The Occoquan Reservoir and Overlay Protection Area (ORPA)

ORPA from PWC

To ensure the protection of the Occoquan Reservoir—a primary drinking water source for eastern Prince William County and eastern Fairfax County—Prince William County established an overlay district in the lower Occoquan Watershed, known as the ORPA. Despite the intent of this protective measure, subsequent rezoning requests from developers has led to exemptions for some of the largest undeveloped parcels, which were recently approved by the Prince William Board of County Supervisors. The fundamental objective of the ORPA is to maintain natural landscapes, limit higher-density development and safeguard the region’s drinking water supply for both well owners and municipal customers; however, recent board actions have not aligned with these priorities.

Urbanization and Watershed Health

Although other parts of the United States may feature higher urban densities, the Occoquan watershed is regarded as the most urbanized while still retaining a functional watershed system. Preserving the watershed and its drinking water resources is essential, but the impact of land use decisions may only become evident over decades due to the slow pace of ecological change.

Groundwater functions as a reserve for rivers and streams, replenished through precipitation and subject to fluctuations stemming from rainfall patterns and land use alterations. Maintaining groundwater recharge within the Occoquan watershed is crucial for sustaining local water supplies and supporting river and stream ecosystems.

Impact of Land Use on Water Resources

Increases in impervious surfaces, such as roads, pavement, and buildings, exceeding 5–10% of an area can have two critical consequences: reduced infiltration of precipitation and accelerated stormwater runoff, both of which hinder groundwater recharge. These effects contribute to increased flooding and diminished water reserves over time. As groundwater levels decline, perennial streams supplying rivers transform into ephemeral flows, disconnected from surface water systems. Research indicates that watersheds experience degradation once 5–10% of their area is developed, although limited restoration potential remains initially. When urbanization surpasses the tipping point, water quality ceases to respond to remediation efforts, and complete restoration of the watershed becomes unattainable. Data from the Northern Virginia Regional Commission (NVRC) shows that by 2015, impervious surfaces in the lower Occoquan area had already reached 11%.

Interconnectedness of Water Resources

The preservation of the Occoquan Reservoir, serving over 800,000 residents, is intrinsically linked with the protection of all water sources in the region. Hydrologic connectivity ensures that precipitation enters the water table, contributes to groundwater, and feeds directly into streams and rivers. Disrupting this hydrological balance has broad consequences. The water supply for the Occoquan Reservoir comes from multiple sources: Bull Run Watershed (25%), Occoquan River Watershed (48%), groundwater and other watershed (21%), and reclaimed water from UOSA (6%). Should the watershed be compromised, only stormwater runoff and UOSA-reclaimed water would remain viable.

The Hydrologic Cycle and Growing Demand

A functioning watershed continually renews freshwater supplies via the hydrologic cycle, supplemented in the case of the Occoquan Reservoir by returning reclaimed wastewater to the Occoquan River. However, population growth, increased affluence, and expanded commercial activity increase water demand. Since the inception of the Occoquan Reservoir management plan nearly five decades ago, the regional population has grown six-fold. The original objectives included implementing advanced water treatment technologies, ensuring all reclaimed water met stringent standards prior to discharge, and capping the basin population at 100,000.

Groundwater Characteristics in Prince William County

Groundwater quantity and quality in Prince William County vary according to the underlying geology and hydrogeology. Generally, recharge occurs at higher elevations, with flow toward streams and estuaries, though these dynamics differ between consolidated rock formations and unconsolidated sediments. The proposed ORPA area, located outside the Culpeper Basin but within the Piedmont geologic province, primarily features hydrogeologic group D (igneous rocks), with transitions toward group E and the Coastal Plain in certain sectors.

Wells located here are highly vulnerable to drought conditions and often yield slightly acidic water. The igneous substrate exhibits subhorizontal sheeting and vertical joints beneath substantial overburden, producing well yields ranging from 1.2 to 100 gallons per minute. These characteristics support the prevalence of private household wells within the area. The only proxy well for Hydrogeologic group D, situated in Prince William Forest Park—the county’s least disturbed area—shows declining groundwater trends, but data remain insufficient to assess long-term sustainability.

Threats to the Region’s Water Supply

Significant threats to the region’s water resources stem from the conversion of woodland and open space to impervious surfaces, which eliminate groundwater recharge and reduce streamflow. The rapid loss of forest cover lowers water tables, degrades habitats, intensifies drought conditions, and raises salinity levels. In just seven years, Prince William County lost nearly 2,000 acres of tree canopy to development.

As highlighted by Dr. Stanley Grant, director of the Occoquan Watershed Monitoring Laboratory, emerging water quality challenges originate from ongoing urban expansion. The sustainability of the water supply for the 800,000 residents dependent on the Occoquan Watershed is jeopardized by increasing population densities and impervious surfaces. Once the density exceeds 100 people per square mile, the rate of impervious cover increases rapidly, threatening the continued existence of the nation’s most urbanized functional watershed and driving up water supply costs.

Rising Salinity and the Cost of Water

Salinity in the reservoir continues to rise, potentially approaching critical thresholds. Increased salt concentrations result primarily from winter road treatments during wet periods and reclaimed UOSA water during dry spells. Projected trends indicate further sodium increases due to cooling discharges from data centers and continued population growth. Expanded impervious coverage exacerbates salt runoff into the watershed.

Currently, removing salt from drinking water requires investing billions in desalination infrastructure, as regional plants lack this capability. Continued suburban sprawl into previously undeveloped areas will first elevate both water infrastructure costs and the cost of water for the residents. Then, diminished water availability will become the growing problem.