Estimation of fossil groundwater mass fraction accounting for endogenic carbon input across California

California is facing water crisis, especially with significant depletion of groundwater resources. Recently, Jasechko proposed a method for estimation of the fossil groundwater mass fraction (Ffₒₛₛᵢₗ) based on ¹⁴C (Jasechko’s method), but it did not account for the endogenic carbon source. Based on carbon isotope and water chemistry balance, the ratio of the concentration of endogenic carbon to dissolved inorganic carbon (Cₑₙdₒgₑₙᵢc/DIC ratio) in groundwater is quantified, and the Jasechko’s method is modified with consideration of endogenic carbon input. Both Jasechko’s and modified methods are applied to estimate the Ffₒₛₛᵢₗ across California, where endogenic CO₂ degases at many locations. The results show that, averagely, the Cₑₙdₒgₑₙᵢc takes up ~23.4% of DIC in groundwater samples, with ~19.4% of samples owning Cₑₙdₒgₑₙᵢc/DIC ratio exceeding 50%. About 8.4% of groundwater samples show smaller Ffₒₛₛᵢₗ calculated by the modified than Jasechko’s method, with the mean value of Ffₒₛₛᵢₗ deceasing by ~3.1%. This indicates obvious mixture of endogenic carbon in DIC of groundwater across California, but it has little influence on the estimation of Ffₒₛₛᵢₗ. Even though, the process-based modified method is more objective than the empirically Jasechko’s method. Groundwater samples with high Cₑₙdₒgₑₙᵢc/DIC ratio and mass fraction difference of fossil groundwater between the two methods are mainly located in the Mojave Desert with wide distribution of main active faults. The average Ffₒₛₛᵢₗ calculated by both methods are ~70%, demonstrating the dominance of fossil groundwater across California. These results are beneficial to better understand the ¹⁴C dating method and the groundwater resources in California.