The geochemistry during management of lake acidification caused by the rewetting of sulfuric (pH < 4) acid sulfate soils

Understanding the geochemistry and kinetics of acidification events arising from acid sulfate soils is important to enable effective management and risk assessment. Large-scale exposure and oxidation of acid sulfate soils occurred during a drought in the Lower Lakes (Murray-Darling Basin) of South Australia. We examined the geochemical changes that occurred in one region (Boggy Lake) that experienced surface water acidification and was subsequently neutralised via aerial limestone (CaCO₃) dosing and dilution via natural lake refill. Very low pH (< 3) and high concentrations (≈10-1000mg/L Fe, Al, Mn) of dissolved metals were initially found in surface water. The water chemistry exhibited pH-dependent enhancement of constituents typically associated with acid sulfate soils (SO₄, Al and Fe). Geochemical speciation calculations indicated that most (60-80%) of the acidity was present as dissolved metal-sulfate complexes at low pH. X-ray diffraction (XRD) analyses showed that the orange-brown precipitates present after an initial limestone dosing were secondary oxyhydroxysulfate minerals (schwertmannite, jarosite). Further limestone dosing resulted in neutralisation of the pH, reduction in dissolved metal concentrations, dissolution of jarosite and schwertmannite precipitates, and formation of other metal oxyhydroxide phases. The results were consistent with a pE-pH diagram constructed for metal-sulfur geochemistry. Assessment of the measured and simulated (using PHREEQC) pH and Ca/Cl ratio during limestone dosing indicated that only about 25% of the limestone dissolved. XRD analyses suggested this passivation of the limestone was due to coating with gypsum and schwertmannite.