Degassing of ³H/³He, CFCs and SF₆ by denitrification: Measurements and two-phase transport simulations

The production of N₂ gas by denitrification may lead to the appearance of a gas phase below the water table prohibiting the conservative transport of tracer gases required for groundwater dating. We used a two-phase flow and transport model (STOMP) to study the reliability of ³H/³He, CFCs and SF₆ as groundwater age tracers under agricultural land where denitrification causes degassing. We were able to reproduce the amount of degassing (R ² =69%), as well as the ³H (R ² =79%) and ³He[low asterisk] (R ² =76%) concentrations observed in a ³H/³He data set using simple 2D models. We found that the TDG correction of the ³H/³He age overestimated the control ³He/³He age by 2.1 years, due to the accumulation of ³He[low asterisk] in the gas phase. The total uncertainty of degassed ³H/³He ages of 6 years (±2 σ) is due to the correction of degassed ³He[low asterisk] using the TDG method, but also due to the travel time in the unsaturated zone and the diffusion of bomb peak ³He[low asterisk]. CFCs appear to be subject to significant degradation in anoxic groundwater and SF₆ is highly susceptible to degassing. We conclude that ³H/³He is the most reliable method to date degassed groundwater and that two-phase flow models such as STOMP are useful tools to assist in the interpretation of degassed groundwater age tracer data.