Numerical simulations of dating young groundwater with multiple atmospheric tracers:: CFC-11, CFC-12, SF6, 3H/3He and 85Kr

Dating groundwater with atmospheric tracers is becoming an important technique for investing hydrology systems. Because the groundwater age dated by a single tracer generally involves high uncertainty and the multiple-tracer approach is assumed to be much more reliable due to cross-validation. the multiple-tracer has recently become the suggested technique. However, the numerical methods used for simulating groundwater ages based oil multiple tracers' concentrations are limited. In addition, the multiple-tracer method requires many evaluations before applications can be considered reliable. In this study we use a rigorous modeling approach. which relies on the backward-in-time random particle tracking solutions of the adjoint of the forward advection-dispersion equations, to simulate the groundwater ages. The atmospheric tracers we selected include Chlorofluorocarbons, sulfur hexafluoride, tritium/helium and krypton 85. In this analysis we build multiple detailed, three-dimensional geostatistical realizations representing complex subsurface heterogeneity. Then we simulate the backward-in-time transport of multiple tracers from the capture monitoring wells to the water table, by considering the advection, mechanical dispersion. molecular diffusion and radioactive decay of tracers represented by high-resolution particles in numerical models. Then the multiple-tracer approach is evaluated systematically by considering various distributions of groundwater age underlying a single water sample. Results show that the simulated Chlorofluorocarbon-based ages match the measured ones. Results also show that there is apparel-it discrepancy between ages dated by different tracers. This discrepancy could contribute to the decreased reliability of the multiple-tracer approach or cause misleading results. The dating discrepancy between multiple tracers results from the variations of growth rate of the historic atmospheric tracer concentration at different recharge ranges. Therefore, the multiple-tracer dating approach is not appropriate for cross-validation unless we are aware of this discrepancy.