Effects of sediment on water quality in the Occoquan Reservoir
To, Yan Pui Samuel
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The purpose of this investigation was to determine the effect of sediments in the Occoquan Reservoir on the water quality. Phosphate determination by the ascorbic acid method was first evaluated for its precision and accuracy. An assay was developed to measure the rate of nutrient phosphate release to the overlying water in a quiescent dark condition. The assay was evaluated to test for its reliability in giving valid conclusions. The assay was an attempt to provide a system similar to the actual system which could be used in the laboratory. Also, the effect of different parameters could be studied by varying conditions one at a time. In this assay, it was found that the rate of release was not affected by normal pH changes, depth of sediment, or depth of overlying water. The rate of release was found to increase with increase in temperature, phosphate content in sediments, and reduced anaerobic condition. The rate was adversely affected by high concentration of salt, drying on sediment surface, thickness of physical barrier, and reduction of bacterial activities. Flushing seemed to decrease the rate of release by a constant ratio. Under normal conditions, sediments release other nutrients besides phosphates. However, it was found that whether the sediment served as a sink or source of phosphate was governed by the critical availability concentration which was different for different sediment. A study of the properties of the reservoir sediment showed that there were variations with respect to time, station, and depth, as well as vicinity within the same station, in this reservoir.
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