Paper No. 21
Presentation Time: 2:00 PM
RECONSTRUCTING PAST SULFUR LOADING IN LITTLE SANDY LAKE, A WILD RICE LAKE IN NORTHERN MINNESOTA
Iron mining has increased dissolved sulfate concentrations in northern Minnesota waterways throughout the 20th century and the proposed copper-nickel mining (“sulfide mining”) in the Duluth Complex may release additional sulfate to the surrounding lakes and rivers. This potential pollution is of concern because of the risks to wild rice (Zizania sp.), which is a sacred plant to the Ojibwe and the state grain of Minnesota. Wild rice is observed to grow poorly in waters with sulfate levels above 10 mg/L, which is the legal standard for Minnesota wild rice water bodies. Little Sandy Lake, located in St. Louis County near Virginia, Minnesota, lies approximately one mile downstream from a taconite tailings basin that was constructed in 1966. Sediment cores were extracted from Little Sandy Lake to determine whether the lake has been impacted by mining activity. The tops of the three surface cores showed a 5-10 cm layer of black sediment, which is an indicator of the presence of iron sulfide compounds. The cores were analyzed for sulfur counts using scanning x-ray fluorescence (XRF) and dated using lead-210 (210Pb) radiometric dating. The data show that there is a significant increase in sulfur levels after the 1960’s, following hundreds of years of very low sulfur levels. Current water sample data show sulfate values in Little Sandy Lake are 80-200 mg/L, down from 300-400 mg/L and even higher levels in the past. Sulfur geochemical data are being used to understand historical sulfur inputs to the lake and the effects of land use changes on wild rice abundance. Sediment accumulation data show earlier periods of erosion in Little Sandy Lake, possibly related to other anthropogenic activities such as logging. The sedimentology of a near shore core shows that lake level has recently increased, but this event cannot be dated directly. The increases in lake level could have contributed to the decline of wild rice, as wild rice grows in shallow waters. Beaver damming of the lake’s outlet stream may have been the cause of the water level rise.