Reconstructed Precipitation Tritium Leads to Overestimated Groundwater Recharge

Abstract

15 m deep, exceptionally well preserved unsaturated zone tritium profiles to quantify the uncertainty in Tritium mass balance from precipitation tritium reconstruction. The tritium profiles have bomb tritium peaks at 7.2–10.5 m below the surface and peaks ranging from 46 to 235 tritium unit. We first estimate the diffuse recharge by the tritium peak method. Since this method is independent of any precipitation tritium reconstruction, the estimated recharge was used as truth to evaluate four precipitation tritium reconstruction methods: two interpolation methods and two reference curve methods. Direct comparison between the observed and simulated precipitation tritium showed that the reference curve methods performed more poorly than the interpolation methods. Indirect evaluation by comparing the recharge rates estimated by the tritium storage method with our truth measurements showed that tritium storage method overestimated recharge by 100% to 200%. Our results suggest that the atmospheric tritium flux was greatly underestimated, especially by the reference curve methods. As such, groundwater recharge at our sites would be overestimated. This has implications for using these standard approaches elsewhere.