
DOI 10.1126/sciadv.abf3503
Volume 7
Issue 17
Year Published 2021
Language English
Publisher AAAS
Contributing office(s) California Water Science Center
Description eabf3503, 9 p.
Country United States
State California
Other Geospatial San Joaquin Valley
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