U-series constraints on aquifer groundwater residence time: the Adour-Garonne district case (southwest France)

摘要

U-series were investigated in groundwaters from two water bodies of the Eocene sands aquifer, southwest France. The ~(234)U/~(238)U activity ratios are always larger than the equilibrium value, some waters being highly enriched in ~(234)U (as large as 13.5). In the southern water body, ~(234)U/~(238)U ratios correlate with ' C ages (with one exception). Such U isotopic signatures may allow the evaluation of the residence time of groundwaters within their aquifer using an open-system model based on U activity ratios and concentrations. It is suggested that the 'oldest' groundwaters from the southern water body are no older than 38 ky, whereas the maximum 'age' of groundwaters from the northern water body, though overall of the same order of magnitude, could be slightly younger (25 ky).