SULFATE REDUCTION IN A ZINC CONTAMINATED BACK WATER LAKE: LAKE DEPUE, IL

We have examined sulfate reduction in a contaminated lake situated in DePue, IL. Sulfate reducers play an important role in cycling of metals and nutrients in aquatic systems. Therefore a study of sulfate reduction can help to elucidate the inner-workings of the lake ecosystem. Since the early 1900s, Lake DePue has been subjected to pollution from a zinc smelting plant, a fertilizer plant, and random sulfuric acid depositions. Often times, the presence of pollutants, especially zinc, can retard microbial processes and in many cases, sulfate reduction ceases. We chose two sites within the lake based on their distance from an abandoned zinc slag deposit. A small stream flows from the slag deposit into the lake.

At each site a sediment core was collected using a plexiglas piston core. Surface water was filtered (0.45 µ PTFE filter) and stored in a Falcon tube at 4˚C until tested. To determine sulfate reduction rates, surface sediments (0-5 cm) from both the sites were dredged using a stainless steel spatula and thoroughly mixed under nitrogen. Individual 50 ml Falcon tubes were completely filled with sediment and were incubated inside a glove bag. At regular time intervals, triplicate sample tubes were sacrificed to measure the concentration of SO₄^2-, Alkalinity, Fe_(Tot), Mn²⁺, NH₄⁺ and PO₄^3- in the pore water.

The results show high rate of sulfate reduction closer to the slag deposit, which is exactly opposite to the expected results considering zinc is supposed to inhibit sulfate reduction. This suggests that the microbes have adapted to high zinc concentrations in the sediment; however, sulfate reduction rates are limited by pore water sulfate concentrations. The results also suggest that iron and manganese reduction also contribute to overall carbon degradation in lake DePue.