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The significance of ground water to the accumulation of iron and
manganese in the sediments of two hydrologically distinct lakes in
North-Central Minnesota: A geological perspective.
Dean, W.E. | Neff, B.P. | Rosenberry, D.O. | Winter, T.C. | Parkhurst,
R.
Ground Water. Vol. 41, no. 7, pp. 951-963. 2003
Williams and Shingobee lakes are at opposite ends of the local
ground water flow system in the Shingobee River Headwaters Area
(SRHA) in north-central Minnesota. Williams Lake, situated near
the highest point in the flow system, has no surface inlet or
outlet, and ground water and precipitation are the only sources of
water. Shingobee Lake, situated at the lowest point in the flow
system, has the Shingobee River as an inlet and outlet. Ground
water directly contributes an estimated one-fourth of the water
input to Shingobee Lake. The Shingobee River also receives large
amounts of ground water discharge along its reach to Shingobee
Lake providing a large, indirect source of ground water to the
lake. Differences in nutrient concentrations reflect the residence
times and nutrient supplies of these two lakes. The average
phosphorus content of Shingobee Lake is about twice that of
Williams Lake. Consequently, phytoplankton productivity in
Shingobee Lake is much higher than in Williams Lake, leading 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 and manganese are present
there during summer stratification. In some years, the manganese
concentration in the hypolimnion of Shingobee Lake remains high
throughout the year. Precipitation of iron and manganese minerals,
presumed to be X-ray amorphous oxyhydroxides, at periods of fall
and spring overturn result in concentrations of iron and manganese
in surface sediments of Shingobee Lake that are seven times and 27
times higher, respectively, than can be explained by contributions
of iron and manganese from detrital aluminosilicates. These
findings indicate that the source and amounts of this excess iron
and manganese found in the sediments are correlated to the amount
of iron and manganese rich ground water discharging to Shingobee
Lake. Because iron and manganese oxyhydroxides are efficient
adsorbers of phosphorus, concentrations of phosphorus are also
high in the sediments of Shingobee Lake. Without this
sequestration of phosphorus, the productivity of Shingobee Lake
would probably be much higher.
 Descriptors: Groundwater | Sediments | Dissolution | Geology
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