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Groundwater age, defined as the time since recharge, is useful for estimating groundwater renewal rates. Radiocarbon (C-14) activity has been widely applied for groundwater age dating since the 1950s. However, this approach can be complicated by other C-14-free carbon sources, which dilute the C-14 content at the time of groundwater recharge. Several corrective models have been introduced to determine the dilution factor using carbon isotopes and chemical mass balances, but these models do not account for the influence of deep crust and/or mantle (endogenic) carbon sources and are hence not applicable for dating groundwater affected by endogenic CO2. Therefore, we developed a new C-14 groundwater dating model including three carbon end-members (biogenically derived sedimentary carbon, carbonate carbon, and deep crust and/or mantle carbon) to quantify the dilution factor. This model was tested by age dating groundwater samples from the United States that were influenced by endogenic carbon (based on their delta C-13 value), showing that the conventional method overestimated the C-14 apparent age by similar to 19% on average. The average uncertainty for the conventional and new method is approximately 1.58% and 11.2%, respectively. The new method proposed here can be applied to groundwater age dating in other regions where aquifers are influenced by endogenic CO2. However, its application requires a priori identification of local geological, hydrogeological, and geochemical conditions.