Occoquan Reservoir, the ICPRB and Your Water

From the Interstate Commission for the Potomac River Basin (ICPRB):

Despite not experiencing drought conditions locally, “Continued low flows in the Potomac River have triggered daily drought monitoring operations by ICPRB’s Section for Cooperative Water Supply Operations on the Potomac .”

During daily drought monitoring protocols, the ICPRB collects river flow, precipitation data and forecasts, and usage data and forecasts from metropolitan area water suppliers....If conditions warrant active management of water supplies and a potential release of stored water to meet demands.

This protocol was last used by ICPRB in September and August 2019, and before that in 2017. The dry conditions are affecting some smaller water systems in the basin. Front Royal, Va., has issued a call for voluntary water conservation because of low flows..,” despite adequate rainfall according to the Drought Monitor.

The Washington, DC, metropolitan area (WMA) is home to almost six million residents and workers. The region’s water suppliers have an important responsibility beyond supplying the needs or the residents: to provide 24/7 water that ensures the federal government, including Congress, the Pentagon, and key agencies can function.  The water suppliers share the Potomac River as the major regional water resource, and so 40 years ago and came together to form the Interstate Commission on the Potomac River Basin (ICPRB) and a cooperative agreement of funding and using the water resources available regionally. 

The Potomac River flow fluctuates with season and weather. The ICPRB helps manage the river’s water resources. The cooperative agreement was created, and the Jennings Randolph Reservoir was built to manage the use of the Potomac River and to ensure that there is enough flow for essential services like wastewater assimilation and habitat maintenance. The ICPRB monitors river flows and water withdrawals to ensure the 100 million gallons per day minimum flow at Little Falls.

That minimum flow level has been maintained since the early 1980's, but during times of drought, natural flows on the Potomac are not always sufficient to allow water withdrawals by the utilities while still maintaining the minimum flow in the river. When necessary, the ICPRB allocates and manages water resources of the river using the jointly owned Jennings Randolph Reservoir, Potomac River Low Flow Allocation Agreement and the Water Supply Coordination Agreement. The reservoir and agreements were part of a water management scheme developed by scientists at Johns Hopkins University. For decades they have been used to jointly improve reliability of the water supply.

The tools available to the ICPRB to manage water use are to have members utilize their in-system storage or the shared system storage and reduce their water withdrawals. Fairfax Water which supplies over 85% of Prince William Service Authority water supply has a reservoir on the Occoquan River that is outside the freshwater drainage area and is supplied by the Occoquan River and recycled wastewater from UOSA (Upper Occoquan Service Authority wastewater treatment plant). So, when necessary, the ICPRB requires Fairfax Water to reduce the water they draw from the Potomac and increase the water drawn from the Occoquan Reservoir.

The reservoir’s current storage capacity is estimated at 8.3 billion gallons. Water from the Occoquan Reservoir can only supply the Griffith treatment plant which predominately serves the customers in the eastern portion of Fairfax Water’s service area and the Eastern Distribution System of Prince William County. However, Fairfax Water has a connector that can transfer water from the Griffith plant to the western portion of its service area (and the Prince William Western Distribution Area) normally supplied by the Corbalis plant using water drawn from the Potomac River.

Two thirds of the Occoquan Watershed that supplies the Occoquan Reservoir is in Prince William County. On November 17, 2020, the Prince William County Board of Supervisors issued Directive No. 20-86 for county staff to develop a protection overlay district for the Occoquan Reservoir. So far county staff have reviewed a recent report prepared by Virginia Tech and their Occoquan Watershed Monitoring Lab of the Occoquan Watershed and the Reservoir System water quality. The county staff has discussed the report-findings with the Northern Virginia Regional Commission. Staff is also reviewing reports and recommendations from local committees and environmental groups and evaluating current design standards and development practices, in relation to water quality trends in the Reservoir.

Staff is expected to recommend a zoning text amendment for an overlay district and/or the process to revise the Design and Construction Standards Manual to provide increased protection for water quality sometime in the future. An overlay district is used to put special restrictions land use or grant special rights to some land. An overlay district could be used to limit the types and amount of development on land within the watershed to protect the Occoquan Reservoir, it could also be ineffective if too loose or constantly overridden by the Board of Supervisors. The problem is not that Prince William County holds about two thirds of the Occoquan Watershed; but that the Occoquan Watershed is more than two thirds of Prince William County. To properly protect the Occoquan Watershed and the regional water supply, the use of the remaining open land must be severely restricted. The rest of the region needs to pay landowners for the protection of the Occoquan Watershed.

This needs to happen now. Sometime in the future may be too late to protect this essential portion of our water supply. Recently, the Prince William County Board of Supervisors approved the development of the Preserve at Long Branch, rezoning a portion of the Rural Crescent adjacent to the Occoquan River. Also approved this spring was the Independent Hill Small Area plan. No analysis was done as to the potential impact of these developments to the hydrology of the Occoquan Watershed.  There is no understanding what the impact this might have to the quality of and supply to the Occoquan Reservoir. Yet the Occoquan Reservoir is irreplaceable for the region.  

Other threats to the watershed are under consideration by the Board of County Supervisors.  The revival of the Bi-County parkway, this time called the Va. 234 Bypass and the proposal from Maryanne Gahaban and Page Snyder. The two Rural Area large landowners are pushing a proposal to convert almost 800 acres of agriculture zoned land (in which they each have significant ownership) to industrial data centers. Once more no analysis was done as to the potential impact of these developments to the hydrology of the Occoquan Watershed.  There is no understanding what the impact this might have to the quality of and supply to the Occoquan Reservoir. 

During development the primary impact is erosion and sediment that are carried by stormwater into the streams. Post-development the primary impact is increased stormwater volume and velocity that is caused by the removal of tree canopy cover and the replacement of pervious surfaces of plants and grass with the impervious surfaces such as roads, parking lots, rooftops, driveways, patios, etc.

Development increases impervious surface area, and this has created in the past and will in the future create a host of concerns for managing the Occoquan Watershed. For instance, the physical condition of the Watershed's tributaries has been measured to fall with development. Increased stormwater runoff from impervious surfaces flows into streams and creeks at a higher volume and velocity. The result is increased erosion of stream banks that leaves a degraded ecosystem.

Development impacts water quality. Minimizing impervious surface cover and maintaining the tree canopy is critical to the protection of the County’s streams which flow to the Occoquan and other reservoirs. There is a direct correlation between stream health and impervious surface cover and tree canopy. According to the Northern Virginia Regional Commission, watersheds with impervious surface cover of 10 to 15% show clear signs of degradation, while watersheds with impervious surface cover greater than 15-25% typically do not support a diverse stream ecology and are dying.