Ferric Oxyhydroxylsulfate Precipitation Improves Water Quality in an Acid Mining Lake: A Hydrogeochemical Investigation

Hydrogeochemistry of a lignite pit lake in Lusatia, Germany, was investigated. Anoxic groundwater from the dump aquifer rich in FeII (average ~5911 µmol/L) and SO4 (average ~14,479 µmol/L) contents enter the lake as subsurface inflow; oxidation and subsequent precipitation of poorly crystallized Fe-oxyhydroxysulfate (schwertmannite) occurs and causes acidification (pH~2.8). However, the removal of dissolved loads as solid phases significantly improves the groundwater quality of the downgradient as an outflow. The rainwater isotopic values (δD ~−8.88‰ and δ18O ~−65.86‰) closely matched with the groundwater showing very little isotopic modification, which points to a short residence time of groundwater. The displacement of δD and δ18O values (slope = 5.3) from the meteoric water line reflected the evaporative enrichment of the lake water. The isotopic signature also revealed longer residence times of epilimnion than the hypolimnion waters which are dominated by groundwater. The lake is dimictic and showed abrupt changes in physicochemical parameters along the interface (~0.30 m thick) when separating the epilimnion (upper 4 m) from the hypolimnion (bottom 1.5 m). Lake sediments were found to be dominated by clay size fraction occurring as laminations (thickness: 1~0.5 mm) that reflect seasonal sedimentation. Higher schwertmannite formation in the south as compared to the north (recharge side) also serves as a scavenger of potentially toxic elements which is probably a natural solution to man-made problems. Schwertmannite transformation to goethite releases sulfate which is reduced and fixed as secondary sulfide minerals over time. Overall, waters are of a Ca–SO4 to Ca–Mg–SO4 type with distinct inflow (FeII/FeIII > 2.5) and outflow (FeII/FeIII < 0.5) of groundwater.