Decadal Shifts in Groundwater Age Detected by Environmental Tracers Across California, USA

Groundwater age offers important insight into recharge, storage, and contamination risk. Although models predict age changes can be driven by pumping and climate variability, direct observational evidence remains limited. Here, we analyzed paired environmental tracer suites (tritium, carbon‐14, and tritiogenic helium‐3) collected a decade apart from 268 wells across California to assess the prevalence of groundwater age transience. Travel‐time distribution models and statistical tests indicated age transience at 29% of sites, occurring most often in agricultural regions, such as the San Joaquin Valley and Southern Coast Ranges, where large carbon‐14 changes coincided with substantial nitrate and chloride shifts. Sites with tritiogenic helium‐3 data showed more frequent age transience, underscoring the value of multi‐tracer data sets. These results provide the first regional evidence of widespread groundwater age change and a method for detecting changing water balances with implications for groundwater sustainability and water quality.