Occoquan Watershed

Recently, the Prince William County Board of Supervisors approved the development of the Preserve at Long Branch, rezoning a portion of the Rural Crescent. No analysis was done as to the potential impact of this development to the hydrology of the Occoquan Watershed.  There is no understanding what the impact this might have to the sustainability of the drinking water supply of adjacent property well owners and the quality of the Occoquan Reservoir itself. 

Occoquan Watershed

The Occoquan Reservoir is an important part of our drinking water supply. The Occoquan supplies about 40% of the clean drinking water for around 2 million people and, in an emergency, can supply all for a short period of time. The reservoir’s current storage capacity is estimated by ICPRB to be 8.3 billion gallons. Prince William land accounts for 40% of the Occoquan watershed which contains 1,300 stream miles, Lake Jackson and Lake Manassas as well as the Occoquan Reservoir.  Water from the Occoquan Reservoir is distributed to customers in Fairfax and Prince William Counties. This water is essential. 

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 25% typically do not support a diverse stream ecology and are dying.

During development the primary impact is erosion and sediment that are carried by stormwater into the streams. The primary post-development 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.

The Occoquan Watershed is more than just a source of water for the Reservoir. In addition to its role as an essential portion of the drinking water system for approximately 1.2 million Northern Virginians, the Reservoir and the Watershed also serves to improve water quality:

• The Reservoir is an essential element in meeting the Chesapeake Bay TMDL by trapping sediments and nutrients. According to the Occoquan Watershed Monitoring Lab (OWML) the Reservoir captured 34% of total nitrogen, 56% of total phosphorus, and 83% of total sediment.
• The downzoned portion of the Watershed within Fairfax and the Rural Crescent serve as a natural water treatment system and high quality ecological habitat.
• The Reservoir is a regional recreational asset.

Prince William has ignored its responsibility to best manage the Occoquan watershed in conjunction with Fairfax County’s management of Occoquan Reservoir (and their side of the Watershed) maintaining the primary benefit of the Reservoir as an essential and reliable source of safe, clean drinking water for Prince William County and the importance of the Reservoir as an integrated ecological and hydrological system with multiple uses.