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Technical Report

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warm water release impoundment, trace metals, water quality, reservoirs


A water quality study of the DeGray Reservoir, Arkansas, was conducted beginning immediately following the beginning of impoundment through the first year in which the reservoir was operated near normal pool elevation. DeGray Reservoir is the first major dam in Arkansas to be equipped with upper level release capabilities. Temperature, dissolved oxygen, and turbidity were measured in situ at stations located over the old river channel. Samples were taken from selected levels within the water column at each station and analyzed for the following parameters: pH, sodium, potassium, calcium, magnesium, nitrate, phosphate, sulfate, chloride, fluoride, alkalinity, COD, iron, manganese, copper, cobalt, nickel, lead, zinc, cadmium, and silver. The heavy metals were determined in both the particualte fraction (retained by a 0.45 micron filter) and the soluble fraction (passed by a 0.45 micron filter). The results of these studies have shown that DeGray Reservoir becomes stratified in the early spring and that oxygen depletion occurs under the thermocline. Turnover or mixing of the reservoir was observed to occur during two years by a series of underflows of water carrying fresh oxygen into the hypolimnion. Density currents, laden with silt were observed near the top of the thermocline and apparently are the origin of metalimnic dissolved oxygen minimum. The total quantity of both iron and manganese in DeGray Reservoir were found to cycle in response to oxygen depletion in the hypolimnion. Although some vague trends in other heavy metals were observed, it was difficult to determine if seasonal fluctuation of these metals was occuring. A comparison of the water discharged from the reservoir with a pre-impoundment water quality study indicate that the upper level release structure is maintaining a temperature regeim in the river which is similar to that present before impoundment. The concentration of other constituents were observed to be significantly different from the river prior to impoundment.

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