Third of three stories
The final speaker at the November 13th, 2008 meeting of the South Fork Reservoir Stewardship Task Force dealt with another angle for the group to consider. If the reservoir is dredged, how would that alter the quality of the drinking water that the Rivanna Water and Sewer Authority supplies to the community?
Bob Wichser, the RWSA’s Operations Director, spent about thirty minutes
exploring that question
. He described how the reservoir is fed by a series of streams. As the water pours into the reservoir, sediments come along for the ride, carrying a variety of chemicals and nutrients that have attached to the sediment particles. These sediments will largely settle to the bottom, but nutrients such as phosphorous will periodically be pulled back into the water due to changes in pressure and temperature.
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“Remember that a reservoir is a physical, chemical biological process,” Wichser said. Operators of the South Fork Water Treatment Plant monitor water quality and modify the way they treat the raw water. “As we get more sediment in, they will be required to add different chemicals to remove those sediments.” Alum, or aluminum sulfate, is used to remove much of the sediment. The amount of sediment in the water is measured by something called turbidity.
Wichser provided an example of how dredging would release chemicals from the reservoir’s floor. The suction dredge pipe would remove solids, but Wichser estimates that as much as 80% of what comes through the pipe would be water. He also said there are biological organisms that live in the section of the reservoir floor called the “sediment/water interface.”
Wichser said there were two potential ways in which water quality would be affected by dredging. First, there would be a short-term increase in the level of suspended sediment in the areas in which the dredge was operating. One way to mitigate this would be to use something called a “double silt curtain,” but Wichser said the curtain might not prevent radical drops in oxygen. As the dredge pipe sucks up water, the disturbance to the bottom of the reservoir could cause plumes of sediment-heavy water to form.
Plumes are columns of liquid that have a different density than the water in which they are suspended, and these oxygen-depleted plumes float through the reservoir. Depending on their size, Wichser said they can create temporary dead-zones which would lead to fish-kills. These plumes would be more likely in warmer months, because water in colder temperatures retains more oxygen. For that reason, Wichser recommended avoiding dredging in April and May, when fish are spawning.
The other potential threat would come from a fuel spill from one of the dredging barges, which Wichser said would affect the water up to ten feet below the surface. The barges carry between 300 and 800 gallons of fuel.
“There is the potential if there was a fuel spill that the water plant would have to be taken off-line until the fuel spill either passed or we could ensure that the [chemicals] from the fuel spill are not being taken in,” Wichser said. He added the water treatment plant does not have the ability to treat water contaminated with petroleum products. Any spill must be immediately reported to the federal National Response Center and the Virginia Department of Health would not allow the plant to go back online until it could be proven that hazardous chemicals such as benzene were not entering the treated water supply. Wichser said that could take as much as 48 hours.
Wichser concluded his presentation by discussing a study that the U.S. Environmental Protection Agency’s Environmental Effects Research Laboratory conducted on the impacts of dredging on the environment.
“It concluded that the environmental impacts of small-scale dredging events in urbanized areas are likely to be localized and of short-term environmental consequences,” Wichser said. “It’s telling you that mother nature is very resilient and the system will repair itself.”
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