SEASONAL VARIATIONS IN TRACE METAL ASSOCIATION WITH IRON AND MANGANESE (HYDR)OXIDES IN THE KALAMAZOO RIVER WATERSHED (KALAMAZOO, MI, USA)

Trace metals in natural sediments are frequently associated with iron and manganese (hydr)oxides. Because seasonal changes in sediment redox, especially near the sediment-water interface (SWI), can lead to alternating periods of reductive dissolution and oxidation/reprecipitation of these phases, trace metal speciation and bioavailability near the SWI may depend strongly on season. Seasonal variability of trace metal speciation associated with seasonal changes in vertical redox stratification was studied in freshwater lake and marsh sediments from the Kalamazoo River watershed. Pore waters and sediments were sampled anaerobically at 1-2 cm intervals from the SWI to ~50 cm depth during fall 2001 and winter and spring 2002. Fresh pore waters were analyzed for pH, alkalinity, dissolved Fe(II)/Fe(III), Mn(II), sulfide, sulfate, ammonium and phosphate. A two-step sequential extraction procedure was used to assess the concentration of trace metals (Cr, Co, Cu, Zn, Cd, Pb and U) associated with amorphous and crystalline Fe and Mn (hydr)oxides. The first step, extraction with ascorbic acid for 24 hours, was intended to liberate primarily amorphous, “readily microbially reducible” phases, and the second step, extraction with 0.5M HCl for 1 hour, was intended to liberate primarily crystalline Fe and Mn (hydr)oxide minerals (Kostka and Luther, 1994, GCA 58, 1701). Consistently higher concentrations of Fe, Mn, Cr and U were present in the ascorbic acid fraction, while Pb, Zn and Cu were always enriched in the HCl fraction. Relative concentrations of ascorbate extractable Fe, Pb, Zn, and sometimes Cu and Co, were typically quite similar, decreasing precipitously over the upper 10-20cm, the same depth interval where pore water Fe(II) concentrations increased. HCl extractable Fe, Co, Cu, Pb and Zn depth profiles were also similar, but were usually constant with depth. Relative concentrations of Mn and Cr in both the ascorbate and HCl fractions were constant or increased slightly with depth. Spatial variations between multiple cores was similar in magnitude to temporal variations among fall, winter and spring. Sampling planned for summer 2002, when microbial reductive dissolution of Fe and Mn (hydr)oxides is expected to be most intense, may shed more light on the seasonal variation of trace metal speciation at these sites.