Diffusive fractionation of ³H and ³He in groundwater and its impact on groundwater age estimates

Abstract

The use of 3 H and 3 He to estimate the apparent age of groundwater presumes that differences in their transport behavior have little effect on 3 H‐ 3 H ages. Here we show that 3 H and 3 He are highly susceptible to diffusive fractionation, which can result in substantial errors in 3 H‐ 3 He apparent ages of post‐1950, prebomb peak groundwater. Previous analyses in homogeneous and weakly heterogeneous systems suggested that significant diffusive fractionation effects on 3 H‐ 3 He ages only manifest at low velocities. We show that the potential for diffusive fractionation depends strongly on the subsurface heterogeneity, flow field, and 3 H source. Results indicate that within aquitards dominated by diffusive mass transfer, diffusive fractionation can lead to artificially old apparent age dates. Along preferential flow paths, diffusive fractionation can lead to artificially young apparent age dates. Along preferential flow paths strongly affected by diffusive mass transfer, this phenomenon can result in the same 3 H‐ 3 He apparent age for postbomb peak and older post‐1950, prebomb peak groundwater. Therefore, where nonunique age dates are plausible, it may be difficult to distinguish postbomb peak groundwater from post‐1950, prebomb peak groundwater affected by diffusive fractionation in the absence of additional hydrogeologic information sufficient to delineate the effects of diffusive fractionation.