Radiocarbon in dissolved inorganic carbon in unsaturated zone soil water of semi-arid western India

Estimating groundwater age is vital for water management, especially in semi-arid regions, where aquifers are the primary freshwater source. Radiocarbon ( 14 C) dating of dissolved inorganic carbon (DIC) is widely used to estimate groundwater age, assuming DIC is derived mainly from modern soil CO 2 , while soil carbonate contributions are considered 14 C-dead. However, in semi-arid regions, soil carbonates contributing to DIC in unsaturated zone soil water (UZSW) may contain measurable 14 C activity. This can impart an apparent age which is different from that assumed in conventional correction models and thus needs to be properly accounted for in groundwater age estimations. Here, we measured 14 C and δ 13 C in DIC in UZSW, soil CO 2 , soil carbonates, and soil organic matter to identify DIC sources and assess their impact on groundwater dating. Results show that the ages of DIC in UZSW range from 90 to 2290 years, with an average of ∼1240 years in semi-arid western India. We estimated that ∼13% of DIC derives from soil carbonates with 14 C ages in the range of 3500–14,000 BP in this region. These findings suggest that 14 C-based groundwater age estimates, particularly for groundwater younger than 10,000 years, may be biased if the 14 C activity of soil carbonates are not adequately accounted for. Our study also highlights the analytical challenges associated with isolating DIC for 14 C measurements. Overall, our results underscore the importance of characterizing the 14 C signature of soil carbonates and incorporating it into groundwater dating frameworks to improve the reliability of groundwater age determinations.