Prince William needs to Protect the Occoquan Watershed

With assistance from the PWCA ORPA workgroup.

The Occoquan Reservoir is a vital drinking water source for 800,000 residents in Northern Virginia including residents on the eastern end of Prince William County. The Occoquan Reservoir watershed spans less than 600 square miles and Prince William County has the largest portion of land area within the Occoquan Watershed in its jurisdiction (40%). Other jurisdictions comprising the watershed include Fauquier County (36%), Fairfax County (17%), and Loudoun County (5%). The City of Manassas and the City of Manassas Park comprise a total of about 2%.

As of the 2020 Census, there were approximately 574,000 people residing within the watershed. About 43% of the population in the Occoquan Watershed resides in Prince William County. As the most populous jurisdiction in the Occoquan watershed and the one with the largest land area, substantial changes in land use patterns in areas of Prince William County will impact water quality in the watershed which will impact the groundwater, the streams and rivers and the Occoquan Reservoir.

To protect the Occoquan Watershed, Fairfax County downzone 41,000 acres of land and protected another 5,000 along the Occoquan Reservoir during the 1980’s. Prince William County adopted a rural area called the Rural Crescent with the adoption of the 1998 Comprehensive Plan which served to protect the headwaters in the fragile Bull Run watershed and Occoquan Watershed by alleviating development pressure in the already heavily urbanized drinking water watershed.

When Prince William County approved their Comprehensive Plan pathway to 2040, the “Rural Area” designation was eliminated. It was replaced it with an "Agricultural Estate" designation covering 55,310 acres and with an "Agricultural and Forestal" designation covering 75,647 acres. These new designations allow for more development in the rural area. Recent rezonings have allowed even more intense development in what was once the rural area. The Comprehensive Plan update also established an Occoquan Reservoir Protection Area (ORPA), to protect the Occoquan Reservoir as a public water supply and meet the requirements of the Chesapeake Bay Watershed Implementation Plan that Virginia is using to meet the US EPA enforced Pollution Reductions mandates.

Protecting the Occoquan Reservoir requires protecting all the water resource in a region because all water on earth is connected. Precipitation moves into the water table (the hyporheic zone) down to groundwater or into rivers and streams. Disrupting the balance of water flow can have dire consequences. The available supply of fresh water is continually renewed by the hydrologic cycle and in the case of the Occoquan Reservoir the actions of mankind. The need for water is constant and grows with population and wealth and business activity. There is also a seasonality to water- we use more in summer.

Many activities of mankind interfere with the hydrologic cycle. Through land change we interrupt the recharge of groundwater which impacts stream flow. Changing the use of the land, covering it with buildings, driveways, roads, walkway and other impervious surfaces will change the hydrology of the site reducing groundwater recharge in the surrounding area increasing stormwater runoff velocity and quantity and reducing streamflow which is feed by groundwater.

As groundwater levels fall, perennial steams that feed the rivers become ephemeral. The groundwater becomes disconnected from the surface water network. Once the hydrology is destroyed by development, it cannot be easily restored, if at all.

The Occoquan Reservoir is fed by the Occoquan River which receives up to 40 million gallons a day of the treated discharge of the Upper Occoquan Sewage Authority treatment plant which discharges to the river upstream of the Occoquan Reservoir so, a significant portion of the flow (especially during dry periods) into the reservoir is recycled sewage. This treated wastewater is from areas supplied by the Corbalis plant or lake Manassas so you do not end up with constantly recycling and concentrating the same impurities into the Occoquan.

In addition, the reservoir receives stormwater runoff, precipitation from the Occoquan Watershed and feeds the streams and creeks that feed Bull Run and the Occoquan River. When generally open rural area is developed, stormwater runoff increases in quantity and velocity washing away stream banks, flooding roads and buildings carrying fertilizers, oil and grease, and road salt to the Occoquan Reservoir. The faster flow of storm water gouges the riverbeds picks up pollutants from impervious surfaces. The cumulative impact of these steps leads to flash floods, unstable banks, heavy pollution and waning life. This is why it is essential to have an ORPA, to ensure both public and private water users continue to have water to drink and use.

Geology, climate, weather, land use and many other factors determine the quality of the groundwater and in turn streamflow. Within Prince William County Virginia there are four distinct geologic provinces: (1) the Blue Ridge, (2) the Culpeper Basin, (3) the Piedmont, and (4) the Coastal Plain. The U.S. Geological Survey divides the four geologic provinces of the county into seven hydrogeologic groups based on the presence and movement of the ground water calling them groups: A, B, B1, C, D, E and F.

The quantity and quality of ground water in Prince William County varies across the county depending on the geologic and hydrogeologic group you are in. The rocks in the Blue Ridge, Piedmont, and Coastal Plain contain minerals that are resistant to weathering, and the ground water tends to be acidic having low concentrations of dissolved constituents. Generally speaking, the groundwater in the county is recharged in elevated areas between stream valleys and channels and discharges to streams and estuaries. However, the paths and duration of groundwater flow are different between consolidated rocks and unconsolidated material. Groundwater in the consolidated rocks flows through the system of fractures following a circuitous path before discharging to a stream or estuary. In unconsolidated material, ground water generally follows a direct path from the recharge area to the discharge area.

In the area of the proposed ORPA is beyond the Culpeper Basin in the Piedmont region. This area of the ORPA is primarily hydrogeologic group D composed of igneous rock formations with limited lenses of hydrogeologic group E that transition at the bounds of the ORPA to group E and then to the Coastal Plain.

Hydrogeologic group D is located within the Piedmont formation and consists of three igneous plutons in the eastern part of Prince William County: the Goldvein, Lake Jackson, and Occoquan Plutons. Rocks within hydrogeologic group D tend to have moderate water-bearing potential and ground-water storage tends to be predominantly in the overburden, which is the soils above the bedrock. Wells in this area are most susceptible to drought and tend to be slightly acidic. The igneous rocks have subhorizontal sheeting and near vertical joints overlain by thick overburden. Groundwater wells in the area tend to have yields range from 1.2 to 100 gal/min which has resulted in the development of homes with wells in the area due to the thickness of the water storing overburden.

Hydrogeologic group E is also in the Piedmont formation in the eastern part of the county, and consists of metasedimentary, metavolcanic, and other metamorphic rocks. Rocks within hydrogeologic group E tend to have poor water-bearing potential, and thin- to thick cover of overburden. Similar to the rocks of hydrogeologic group D, ground-water storage tends to be predominantly in the overburden. Some of the poorest yielding wells in Prince William County are located in this hydrogeologic group and can be as low a 0.25 gallons per minute upto 70 gallons per minute-, but tending towards the low end because of the thinness of the overburden beyond the limits of what is the proposed ORPA. Homes and businesses in this area have depended on public water supply due to the limitations on well development and that water comes from the Occoquan Reservoir.

Protecting groundwater serves to protect all of the water resources in the watershed. Today, the Occoquan watershed is often described as the most urbanized watershed in the nation. Certainly there are far more urbanized areas in the United States, but they do not have functioning watersheds. We need to effectively protect ours.