Mine water hydrogeochemistry of abandoned coal mines in the outcropped Carboniferous formations, Ruhr Area, Germany

A hydrogeochemical study was conducted in the outcropped part of the coal-bearing Upper Carboniferous formations in the Ruhr area to investigate the characteristics of regional mine water and understand the hydrogeochemical conditions. Twenty-eight mine water samples from the adits of different abandoned coal mines were collected and analyzed for major ions, iron (Fe2+, Fetotal) and sulfide (H2S). The water samples had a pH of 6.39–7.65. Total dissolved solids (TDS) ranged from 157 to 1806 mg L−1. Mine water samples were dominated by HCO3− > Ca2+ > SO42− > Mg2+ > Na+ > Cl− > K+ > NO3− > (F−, Li+). Several water samples had elevated SO42−, Fetotal, and H2S concentrations up to 354 mg L−1, 4.2 mg L−1, and 10 µg L−1, respectively. The water types (Ca–Mg–HCO3–SO4, Na–Ca–HCO3–SO4, Ca–Mg–Na–HCO3–SO4, and Na–Ca–Mg–HCO3–SO4) were the dominant hydrogeochemical facies. Gypsum and halite were always undersaturated, while calcite and dolomite were undersaturated in about 82% of water samples. The chemical weathering processes of pyrite, calcite, dolomite, gypsum, and halite, as well as ion exchange, appear to be the dominant hydrogeochemical processes controlling adit mine water chemistry. In general, there was an increasing trend of TDS and major ion concentrations from south to northeast in the study area. This study may aid in the improvement of water management in the Ruhr area since the adits discharge into the river Ruhr which is strongly used for the drinking water supply of a large metropolitan region in western Germany.