Where Our Water Comes From

The Potomac River, its tributaries, reservoirs and the associated groundwater resources are the source of drinking water for the over 6,000,000 people in the Washington Metropolitan area. The Washington Aqueduct, WSSC, Fairfax Water and Loudoun Water directly draw from the Potomac River. The Occoquan River is a downstream tributary to the Potomac River.

The Interstate Commission on the Potomac River Basin (ICPRB) coordinates water supply/withdrawal operations for the Potomac River during times of drought and recommends releases of stored water from the jointly owned reservoirs. This is to ensure sharing of the water resources among the large Washington metropolitan area water companies and to meet minimum environmental flow levels that were jointly agreed to.

 

Water management in the Potomac basins requires preparation for summers and autumns when river flow is typically lowest and water demand is at its highest. This seasonality may be made worse by the changing climate and  expansion of data centers that require more cooling in the summer-typically using water which is more cost effective than air cooling.

The need for water is constant and grows with population, industry and wealth. A community or society becomes unstable if water resources are “used up” –inadequate to meet the needs of the community by growth in demand or reduction in supply- often both. Land use decisions that result in increasing impervious cover impacts our water resources. 

Fairfax Water owns and operates the two largest water treatment facilities in Virginia with an average daily water production of 167 million gallons and a combined maximum capacity of 376 million gallons per day. The James J. Corbalis Jr. treatment plant is at the northern tip of Fairfax County and the Frederick P. Griffith Jr. treatment plant is on the northern edge of the Occoquan Reservoir in the southeast part of Fairfax County.

Within Fairfax, Prince William and Loudoun Counties are a number of residents who still obtain their water from private wells or community wells that draw from the groundwater. However, most residents are served by the large pubic water companies and that water comes from two sources: the Potomac River and the Occoquan Reservoir.

The Occoquan Reservoir is an essential part of our water supply. It is fed by Bull Run and the Occoquan River which receives up to 40 million gallons a day of the treated discharge of the Upper Occoquan Sewage Authority treatment plant (UOSA). The UOSA treatment plant  is located south of Centreville and west of Route 123. It discharges all the treated water upstream of the Occoquan Reservoir so, a significant portion of the flow (especially during dry periods) into the reservoir is recycled sewer water. 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, but it also makes the water in the Occoquan Reservoir dependent on the Potomac River.  

In addition, the Occoquan Reservoir receives stormwater – snow and rain from the Occoquan Watershed which covers portions of Loudoun, Fairfax, Fauquier, and Prince William counties and feeds the streams and creeks that feed Bull Run and the Occoquan River. As we change the land use in the region, it has a significant impact on our regional water resources. The extent of development in a watershed impacts stream health. Development results in increased impervious surface area, as new roads are built to access new buildings. Urbanization and industrial development brings a variety of pollutant sources such as oils, paints, salts, loose sediment, and other contaminants which are deposited on the impervious surfaces. Rainwater then washes these materials and other contaminants (e.g., heavy metals, pesticides, fertilizers, etc) from the land into nearby streams, either directly or through the storm drain network.

During development the primary impact is erosion and sediment that are carried by stormwater into the streams. Post-development the primary 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. The increased impervious surface area result in increased stormwater runoff that increases erosion of stream banks and reduces infiltration of water in to the ground. 

Generally, groundwater in the regional Culpeper Basin is renewed each year through precipitation. The water stored in the watershed has always been able to provide adequate base flow to the rivers and streams in droughts because historically the withdrawal of water was within the average recharge rate. However, the nearby US Geological Survey groundwater monitoring well in the Occoquan watershed is no longer stable. The water level has been slowly falling since before the last drought- despite a series of wet years.

The Potomac River and the Occoquan Reservoir are the main supply of water for Fairfax Water which also supplies Prince William Service Authority, Loudoun Water and American Water. Public water supply in Prince William County is a mix of water from the Occoquan Reservoir, the Potomac River, groundwater and Lake Manassas. Public water supply in Loudoun County is a mix of groundwater, Goose Creek, and the Potomac River. The groundwater is an essential part of that mix. 

Never forget our water supplies are connected to each other and the land. How we treat the land will impact our water supplies. As the climate changes, our region is predicted to grow wetter with more intense storms and also have more severe droughts. More water storage (additional reservoirs) and better management of our water supplies and land are required to ensure there will be enough water for our region into the future.