Long Term BOD Analysis of the Big Eau Pleine Reservoir, Wisconsin, as Related to BOD Source
Swalby, Louie J.
University of Wisconsin-Stevens Point, College of Natural Resources
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Long term BOD data were collected from the Big Eau Pleine Reservoir, Wisconsin to: 1) determine the source and composition of the BOD; 2) determine the role winter BOD plays in low oxygen levels and 3) provide deoxygenation coefficients and "ultimate BOD" values for the calibration of a limnological model. These studies and related water chemistry were measured in the fall of 1974 through the spring of 1976. BOD was initially characterized by its carbonaceous, nitrogenous and filtrable fractions. Further characterization involved differentiating and quantifying the suspended solids into living biomass and detrital fractions. The highest reservoir BOD was found in the summer. This is attributable to algal blooms and subsequent decay. Within the reservoir, on a yearly basis, filtrable BOD comprised 84% of the total BOD; nitrogenous BOD comprised 31% of the total BOD. Nitrogenous BOD from inflowing waters comprised 31% of the total BOD while the filtrable fraction was approximately 87% of the total BOD. On the average, 90% of the nitrogenous BOD for both inflow and reservoir waters was filtrable. Under nondrawdown conditions, the typical fall and winter BOD is insufficient to cause the extreme low oxygen conditions in the reservoir. Winter reservoir deoxygenation appears to be induced by decreasing water depths and resultant resuspension of organics and reduced chemical compounds. Deoxygenation coefficients are generally low (0.05) suggesting complex organics. Only in the spring inflow waters does this rate exceed 0.1. This was found to be characteristic of agricultural runoff.