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The Occoquan Watershed

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The Occoquan Watershed, is located in northern Virginia and is situated on the southwestern periphery of the Virginia suburbs of the City of Washington, D.C. The basin encompasses six political subdivisions of the Commonwealth of Virginia, including portions of four counties, and the entire land area of two independent cities, as follows: County of Fairfax, County of Fauquier, County of Loudoun, County of Prince William, City of Manassas, and the City of Manassas Park.

The watershed lies to the south and west of the U.S. National Capital, Washington, D.C. It is bounded by the Potomac Estuary to the east and Bull Run Mountain to the west. The northern and southern boundaries lie in the Counties of Fairfax and Fauquier-Prince William, respectively. At the location of the Occoquan High Dam, the watershed drains 570 square miles (mi2).

The major drainages tributary to the Occoquan Reservoir may be dividedinto two principal sub-basins: Bull Run and Occoquan Creek. Bull Run lies in the northern portion of the basin, and constitutes the principal drainage bounded by Bull Run Mountain on the west, Dulles Airport on the north, and the Manassas urban area on the south. To the east lies the confluence with Occoquan Creek in the upper reaches of the Occoquan Reservoir. The aggregate drainage of Bull Run and its tributaries above the Reservoir is 185 mi2.

Occoquan Creek is formed by the confluence of Broad Run and Cedar Run. Broad Run, as noted previously, drains the western extreme of the basin. Lake Manassas, which is the principal water supply of the City of Manassas, and is also an artificial impoundment, lies within the Broad Run drainage. Below the confluence of Cedar Run and Broad Run lies Lake Jackson, which is an impoundment of Occoquan Creek. Lake Jackson was originally constructed in the 1930's as a hydroelectric power production facility. At the present time, however, the lake is the centerpiece of a residential area lying to the south of the City of Manassas, and is maintained for recreational purposes only. It should be noted that substantial pollutant loading reductions to the Occoquan Reservoir result from the presence of these upper-basin impoundments. Below Lake Jackson, Occoquan Creek flows directly into the tail waters of the Occoquan Reservoir, and drains an aggregate of 343 mi2.

The remaining direct drainage to the Occoquan Reservoir originates in small streams in both the Counties of Fairfax and Prince William. The total drainage distributed among these small tributaries is 42 mi2., which is slightly over seven percent of the total drainage area. In Fairfax County, these small tributaries include Pope's Head Creek, Wolf Run, and Sandy Run. Similarly, in Prince William County, one may identify Hooes Run, which enters the Reservoir directly upstream of the high dam.

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Water Quality Considerations
Early in the decade of the 1960's, the urban growth began to reach into the upper Occoquan Watershed in unprecedented (and unanticipated) proportions. Coincident with the onset of accelerated population growth, a number of wastewater treatment plants were constructed and/or expanded in western Fairfax County and central Prince William County, resulting in substantial increases in the discharge of domestic wastes to the receiving waters of the basin. By the latter part of the decade of the 1960's, eleven (11) publicly owned treatment works (POTW's) of conventional secondary design were discharging an average of nearly three (3) million gallons per day (MGD) of treated wastewater to the basin. The quality of the plant effluents was quite variable, and no provisions were made for the removal of plant nutrients from the discharges. In addition, the percentage of basin area devoted to urban land uses began to increase substantially, raising the input of urban stormwater into the system. Increased conventional agricultural activity in the western basin, along with the application of chemical fertilizers, resulted in greater soil erosion, and the accompanying loss of nutrients in surface runoff.

Because the Occoquan Reservoir had become an irreplaceable resource for the citizens of northern Virginia, it was apparent that steps would be required to insure the long-term viability of the reservoir as a public water supply. In 1968, the Virginia State Water Control Board (SWCB) commissioned a study of the Reservoir and its tributary streams by the consulting engineering firm of Metcalf and Eddy (1969), with the goal of developing a management plan for the surface waters of the basin. That study, completed in 1970, stated that the reservoir was "highly eutrophic...", and further, that "the sewage plant effluents are mainly responsible for the advanced stage of eutrophication occurring..." Metcalf and Eddy study concluded with the recommendation that three alternatives be considered for future management of water quality in the reservoir:

-Wastewaters from the basin be exported to another watershed.

-Advanced wastewater treatment practices be adopted; treated waters be exported for re-use, and basin population be limited.

-Advance wastewater treatment practices be adopted with effluents remaining in the watershed, and basin population be limited.

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The Occoquan Policy
In July of 1971, after considering the recommendations of the Metcalf and Eddy report, the SWCB adopted A Policy for Waste Treatment and Water Quality Management in the Occoquan Watershed (VSWCB, 1971). Recognizing the practical limitations imposed by both the inter-basin transport of wastewaters and the imposition of population limitations, the Occoquan Policy, as it has come to be known, was based on a modification of the third option shown above. A milestone in water quality management in the Commonwealth of Virginia, the Policy included an implicit recognition that an indirect re-use of treated wastewater would become the operational norm in the Occoquan Watershed. It also recognized that extraordinary measures would be required to protect the public health in a situation where a water body was to be subjected to the competing uses of wastewater disposal and public water supply. In addressing this, the document not only specified the type of waste treatment practice to be adopted on a basin-wide scale, but it provided for an ongoing program of water quality surveillance to quantify the success of the water quality protection effort.

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The Occoquan Watershed Monitoring Program
The Occoquan Policy, in addition to mandating the adoption of regional advanced wastewater treatment practices at all new regional wastewater treatment plants in the Basin, went so far as to establish an innovative requirement for the establishment of an independent entity for the purpose of water quality surveillance and evaluation, and "to insure that performance levels are maintained at the ... plant, and that the effects of discharges and urban run-off (sic) are known." The entity charged with the creation and governance of the monitoring program was the Occoquan Watershed Monitoring Subcommittee (OWMS).

The OWMS was given the authority to create an independent facility to conduct the required monitoring program, using funds contributed by the wastewater generators and the finished water purveyor. In practical terms, this means that funds were contributed by the counties and cities in the Basin, and by the Fairfax County Water Authority. The resulting facility, the Occoquan Watershed Monitoring Laboratory (OWML), was established by the Virginia Polytechnic Institute Department of Civil Engineering. The laboratory began its on-site operations in 1972, and has conducted comprehensive studies of receiving water quality, and effects of the AWT discharges to the present time.

In the course of its studies, OWML has developed a comprehensive database of water quality in the Occoquan Basin, and has been instrumental in making determinations in a number of areas which have proven to be critical to the ongoing management of water quality:

Determining the suitability of AWT effluent for indirect discharge into a public water supply;

Providing information required for consideration of alternative treatment practices at the AWT plant;

Providing receiving water data for use in contemplating AWT plant expansions;

Providing information on water quality effects and cost-effective control of nonpoint sources of pollution.

Timeline

 

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