Hydrogeochemical processes of critical zone developed in Tégulines Clay, Paris Basin: Hydrogeochemical and multi-isotopic approach (δ13C, δD, δ18O, 87Sr/86Sr and 14C)
Résumé
Water transfers and processes governing the chemistry of groundwaters and clay pore waters were investigated in the critical zone developed in Tégulines Clay in the area of Brienne-Le-chateau, at east of the Aube alluvium plain (France). A pit-digging campaign along two-stepped west-facing hillslopes along the Brevonne valley and the Aube/Brevonne paleo-valley gives evidence of a thin Quaternary clay silty and carbonate-free loams overlying Tégulines Clay, mobile regolith along the slopes and the presence of a carbonate alluvium layer in pits of the Brevonne valley. Groundwaters at the first ridgetop attest of a temporary perched water table, while groundwaters at top of Tégulines Clay along the first west-facing steep slope toward the Brevonne River provide evidence of lateral groundwater transfers and runoff of waters toward the valley which is a discharge zone belonging to the present-day Aube alluvium plain. Groundwaters have low concentrations of Total Dissolved Solids (TDS < 600 mg/L) and are calcic-carbonate waters. Their δ18O and δD of groundwaters are consistent with local meteoric values. An 18O and D enrichment in groundwaters at the bottom of the second hillslope and in waters from reservoirs indicates evaporation processes. The δ13CCO2 of dissolved inorganic carbone in groundwaters is essentially due to degradation of organic matter. The composition of the pore waters from Tégulines Clay in boreholes are chemically heterogeneous and different from groundwaters. They are Ca-Mg-SO4-rich and show a large range of TDS concentrations (592–6457 mg/L). The highest values are measured in the most intensely weathered clay developed in the first 10–15 m under the first ridge top and the low east-facing slope of the Aube/Brevonne paleo-valley. This latter seems to represent a large intermediate discharge zone between the coarse ridgetops (recharge zones) and the current Aube alluvium plain. The δ18O and δD of pore waters are alined along the local meteoric water line, indicating that meteoric waters have diffused through the clay formation during post-depositional history and replaced original connate seawater. On contrary the 87Sr/86Sr ratios of pore waters remain almost similar to that of Lower Cretaceous seawater, suggesting a limited diffusion of Sr since the formation deposition. Toward the top of the formation in the Aube/Brevonne paleovalley, major cation and anion concentrations, 18O and D enrichment and a slight increase of the 87Sr/86Sr ratios suggest the diffusion of evaporated waters. The δ13C, δ18O, 87Sr/86Sr ratios and 14C activity of concretions at top of Tégulines Clay show that they precipitated from evaporated old groundwaters, the oldest one at ∼ 34 ky in the TPH2-1 pit at the ridgetop of the second hillslope and a younger age of ∼ 8 ky in the TV4-1 pit.