Effects of The UOSA Final Discharge On The Occoquan Reservoir Water Quality
The Occoquan Reservoir is a component in a water supply system that currently serves over 1.4 million residents of Northern Virginia. During the suburban expansion of the Northern Virginia region in the 1960s, the reservoir’s water quality degraded resulting in
- Massive algal blooms (including blue-green algae of the genus cyanobacter) due to over enrichment with plant nutrients;
- Periodic episodes of taste and odor in the finished drinking water;
- Low dissolved oxygen levels;
- Periodic fish kills; and
- Generation of hydrogen sulfide in the bottoms waters of the waters of the reservoir.
In 1968 - 1969 a study conducted by Metcalf and Eddy, under the auspices of the State Water Control Board, determined that water quality deterioration in the reservoir was caused by substandard wastewater discharges from 11 secondary wastewater treatment plants and non-point sources of pollution. Following the commissioning of UOSA, the water quality of the Occoquan Reservoir has greatly improved.
UOSA discharges a highly nitrified reclaimed water into Bull Run, which is a major tributary of the Occoquan Reservoir. Research conducted by OWML has shown that the nitrate in the UOSA reclaimed water has benefited the Occoquan Reservoir water quality specifically by:
- Preventing the release of sediment bound phosphorus;
- Reducing the release of ammonia from reservoir sediments;
- Preventing the reduction of sulfate to sulfide in the bottom layers of the reservoir;
- Possibly preventing the release of manganese (II) from sediments; and
- Maintaining green algae and diatoms species dominance and preventing the proliferation of less desirable blue-green algae.
According to OWML’s research, phosphorus releases are prevented when, in the absence of molecular oxygen in the lower levels of the reservoir, nitrate acts as an alternate terminal electron acceptor for certain bacteria delaying the onset of anaerobic conditions and inhibiting the release of iron-bound phosphorus from the sediments. During this process the nitrate is converted to nitrogen gas, which is released to the atmosphere. This is a fortuitous process that occurs because, during periods of thermal stratification, the waters of Bull Run enter the reservoir at lower temperatures than the surface waters causing the high nitrate water to sink to the oxygen-deficient lower reservoir depths.
The Virginia DEQ recognizes the water quality protection benefits of the UOSA nitrified discharge on the Occoquan Reservoir in State permits.
Read more about this topic: Upper Occoquan Sewage Authority
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