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 delta O-18 values of groundwater fall in the range -5.6 to -10.6 parts per thousand vs. SMOW, with delta H-2 values varying between and -34.3 to -72.3 parts per thousand vs. SMOW. There is no relationship between delta O-18 and delta H-2 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 2550 mu g L-1) and boron isotopic compositions, the lowest delta B-11 value being close to -18.0 parts per thousand and the highest one near + 36.2 parts per thousand. The scatter of B concentrations and delta B-11 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 253 mu g L-1) as do the lithium-isotopic signatures that range between + 6.5 parts per thousand and + 28.6 parts per thousand. We can evidence there that the B isotopic compositions are controlled by water/rock interactions, while Li and delta Li-2 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.