Short-Term Changes in the Chemistry of Trace Metals Following Calcium Carbonate Treatment of Acidic Lakes

Abstract

Marked changes were evident in the concentrations of trace metals (aluminum, manganese, and zinc) following base application to two acidic lakes in the Adirondack region of New York. Immediately after calcite treatment, epilimnitic pH values increased above 9.0. This response coincided with a shift in the speciation of aluminum from acid-soluble to labile (inorganic) monomeric forms. Following the manipulation, pH values decreased below 8.0 as the epilimnion equilibrated with atmospheric CO2. Within 1–2 mo, aluminum, manganese, and zinc concentrations were greatly reduced, typically by an order of magnitude. The mechanism(s) for immobilization of trace metals following base treatment is not entirely clear. However, chemical equilibrium calculations suggest that at the higher pH resulting from base addition, lake water was oversaturated with readily forming aluminum and manganese minerals. Therefore, removal of these metals from the water column may have been due to direct precipitation. Solutions were highly undersaturated with zinc minerals, so zinc retention probably occurred by adsorption or coprecipitation reactions. Acidic Adirondack Lakes are generally net sinks for aluminum, and conservative with respect to transport of manganese and zinc. Base addition greatly enhanced retention of all three metals.