The Fairfax County Water Authority applies copper sulfate as an algaecide in the
Occoquan Reservoir to control the growth of taste and odor causing algae. The
Occoquan Water Treatment Plant removes water from the reservoir for distribution to
portions of Fairfax County. Approximately once a day, the treatment plant backwashes
one of its five clarifier/filters and discharges this backwash water directly into the
Occoquan River. Concern has risen that the plant may be concentrating this copper during
treatment, and therefore may be responsible for violations in the toxicity standards for
copper in the Occoquan River below the outfall. This investigation has attempted to define
the extent of the mixing zone around the Occoquan Water Treatment Plant.
A dynamic computer model has been developed based upon the 1988 version of
the Tidal Prism Model. Simulations were run on the "worst case scenarios" in the
Occoquan River as an attempt to define the mixing zone. It is our initial conclusion that
the treatment plant discharge has a negligible effect upon the concentrations of dissolved
copper that may exist in the Occoquan River. The treatment plant appears to be removing
copper from the raw water rather than concentrating it as initially assumed. Because both the flow and the background dissolved copper in the Occoquan River seem to be
controlled by the Occoquan Reservoir, the definition of a static mixing zone boundary is
not practical. A dynamic boundary was proposed as an alternative which incorporates the
many variables in this system.
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