HELIUM ISOTOPE FLUXES AND GROUNDWATER AGES IN THE DOGGER AQUIFER, PARIS BASIN

Helium isotopic and rare gas measurements in the Dogger aquifer of the Paris Basin have shown the helium concentration and helium isotopic ratio to be first-order homogeneous with only slight differences in relation to specific chemical water types. Helium in the Dogger aquifer contains quantities of He-3 3-4 times in excess of radiogenic production/accumulation from natural decay of U, Th series elements. Calculations of cosmogenic production/accumulation of He-3 cannot account for the observed excesses, and He-3 excess is attributed to the addition of a mantle-derived He magmatic component. Calculations of the isotopic fluxes of helium out of the Dogger (He-4 flux, 2 x 10(-7) mol He-4 m-2 yr-1; He-3 flux, 1.8 x 10(-14) mol He-3 m-2 yr 1 (+/-50%)) suggest that the helium residence time and the groundwater residence time in the Dogger aquifer are 4 Myr (+/-50%). This age estimate is at least 10 times as great as estimates derived from hydrologic models but is more consistent with the geochemistry of the water.