Groundwater residence time estimates obscured by anthropogenic carbonate

Abstract

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.

Estimation of Groundwater Residence Time Using Radiocarbon and Stable Isotope Ratio in Dissolved Inorganic Carbon and Soil CO2

“…There is no standard way to account all the complexities discussed above. Researchers address these complexities using different models (Seltzer et al 2021; Starn et al 2021; Fareze et al 2022). One approach is to use Lumped Parameter Models (LPM) (Maloszewski and Zuber 1982; Maloszewski and Zuber 1992; Maloszewski 2000; McGuire and McDonnell 2006; Jurgens et al 2012; Cartwright et al 2020) to estimate the groundwater residence times.…”

Radiocarbon signatures of carbon phases exported by Swiss rivers in the Anthropocene

“…However, the 14 C value of DIC at station 16 is much higher than that of DOC, whereas the opposite is the case for station 18, contradicting this well mixed behaviour observed at station 21. The agricultural practice of liming could also serve as a further source of DIC with low 14 C value [89,90], although this remains speculative as we lack information on such practices in Switzerland. We note again that 14 C values for DIC at the three most agriculturally impacted stations 6, 11 and 13 are within a similar range from −138‰ to −177‰ (table 1), suggesting that this mode of land-use may modulate riverine DI 14 C signatures.…”

THE VERSATILE USES OF THE 14C BOMB PEAK

“…Groundwater is the largest freshwater resource on earth. Since our life depends on the availability of freshwater, it is important to understand the residence time of a groundwater system (Seltzer et al 2021), and how fast it is being recharged by precipitation. The age of groundwater systems can provide a first estimate of this information.…”