the dam built in 1957 by the Alexandria Water Company forms the current Occoquan Reservoir
Source: Google Maps
In the 1950's, two dams were built on the Occoquan River by the Alexandria Water Company. That company was providing water to Alexandria from Lake Barcroft, which it constructed in 1915. In 1942 the dam was raised five feet by adding gates at the top, doublig the size of the lake and increasing the storage capacity to supply the population boom due to World War II.
With the increased growth at the start of the Cold War, the Alexandria Water Company projected additional demand that exceeded the storage capacity of Lake Barcroft. The company sold the lake to developers who constructed 1,000 houses around it.1
The water company chose to build a new reservoir on the Occoquan River, which had a larger (590 square mile) watershed. A 30-high dam constructed in 1950 impounded 55 million gallons. In 1957, the company built a 70-feet high dam upstream to impound almost 10 billion gallons.2
the first Alexandria Water Company dam on the Occoquan River was built in 1950 at the Fall Line, upstream of the Town of Occoquan
Source: ESRI, ArcGIS Online
the Occoquan River drains a watershed of 590 square miles
Source: Northern Virginia Regional Commission, Occoquan Watershed: "Where Is It and What�s In It"
As Fairfax County developed in the 1950's, local officials sought to ensure an adequate water supply. The county created a water authority and purchased the Annandale Water Company, then notified the Alexandria Water Company that it would acquire the private company's land and facilities at the Occoquan Reservoir. When the company refused to sell, the county condemned the property and acquired it trough the right of eminent domain.
A lawsuit filed in state court determined the price the county had to pay for the assets. The county proposed a value of $28 million, while the company claimed its property was worth $55-65 million.
the Occoquan River in 1937, prior to construction of two dams creating drinking water reservoirs
Source: Prince William County, County Mapper
the 1950 low dam is downstream of the 1957 high dam
Source: Historic Prince William, Occoquan - #77
The jury determining the award visited the water treatment plant and dams, and the Alexandria Water Company ensured all the facilities looked their best before the visit. A common comment when walking through the plant was:3
The final award was nearly $50 million, and the county acquired the property in 1967.4
Fairfax Water controls water levels of the Occoquan Reservoir, which backs up into Prince William County
Source: Historic Prince William, Clearing at the end of Lake Occoquan Dr.
After Hurricane Isabel in 2003 knocked the water system offline for the online time in its history, Fairfax Water built the new Griffith Water Plant and opened it in 2006.5
In 2016, the public utility allowed the Town of Occoquan to open up the site of the old treatment plant as River Mill Park.
the old water treatment facilities on the Occoquan River were demolished (except for the underground storage tanks) and the site was opened as River Mill Park on July 30, 2016
The Alexandria Water Company morphed into Virginia-American Water. That private utility continues to supply Alexandria and eastern Prince William County with treated drinking water, which it purchases at wholesale prices from Fairfax Water.
As Fairfax Water expanded its system of pipes to supply drinking water throughout the county, it also had to address eutrophication of the reservoir. Occoquan Reservoir received the treated wastewater discharged from almost a dozen sewage treatment plants in the watershed. High levels of nitrogen and phosphorous in the wastewater allowed excessive growth of cyanobacteria (blue-green algae). Oxygen levels in the water dropped to near-zero and the reservoir literally stank in the late summer months, as massive amounts of cyanobacteria decayed.
It was too expensive to divert the sewage out of the watershed, pumping it to treatment plants that discharged into the Potomac River. In addition, that option just transferred the problem rather than solved it.
The solution was to create the Upper Occoquan Sewage Authority and build a state-of-the-art wastewater treatment plant in 1978, removing the nutrients and discharging water clean enough to drink. The old wastewater plants were closed, and the reservoir became an attractive artificial lake again. When necessary, excessive levels of cyanobacteria are kept in check through application of copper sulphate on the surface of the reservoir.
The Upper Occoquan Sewage Authority treatment plant was located south of Centreville and west of Route 123. That placed the discharge pipe upstream of the Occoquan Reservoir, so much of the flow in Bull Run and water in the reservoir is recycled sewage.
Wastewater flows from toilets, kitchen sinks, showers, and other sources to the Upper Occoquan Sewage Authority plant. There the sewage is treated and discharged into Bull Run. Bull Run flows down into the Occoquan Reservoir. Water is pumped from the Occoquan Reservoir and processed to meet drinking water standards at the Griffith Treatment Plant, then piped to customers.
water is recycled in the Occoquan watershed between the drinking water and watstewater treatment plants
Source: Upper Occoquan Service Authority, Using UV-254 as a Surrogate to SCOD and SOC Laboratory Methods to Improve Plant Process Performance
For customers with pipes connected to the Upper Occoquan Sewage Authority, the cycle of water is continuous. Most customers of the Virginia-American Water utility live outside the Occoquan watershed. For those customers, the cycle is broken and water molecules are not recycled back continuously to the Upper Occoquan Sewage Authority wastewater treatment plant.
The 305(b)/303(d) Water Quality Assessment Integrated Report issued by the Virginia Department of Environmental Quality (DEQ) listed the upper Occoquan Reservoir (above the second dam) as "impaired." Fish tissue samples have revealed excessively high levels of polychlorinated biphenyls (PCB's), so the reservoir is Not Supporting for fish consumption.6
the Occoquan Reservoir is listed as "impaired" due to excessively high levels of polychlorinated biphenyls (PCB's) in fish tissue samples
Source: Virginia Department of Environmental Quality (DEQ), Environmental Data Mapper
the Occoquan Reservoir receives wastewater from the Upper Occoquan Service Authority
Source: ESRI, ArcGIS Online
wastewater treatment tanks at Upper Occoquan Sewage Authority
Source: Historic Prince William, Aerial Photo Survey 2019
Fairfax Water maintains a warning system, in case the dams on the Occoquan River are at risk of breaking
Source: Fairfax Water, Occoquan Dam Siren
Source: Northern Virginia Regional Commission, Reclaiming Our Water - The Occoquan River Watershed
the Occoquan Reservoir receives stormwater runoff from Loudoun, Fairfax, Fauquier, and Prince William counties
Source: Northern Virginia Regional Commission, Occoquan Watershed: "Where Is It and What's In It"
Occoquan reservoir in 2010, showing how land use planning in 1980-2000 led to contrasting levels of development on Fairfax County vs. Prince William County sides
Source: US Geological Survey (USGS), Occoquan 7.5x7.5 topographic quadrangle (2010)