Estimation of groundwater residence time with deeply-derived carbon mixture considered in California

Groundwater has experienced long-term overdraft due to drought and human activities in California, resulting in issues of land subsidence and groundwater anthropogenic contamination. As a useful indicator of groundwater renewal rate and its residence time, groundwater age has been conventionally investigated based on ¹⁴C content and ³H concentration. However, the influence of deeply-derived (endogenic) CO₂ mixture is not fully and quantitatively considered, although endogenic carbon contributes a large part of the dissolved inorganic carbon in groundwater. Combined with ³H concentration, both conventional and modified ¹⁴C-dating methods with endogenic CO₂ mixture considered are employed for groundwater age determination in California. On average, the conventional groundwater ¹⁴C apparent age is overestimated by ~4.9 kyr or ~26.2 %, causing groundwater recharge rate underestimation and aquifer recovery time overestimation by ~46.0 % and ~26.2 %, respectively. High ³H concentration indicates modern water mixture in more than one fifth of groundwater samples, including those with high modified ¹⁴C apparent age (> 12 kyr, i.e., fossil groundwater) in the Central Valley and southern California, which are generally considered not to be recharged by modern water. Modern water mixture in old groundwater can potentially bring anthropogenic contamination to these groundwater resources, which should be paid attentions by the government and the managers. The results have important implications in evaluation of groundwater replenishment and its susceptibility to modern contamination in California, and in groundwater resources estimation globally.