3.6 Discussion of Tracer‑Specific Issues

Because it is part of the water molecule, [latex]_{}^{3}H[/latex] is nearly an ideal tracer for groundwater, and advection and dispersion will have essentially the same effects on tritiated water [latex](HTO)[/latex] as [latex]H_{2}O[/latex]. As previously discussed, a small fractionation between water vapor and liquid results in the vapor phase being depleted in [latex]_{}^{3}H[/latex] relative to the liquid. Nevertheless, [latex]_{}^{3}H[/latex] can be transported in the vapor phase and this results in important differences between [latex]_{}^{3}H[/latex] transport and dissolved ions in the unsaturated zone. For example, Phillips and others (1988) showed that [latex]_{}^{3}H[/latex] in the unsaturated zone in arid regions was transported at a faster rate than [latex]_{}^{36}{Cl}[/latex] because [latex]_{}^{3}H[/latex] was transported in both vapor and liquid phases, while [latex]_{}^{36}{Cl}[/latex] was only transported in liquid phase. Significant [latex]_{}^{3}H[/latex] transport in the vapor phase has also been observed in the vicinity of nuclear reactors (Noguchi et al., 2001).