Heterogeneities and interconnections in groundwaters: Coupled B, Li and stable-isotope variations in a large aquifer system (Eocene Sand aquifer, Southwestern France)

Abstract

The study of stable-isotope of the water molecule and elemental data for groundwater in the large Eocene sand aquifer system of SW France, showed that the δ18O values of groundwater fall in the range −5.6 to −10.6‰ vs. SMOW, with δ2H values varying between and −34.3 to −72.3‰ vs. SMOW. There is no relationship between δ18O and δ2H values and the salinity, but the stable isotopes data plot on or near the global meteoric-water line. Taken as a whole, the enriched samples clearly correlate with present-day recharge while the most depleted samples reflect an old recharge.Groundwaters in the present study display large ranges of both boron concentrations (7 to 2550μgL−1) and boron isotopic compositions, the lowest δ11B value being close to −18.0‰ and the highest one near +36.2‰. The scatter of B concentrations and δ11B values reflects the different rock types contributing to the supply of dissolved boron (carbonates, evaporites and silicates), as well as an input from rainwater and fractionation during water/rock interaction with clay minerals. Lithium concentrations in groundwater also span a wide range (0.6 to 253μgL−1) as do the lithium-isotopic signatures that range between +6.5‰ and +28.6‰. We can evidence there that the B isotopic compositions are controlled by water/rock interactions, while Li and δ7Li variations are also controlled by water/rock interaction processes (dissolution and formation of alteration minerals) that can occur at different temperatures within the groundwater system.