Geochemistry and age of groundwater in a hydrochemically diversified aquifer (Permo-Carboniferous, the Intra- Sudetic Synclinorium, SW Poland) derived from geochemical modelling and isotopic studies

Abstract

Comprehensive investigations of groundwater were performed in a sedimentary aquifer of Permo-Carboniferous, Intra-Sudetic Synclinorium, in SWPoland. The investigation included aqueous chemical and isotopic composition, chemistry of mineral phases, geochemical modelling, and tritium and radiocarbon groundwater dating. Chemical diversity in the groundwater system is created by the mixing of modern fresh water and older sulphate water with higher dissolved solids. The system is treated as a system of flows of two end-member water types. Geochemical modelling is used for: (1) explaining the origin of the chemistry of both water components, (2) quantifying the groundwater mixing, (3) correcting the radiocarbon age of the groundwater for the effects of chemical water-rock interactions, and (4) calculating reaction rates. Study of stable (C, S, O, H) and unstable (³H, ¹⁴C) isotopes allowed the inverse mass balance geochemical models to be verified and specified, and the groundwater to be dated. The chemistry of the modern, tritium-bearing, fresh water is a result of dissolution of limestones, dolomites and gypsum. The mean tritium-age of this water, based on the lumped-parameter approach, varies between 10 and 200 years. The sulphate mineral water owes its chemistry to the process of dedolomitization driven by gypsum dissolution. Its radiocarbon age is about 5.9 ka BP, i.e., during the Mid-Holocene Climatic Optimum. Rates of chemical reactions responsible for the formation of sulphate type water are estimated to be: dissolution of gypsum (2.85 μmol/L/year) and dolomite (0.21 μmol/L/year), calcite precipitation (0.20 μmol/L/year), organic matter decomposition (0.08 μmol/L/year).