GROUNDWATER AGE DATING WITH GAS TRACERS: THE ROLE OF GAS PARTITIONING

Several environmental tracer methods that are used for groundwater age dating are based on gases, e.g., CFCs, SF₆, or He. The partitioning of gases between groundwater and any gas phase in the subsurface is therefore of central importance for these methods. The atmospheric noble gases are powerful tools to trace and study the gas partitioning because of their ideal conservative behavior and wide range of solubilities. Many noble gas studies have clearly shown the widespread presence of a gas excess above atmospheric solubility equilibrium (so-called “excess air”) in groundwater. Sometimes, however, undersaturations are also found, which are attributed to degassing.

Both excesses and deficits of dissolved gases in groundwater can be explained by interactions with trapped gas bubbles. Excess air appears to be closely related to water table rises and related pressure increases in the recharge zone. Degassing could be due to the accumulation of geogenic or biogenic gases and/or a pressure decrease in the groundwater discharge area. The concept of closed-system equilibration between groundwater and trapped bubbles (CE-model) proved successful in modeling excess air. It can also be used to model the loss of dissolved gases that occurs if gas bubbles form in the subsurface. This approach might be more appropriate than alternative models assuming a diffusion-controlled degassing process.

Measurements of dissolved noble gases in groundwater enable a reliable description of the atmospheric gas components, i.e. the equilibrium concentrations modified by excess air or degassing. This provides an ideal basis for the determination of recharge temperatures and gas tracer ages. Even if such complete information is not available, the gas components relevant for groundwater age dating can be estimated within reasonable uncertainty ranges. The susceptibility of the different gas tracer methods to the effects of gas partitioning and ways to account for these effects are discussed.