『Abstract
　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, CDCs and SF6 as groundwater age tracers under agricultural and where denitrification causes degassing. We were able to reproduce the amount of degassing (R²=69％), as well as the ³H(R²=79％) and ³He^*(R²=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 ³H/³He age by 2.1 years, due to the accumulation of ³He^* in the gas phase. The total uncertainty of degassed ³H/³He ages of 6 years (±2σ) is due to the correction of degassed ³He^* using the TDG method, but also due to the travel time in the unsaturated zone and the diffusion of bomb peak ³He^*. CFCs appear to be subject to significant degradation in anoxic groundwater and SF6 is highly susceptible to degassing. We conclude that ³H/³ 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.

Keywords: ³H/³; CFCs; SF6; Groundwater dating; Two-phase transport modeling; Denitrification; Degassing』

1. Introduction
2. Study area
3. Model
　3.1. Model description
　3.2. Model limitations
　3.3. １D simulation
　3.4. 2D simulations
　3.5. Model summary
4. Accuracy of ³H/³He groundwater ages
5. Results
　5.1. 1D model
　5.2. 2D model calibration
　5.3. Model performance at all 14 locations
　5.4. Hydrological effect of degassing on groundwater age
　5.5. Accuracy of ³H/³He ages of degassed groundwater
6. Discussion and conclusions
Acknowledgements
References