THE SIGNIFICANCE OF GROUNDWATER-SURFACE WATER INTERACTIONS TO THE ACCUMULATION OF IRON AND MANGANESE IN THE SEDIMENTS OF LAKES IN NORTH-CENTRAL MINNESOTA: A GEOLOGICAL PERSPECTIVE

Shingobee Lake, situated at the lowest point in the local groundwater flow system in the Shingobee River Headwaters Area (SRHA) in north-central Minnesota has the Shingobee River as its inlet and outlet. Groundwater directly contributes an estimated one fourth of the water input to Shingobee Lake. High productivity in Shingobee Lake leads to an oxygen-deficient (<1 ppm dissolved oxygen) hypolimnion within a month after overturn in both the spring and fall. Because of the extreme reducing conditions in the hypolimnion of Shingobee Lake, high concentrations of dissolved iron (Fe) and manganese (Mn) are present there during summer stratification. Concentration of Fe and Mn in the shallow springs and seeps around Shingobee Lake and along the Shingobee River are too low to explain these high concentrations. However, analyses of waters from deep wells (up to 130 m) show that concentrations of Fe and Mn are considerably higher, and it appears that deep groundwater is the most likely source of Fe and Mn that accumulate in the hypolimnion of Shingobee Lake during summer and winter stratification. Precipitation of Fe and Mn minerals, presumed to be endogenic X-ray amorphous oxyhydroxides, at periods of fall and spring overturn results in concentrations of Fe and Mn in surface sediments of Shingobee Lake that are 7 times and 27 times higher, respectively, than can be explained by contributions of Fe and Mn from detrital aluminosilicates. Holocene sediments in cores from Shingobee Lake, and from nearby Little Shingobee Lake and Little Shingobee Fen, contain even higher concentrations of Fe and Mn, In fact, almost all of the iron (Fe) and manganese (Mn) in the sediments in these cores are incorporated in endogenic minerals; almost no Fe and Mn are contributed from detrital aluminosilicate minerals. These findings indicate that the source and amounts of excess Fe and Mn found in the sediments of these lakes are correlated to the amount of Fe- and Mn-rich groundwater discharging to the lakes.