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.
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 (mi.2).
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.
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.
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.
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.
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:
from the basin be exported to another watershed.
wastewater treatment practices be adopted; treated waters be exported
for re-use, and basin population be limited.
wastewater treatment practices be adopted with effluents remaining
in the watershed, and basin population be limited.
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).
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.
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).
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.
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:
the suitability of AWT effluent for indirect discharge into a public
information required for consideration of alternative treatment
practices at the AWT plant;
receiving water data for use in contemplating AWT plant expansions;
information on water quality effects and cost-effective control
of nonpoint sources of pollution.