A general reactive transport modeling framework for simulating and interpreting groundwater ¹⁴C age and δ ¹³C

A reactive transport modeling framework is presented that allows simultaneous assessment of groundwater flow, water quality evolution including δ¹³C, and ¹⁴C activity or "age". Through application of this framework, simulated ¹⁴C activities can be directly compared with measured ¹⁴C activities. This bypasses the need for interpretation of a ¹⁴C age prior to flow simulation through factoring out processes other than radioactive decay, which typically involves simplifying assumptions regarding spatial and temporal variability in reactions, flow, and mixing. The utility of the approach is demonstrated for an aquifer system with spatially variable carbonate mineral distribution, multiple organic carbon sources, and transient boundary conditions for ¹⁴C activity in the recharge water. In this case, the simulated ¹⁴C age was shown to be relatively insensitive to isotopic fractionation during DOC oxidation and variations in assumed DOC degradation behavior. We demonstrate that the model allows quantitative testing of hypotheses regarding controls on groundwater age and water quality evolution for all three carbon isotopes. The approach also facilitates incorporation of multiple environmental tracers and combination with parameter optimization techniques.