Significance of the Occoquan Watershed

Prince William County encompasses approximately 44% of the Occoquan Watershed. More importantly, the watershed covers over two-thirds of the county’s land. Therefore, decisions made within Prince William County have wide-reaching consequences for all Fairfax Water customers and nearby groundwater users. Protecting the Occoquan Watershed and the regional water supply requires a thorough understanding of the impacts of ongoing and planned development.

The Occoquan Watershed Model was developed over several decades to assess how land use changes affect the water quality of the Occoquan Reservoir. It is currently being updated and new The Northern Virginia Regional Commission has operated this model and plans to release its findings later this year. Despite this, the Board of County Supervisors recently approved comprehensive plan changes that eliminated the Rural Crescent and made sweeping land use changes without considering the potential impacts on the water supply.

Rising Salinity and Its Implications

Salinity levels in the Occoquan Reservoir have been steadily increasing and may be approaching a critical threshold. This rise in salt is primarily attributed to watershed runoff—especially from road salting during wet weather—and reclaimed water from the Upper Occoquan Service Authority (UOSA) during dry weather. The sodium content in reclaimed water from UOSA exceeds that of outflows from the two watersheds. As paved areas expand, salt runoff into the watershed also increases.

The removal of salt from the drinking water supply would require a major investment in desalination technology at regional water treatment plants, which currently lack this capability. The estimated cost of adding treatment lines at Fairfax Water to keep the Occoquan potable is between $1 and $2 billion, a financial burden that would ultimately fall on water rate payers, including the 350,000 public water users in Prince William County.

Urbanization and Watershed Health

The Occoquan watershed is frequently described as the most urbanized watershed in the nation. Although more heavily urbanized areas exist in the United States, these do not have functioning watersheds. Historically, cities like New York, Philadelphia, Baltimore, and Washington have confined and subsumed countless streams, erasing them and destroying their watersheds.

Scientific studies indicate that when 5–10% of a watershed is developed, it begins to decline, though restoration is still possible for a time. However, if urban land use surpasses this tipping point, water quality ceases to respond to restoration efforts. Once a watershed is destroyed, restoration may be impossible.

To avoid irreversible harm to the local ecology and regional drinking water supply, it is essential to understand the impacts of planned changes. The cost of restoring the basin and treating the water could reach billions of dollars, a financial responsibility for residents. The Occoquan Reservoir is a vital and irreplaceable component of the region’s water supply.

Average Water Flows to the Reservoir

Source	Percentage (%)
Bull Run Watershed	25%
Occoquan River Watershed	48%
Groundwater and Other Watershed	20%
UOSA Reclamation Facility	6%

Initial development of previously open areas mainly causes erosion and sediment transport via stormwater into local streams. After development, the primary concern shifts to higher stormwater volume and velocity, resulting from loss of tree canopy and replacement of pervious surfaces with impervious ones such as roads, parking lots, rooftops, driveways, and patios. This leads to the disappearance of small streams and a reduction in groundwater recharge. Current stream monitoring by the Conservation District already shows troubling signs of declining tributary health.

Connection Between Surface Water and Groundwater

The US Geological Survey emphasizes that virtually all surface-water features—streams, lakes, reservoirs, wetlands, and estuaries—interact with groundwater. All water is interconnected, and groundwater flow and storage are dynamic, constantly changing due to human and climatic stress. Altering the land changes both the quality and quantity of groundwater and streamflow.

Land use changes that increase impervious cover beyond 5–10% from roads, pavement, and buildings have two significant effects: they reduce open areas where rain and snow can infiltrate the ground and recharge groundwater, and they increase stormwater velocity, preventing water from percolating into the earth. This leads to more frequent flooding and less groundwater recharge.

Over time, reduced groundwater levels transform perennial streams into ephemeral ones, disconnecting groundwater from the surface water network. Once watershed hydrology is destroyed by development, restoration is extremely difficult, if not impossible. The Occoquan watershed is essential for the region’s drinking water supply.

Emerging Water Quality Issues

According to Dr. Stanley Grant, emerging water quality issues are mainly a result of the built environment. Ongoing development within the Occoquan Watershed threatens the sustainability of the water supply for up to one million people in northern Virginia. Population density increases lead to a rapid rise in impervious surfaces once the density reaches 100 people per square mile, accelerating the decline of the most urbanized watershed in the United States.

Water’s Journey Through the Region

Water originates in the Potomac River and is treated by Fairfax Water at the James J. Corbalis, Jr. Water Treatment Plant. It is then pumped to the western portion of the PW Water service area, including Haymarket, Gainesville, and Manassas, and supplemented by water from Lake Manassas treated at the City of Manassas Water Treatment Plant. After potable water is used in the western system, wastewater is sent to the Upper Occoquan Service Authority’s wastewater treatment plant in Centreville, where it is treated and discharged to Bull Run, a tributary of the Occoquan Reservoir. Water from the reservoir is then withdrawn by Fairfax Water and treated at the Frederick P. Griffith, Jr. Water Treatment Plant in Lorton before being pumped into the eastern PW Water service area. Wastewater from this area is sent to the H.L. Mooney Advanced Water Reclamation Facility, treated to Chesapeake Bay clean water standards, and discharged to Neabsco Creek, a tributary of the Potomac River.