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.