Geochemical controls for Al, Fe, Mn, Cd, Cu, Pb, and Zn during experimental acidification and recovery of Little Rock Lake, WI, USA

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Concentrations of Al, Fe, Mn, Cd, Cu, Pb, and Zn were measured in thereference and treatment basins of Little Rock Lake (Vilas County, Wisconsin), alow-alkalinity, seepage system (pH ∼6.1, alkalinity25 μeq/L) during six years of a whole-basinacidificationand the first four years of the lake's recovery. The treatment basin wasacidified with H2SO4 in three two-year steps to pH∼5.6, ∼5.1, and ∼4.7. By the end of year 4 of recovery, treatmentbasin pH increased to ∼5.3 as a result of internal alkalinity generation.During acidification, dissolved Mn and Fe (0.4 μmpore-size filters) increased at pH ∼5.6; dissolved Al, Cd, and Zn becameelevated at pH ∼5.1; and dissolved Pb at pH ∼4.7. Dissolved Cu remainedsimilar in both basins to pH ∼4.7. Al, Fe and Mn levels declinedsignificantly during the recovery period, approaching values at pH 5.3intermediate between the concentrations at pH 5.6 and 5.1 during acidification.Dissolved Al and Fe in the reference basin were near the equilibrium levels forsolubility of gibbsite (Al(OH)3) and amorphousFe(OH)3(s).The acidified basin was undersaturated relative to gibbsite, and dissolved Alwas limited by pH disequilibrium between the water column and sediments andpossibly by Al-DOC precipitation. Dissolved Fe apparently was controlled bysolubility of amorphous Fe(OH)3(s) and Fe-DOC precipitation.Dissolved Mn levels in both basins were consistent with manganite[γ-MnOOH(s)] solubility. Elevated levels of Cd, Pb, and Zn in thetreatment basin during acidification probably resulted from less efficientscavenging of atmospherically-deposited Cd, Pb, and Zn by settling particles.