Geochemical Controls for Al, Fe, Mn, Cd, Cu, Pb, and Zn during Experimental Acidification and Recovery of Little Rock Lake, WI, USA

• Published in: Biogeochemistry 2003-02, Vol.62 (2), p.119-143
• Main Authors: Brezonik, Patrick L., Mach, Carl E., Sampson, Carolyn J.
• Format: Article
• Language: English
• Subjects: Acidification; Alkalinity; Aluminum; Basins; Earth sciences; Earth, ocean, space; Exact sciences and technology; Freshwater; Geochemistry; Gibbsite; Hydrology; Hydrology. Hydrogeology; Lakes; Mineralogy; Minerals; Rocks; Sediments; Silicates; Soil water; Solubility; Surface water; Water column; Water geochemistry; Zinc
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1023/A:1021167917839

Concentrations of Al, Fe, Mn, Cd, Cu, Pb, and Zn were measured in the reference and treatment basins of Little Rock Lake (Vilas County, Wisconsin), a low-alkalinity, seepage system (pH ∼6.1, alkalinity 25 μeq/L) during six years of a whole-basin acidification and the first four years of the lake's recovery. The treatment basin was acidified with H2SO4 in three two-year steps to pH ∼5.6, ∼5.1, and ∼4.7. By the end of year 4 of recovery, treatment basin pH increased to ∼5.3 as a result of internal alkalinity generation. During acidification, dissolved Mn and Fe (0.4 μm pore-size filters) increased at pH ∼5.6; dissolved Al, Cd, and Zn became elevated at pH ∼5.1; and dissolved Pb at pH ∼4.7. Dissolved Cu remained similar in both basins to pH ∼4.7. Al, Fe and Mn levels declined significantly during the recovery period, approaching values at pH 5.3 intermediate 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 for solubility of gibbsite (Al(OH)3) and amorphous Fe(OH)3 (s). The acidified basin was undersaturated relative to gibbsite, and dissolved Al was limited by pH disequilibrium between the water column and sediments and possibly by Al-DOC precipitation. Dissolved Fe apparently was controlled by solubility 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 the treatment basin during acidification probably resulted from less efficient scavenging of atmospherically-deposited Cd, Pb, and Zn by settling particles.