The Occoquan Reservoir and Development in the Rural Crescent

 Last Monday the Prince William Conservation Alliance sponsored Protecting the Occoquan Reservoir: Our Shared Water Source! with special guest Stephen Souza PhD, past president of the North American Lake Management Society, the article below is based on his presentation.  Dr. Souza addressed how development of the Rural Crescent and essential portion of the Occoquan Reservoir watershed will negatively affect the reservoir and drinking water in our region.

The Occoquan Reservoir is 1,400 acres containing 8.5 billion gallons of water that provides 40% of the daily water supply for Fairfax Water which in turn supplies Prince William Service Authority and a significant portion of Loudoun County. The reservoir is a reflection of its watershed.  The Occoquan Watershed is 590 square miles two thirds of which is in Prince William County including the all important headwaters of the Occoquan.

When the Occoquan Reservoir was first built 1957 it was located in a rural and forested area and the water was pristine. The unrelenting growth and development in this region has changed that.  In 1982 to protect the Occoquan from contamination, Fairfax County turned 5,000 acres bordering the reservoir into parkland and down zoned 41,000 acres. Today in Fairfax there are 51 homes on the shore of the Reservoir. In Prince William County there are 450 homes along the shore of the Occoquan Reservoir and Reservoir and watershed are under threat from the development of the Rural Crescent  as development pressures have moved further out with urbanization of the region. 

 Development in the watershed triggers a number of problems that begin with storm water runoff as the primary driver, though waste water point sources and non-point sources also contribute to the deterioration of the water quality. Pollution from runoff, called non-point source pollution is threatening the health of the Occoquan Reservoir and our drinking water supply. As you can see in the diagram below which originally came from the U.S. EPA and Dr. Souza used in his presentation, the hydrology of an area changes with development.

US EPA 2003

One acre of wooded area produces 1,360 gallons of stormwater runoff. While one ace of parking lot and roadways produces 25,800 gallons of stormwater runoff. According to Dr. Souza, the typical suburban lawn is almost as compacted as a parking lot and produces similar runoff. This increased runoff not only reduces groundwater recharge and stream base flow to the region, but also increases flooding, scour and erosion of the stream banks.

Increased runoff reduces water quality and increases the costs to treat the water to meet drinking water standards. Stormwater can also impact groundwater quality. Contaminants are mobilized by runoff and can infiltrate into the surficial groundwater and the fractured rock system that predominates the northwestern portion of the Rural Crescent. This can carry road salt, petroleum hydrocarbons and heavy metals into the groundwater which is the drinking water supply for the Rural Crescent.

Development of the watershed impacts stream flow and health, water quality, the regional ecology and stability of water supply to our region. Prince William County is in the very early stages of studying whether an “Occoquan overlay district” is needed to help protect the reservoir from becoming further polluted, although the County Board of Supervisors is much further along in planning to develop the rural area that is an essential protection and buffer for the Occoquan. 

According to Dr. Souza, development of the rural area of the county could push the Occoquan Reservoir to a water quality tipping point and the next generation will have to address eutrophication of the Occoquan Reservoir. A memorable legacy.