Groundwater geochemistry in shallow aquifers above longwall mines in Illinois, USA

Aquifers above high-extraction underground coal mines are not affected by m ine drainage, but they may still exhibit changes in groundwater chemistry d ue to alterations in groundwater flow induced by mine subsidence. At two ac tive longwall mine sites in Illinois, USA, glacial-drift aquifers were larg ely unaffected by mining, but the geochemistry of the bedrock aquifers chan ged during the post-mining water-level recovery. At the Jefferson site, bra ckish, high-sulfate water present in the upper bedrock shale briefly had lo wer values of total dissolved solids (TDS) after mining due to increased re charge from the overlying drift, whereas TDS and sulfate increased in the s odium-bicarbonate water present in the underlying sandstone due to downward leakage from the shale and lateral inflow of water through the sandstone. At the Saline; site, sandstones contained water ranging from brackish sodiu m-chloride to fresh sodium-bicarbonate type. Post-mining recovery of the po tentiometric levels was minimal, and the water had minor quality changes. L ongwall mining affects geochemistry due to subsidence-related fracturing, w hich increases downward leakage from overlying units, and due to the tempor ary potentiometric depression and subsequent recovery, whereby water from s urrounding areas of the aquifer recharges the affected zone above and adjac ent to the mine.