2. Dissolved Gases in Groundwater

Except for [latex]_{}^{3}H[/latex], the age‑dating tracer methods in this book use dissolved gases. In this section, we discuss the basic concepts of equilibrium gas solubility as a function of temperature and pressure and the occurrence of gas that is not in equilibrium with the fluid due to trapped bubbles. These concepts are illustrated using the noble gases, but they apply to [latex]{SF}_{6}[/latex] and CFCs.

Many gases are relatively inert with respect to geochemical reactions in the subsurface; thus, they behave as conservative tracers. However, when both gas and liquid phases are present, all gases will partition into both phases, which can lead to very different transport behavior than that of conservative ionic tracers in groundwater. As such, the existence of both gas and liquid phases and the solubility of a given gas are important for relating the movement of dissolved gas tracers to the movement of groundwater.