A method for groundwater ¹⁴C age correction accounting for mixing with young and old groundwaters: application to an alluvial aquifer in the Jianghan Plain, China

The distribution of groundwater ¹⁴C ages can indicate the direction of groundwater flow. Groundwater ¹⁴C age is based on ¹⁴C radioactive decay and the ¹⁴C activity (a¹⁴C) of dissolved atmospheric carbon, but hydrochemical processes can change the groundwater a¹⁴C. A series of ¹⁴C correction models have been proposed to eliminate the influence of a¹⁴C change on ¹⁴C dating. However, mixing in the groundwater flow path and in the groundwater discharge area can also change the groundwater a¹⁴C. The classical ¹⁴C correction models do not clearly identify groundwater mixing, and their results provide mixed ¹⁴C ages. Hydrochemical components that have gradually accumulated along a groundwater flow path can be used to identify mixing and to correct a¹⁴C values. The relationship between SO₄²⁻ concentrations and a¹⁴C values is used to identify groundwater mixing in the Jianghan Plain, central China. The plotted ¹⁴C vs. δ¹³C relationship indicates that the classical IAEA model is suitable for most samples taken from a confined aquifer. The a¹⁴C values of some samples are corrected by projecting them onto a mixing line between young groundwater and old groundwater. Having been corrected by the Mixing-IAEA model, the ¹⁴C ages of groundwater mixed with young groundwater are found to have been underestimated and were recalculated to be 3.41–12.11 ka, and the ¹⁴C ages of groundwater mixed with old groundwater are found to have been overestimated and were recalculated to be 4.97–10.78 ka. Comparison of ¹⁴C ages corrected by the Mixing-IAEA model with the IAEA model allows improved interpretation of groundwater flow directions.