Uncertainties in tritium mass balance models for groundwater recharge estimation

Understanding the time that water takes to traverse the vadose zone is important to quantify groundwater recharge rates and to assess the vulnerability of aquifers to contamination. Tritium mass balance models provide measurement-driven estimates of soil water velocities, yet uncertainties embedded in these groundwater recharge estimates are rarely investigated. Here we analyze six > 15-m tritium profiles collected from thick unsaturated zones located on China's Loess Plateau and compare recharge rates obtained from various calculation approaches. Models used to estimate recharge based on unsaturated zone data include the tritium peak method and the tritium storage method; models used to estimate recharge using saturated zone data include the 'modern groundwater' method and a method that assumes groundwater is well-mixed in the aquifer. The tritium peak method presents reliable recharge; however, the recharge from the tritium storage method is two times of that from the tritium peak method. The well-mixed reservoir method overestimates groundwater renewal rates because the implicit assumption that groundwater is well-mixed in aquifers is not realistic. For potential recharge, considerable uncertainties arise from precipitation tritium reconstruction; while for actual recharge estimation, uncertainties come from the mismatched recharge mechanism between the model assumption and local characteristics. Selection of appropriate tritium mass balance models is of utmost importance for recharge estimation.