Tritium-Helium Dating of Groundwater in a Fractured and Karstified Carbonate Rock Aquifer

Fractured and karstified aquifers are often vulnerable to pollution by nitrate or other anthropogenic compounds. Therefore, a better understanding of the flow and transport processes in these aquifers is imperative for effective drinking water management. In this study, we used the analysis of tritium and tritiogenic helium-3 concentrations to estimate the residence and exposure time of nitrate transported in a fractured groundwater system of the Upper Muschelkalk in southwest Germany.

The recharge area is characterised by elevated nitrate concentrations of up to 60 mg/L which are in accordance with dominating agricultural landuse in the catchment. Further along the groundwater flow direction a significant decrease in dissolved oxygen as well as nitrate concentrations to values close to the detection limit is observed.

Tritium/³He ages were found to be in the range of zero to forty years. However, in the fractured aquifer the age tracers were most probably affected by mixing and exchange processes that might change the concentration as well as the ratio of tritium and helium-3 in addition to radioactive decay. Therefore, we investigated the impact of different transport processes such as mixing of water parcels at fracture joints or exchange between mobile water on fractures and the pore matrix using forward convolution approaches for both isotopes separately.

In combination with hydrochemical, multi-isotopic, petrographical, and molecular biological data, the groundwater residence time data was intended to gain crucial insight into the processes and limiting factors of autotrophic denitrification found within the Muschelkalk aquifer.