Chemical and isotopic evidence to clarify the salinity of the bedrock groundwater at Finnish mining site

Lithium mine in Ostrobothnia, Western Finland. Clear anomalous electrical conductivity horizons and high salinity (Cl, Na, Ca) of groundwater were identified in several exploration boreholes. As these could not be explained thoroughly with geological variations, further studies on the origin of the anomalies were suggested. It was speculated that the abrupt rise in electrical conductivity shows a horizon of waters of different ages, the lower being either old brine, or having ancient marine origin. To understand the processes controlling the hydrochemistry and the evolution of the salinity of the groundwater, the hydrogeochemistry and isotopic composition (δ2H, δ18O, 87 Sr/86 Sr, 3 H, 14 C, δ13C) was analysed in 13 boreholes, two lakes, a test pit and a stream. The isotope and geochemistry results indicate long residence times (up to 10,000 years BP), but no strong water-rock interaction and a typical Litorina SO4 layer is missing. The mismatching results could be explained by 1) the fact that even a small amount of ancient brine or ancient sea water mixing with fresh groundwater would affect water geochemistry, while the isotopic composition would stay immutable, or 2) the salinity originates from pre-Litorina SO4-poor brackish groundwater due to an earlier uplift from the Baltic Sea. Therefore, the groundwaters of the site are interpreted as a mixture of modern meteoric and fresh HCO3 water, brackish pre-Litorina subglacial groundwater, and glacial melt water.