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

Abstract

Aquifers above high-extraction underground coal mines are not affected by mine drainage, but they may still exhibit changes in groundwater chemistry due to alterations in groundwater flow induced by mine subsidence. At two active longwall mine sites in Illinois, USA, glacial-drift aquifers were largely unaffected by mining, but the geochemistry of the bedrock aquifers changed during the post-mining water-level recovery. At the Jefferson site, brackish, high-sulfate water present in the upper bedrock shale briefly had lower values of total dissolved solids (TDS) after mining due to increased recharge from the overlying drift, whereas TDS and sulfate increased in the sodium-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 sodium-chloride to fresh sodium-bicarbonate type. Post-mining recovery of the potentiometric levels was minimal, and the water had minor quality changes. Longwall mining affects geochemistry due to subsidence-related fracturing, which increases downward leakage from overlying units, and due to the temporary potentiometric depression and subsequent recovery, whereby water from surrounding areas of the aquifer recharges the affected zone above and adjacent to the mine.