the dam built in 1957 by the Alexandria Water Company forms the current Occoquan Reservoir
Source: Google Maps
the dam built in 1957 by the Alexandria Water Company forms the current Occoquan Reservoir
Source: Google Maps
In 1947, the Alexandria Water Company purchased the Occoquan Hydro Electric Company. It's founder, Lt. Col. Fred Butterfield Ryons, had built a hydroelectric dam on the Occoquan River between 1928-1935. That purchase enabled the company to build two dams on the Occoquan River in the 1940's.
The Alexandria Water Company had been providing water to Alexandria from Lake Barcroft, which had been created in 1915 by building a dam across Holmes Run. In 1942 the Lake Barcroft dam was raised five feet by adding gates at the top. That doubled the size of the lake and increased the storage capacity, enabling the Alexandria Water Company to supply drinking water during the World War II population boom.
Population continued to grow at the start of the Cold War, and the Alexandria Water Company projected additional demand would exceed the potential storage capacity of Lake Barcroft. The water company chose to build a new reservoir on the Occoquan River, which had a larger (590 square mile) watershed and therefore could provide more water. Lake Barcroft was sold to developers, who constructed 1,000 houses around it.1
A 30-high dam on the Occoquan River was completed in 1950. That dam create the first Occoquan Reservoir, and 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 through 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 (UOSA), now known as the Upper Occoquan Service Authority. It built a state-of-the-art wastewater treatment plant in 1978, capable of removing nitrogen/phosphorus 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 Service Authority treatment plant is located south of Centreville and west of Route 123. That places the discharge pipe upstream of the Occoquan Reservoir. Much of the flow in Bull Run, particularly in the summer, is recycled sewage.
Wastewater flows from toilets, kitchen sinks, showers, and other sources to the Upper Occoquan Service 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 Frederick P. Griffith Jr. Water Treatment Plant, then piped to customers.
water is recycled in the Occoquan watershed between the drinking water and wastewater treatment plants
Source: Upper Occoquan Service Authority, Using UV-254 as a Surrogate to SCOD and SOC Laboratory Methods to Improve Plant Process Performance
Customers in eastern Fairfax and Prince William counties drink treated wastewater, brought to drinking water standards at both the Upper Occoquan Service Authority and then processed again at the Griffith Treatment Plant operated by Fairfax Water. Because such a high percentage of the water in the Occoquan Reservoir comes from Upper Occoquan Service Authority treatment plant, the Virgnia General Assembly established an Occoquan Policy with higher-than-average standards for treatment.
That treated wastewater is not recycled continuously. After consumers in eastern Fairfax County send wastewater into the pipes, almost all of it is processed at the Noman M. Cole, Jr. Pollution Control Plant near Lorton. Wastewater from eastern Prince William County is processed at the H.L. Mooney Advanced Water Reclamation Facility or the Virginia-American wastewater treatment plant. All three facilities discharge into tributaries of the Potomac River, and the processed wastewater flows down to the Chesapeake Bay.
Many customers in western Fairfax and Prince William counties also drink treated wastewater supplied by Fairfax Water, though the sewage discharge is further upstream from the intake on the Potomac River for the James J. Corbalis Jr. Water Treatment Plant. Multiple cities upstream from that intake, including Staunton on the North Fork of the Shenandoah River, discharge treated sewage (and untreated stormwater) into the Potomac River.
As the river flows downstream, sunlight and oxygen naturally kill harmful bacteria and improve the water quality of the discharged sewage. Because of the distance and dilution within the Potomac River, wastewater treatment plants upstream of the Fairfax Water intake are not required to meet the higher standards established for the Upper Occoquan Service Authority facility.
There are water pollution threats to the Occoquan Reservoir beyond bacteria from human use. Uranium in Fauquier County could contribute unacceptable levels of radioactivity, if that resurce was ever mined.6
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.7
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
Water quality is also at risk from perfluoroalkyl (PFAS) substances and polyfluoroalkyl (PFOA) substances, also known as "forever chemicals" because natural degradation is so slow.
In 2023, the Environmental Protection Agency proposed to lower the Maximum Contaminant Level (MCL) standards for PFAS from 70 parts per trillion (ppt) set in 2016. If dropped to 4 parts per trillion as initially proposed, a coalition of water treatment agencies claimed:8
"Forever chemical" contamination of the Occoquan Reservoir comes from multiple sources, but two particular sites were potential significant contributors.
In a groundwater test well at the former U.S. Department of Defense base at Vint Hill in Fauquier County, the PFAS level reached 1,200 (parts per trillion). A drinking water well there measured over 36 parts per trillion. The likely source was the burn pit where firefighting foam was used in training exercises.
Another source could be Mannassas Regional Airport. In 2020, a sprinkler system released firefighting foam that reached Broad Run.9
PFAS contamination in the Occoquan Reservoir can be traced in part to firefighting foam used at Vint Hill and Manassas Regional Airport
Source: L.C. Nottaasen, Practice with powder and foam extinguishers
The other significant risk to water quality in the Occoquan Reservoir is increasing salinity. In 2003, the Environmental Protection Agency established an advisory level for individuals with a very restricted total sodium intake. The drinking water threshold for those already at risk of high blood pressure and hardening of the arteries was set at 20 milligrams/liter. For aquatic life, the standard is 230 mg/l. "Freshwater" typically has no more than 500 mg/l, while oceans have 35,000 mg/L (35 parts/thousand) of various salts.10
By 2022, salinity levels in the Occoquan Reservoir were consistently above 20 mg/l and occasionally above 30 mg/l, at which point people can taste it. Blending Occoquan Reservoir and Potomac River water was one possible solution, but at the Fairfax Water intake the Potomac River salinity was reaching 16 mg/l.
The "freshwater salinization syndrome" is a nationwide issue, caused by use of laundry/dishwasher detergents and brine/salt crystals in winter to reduce ice on highways and sidewalks. Humans with a high-sodium diet also flush high-sodium waste from bathrooms.
Source: Prince William Conservation Alliance, https://youtu.be/g3bTK_kwNSA?si=TC5P9smBKGyPxZfe
A major source of salt in the Occoquan watershed is runoff from impervious surfaces, especially as snow/ice melts and washes salt applied by homeowners, businesses, and the Virginia Department of Transportation (VDOT) to improve traction. Discharge from the Upper Occoquan Service Authority (UOSA) into the reservoir has a higher percentage of sodium ions than the two major tributaries, Bull Run and the Occoquan River.
The relatively high-sodium wastewater from the Micron computer chip manufacturing plant in Manassas is included in that discharge, but the vast majority of that flow comes from homes and other commercial sources. Salt-based water softeners in homes replace chloride ions with sodium ions. Many homeowners relying upon wells for drinking water use a water softener. Sodium discharged into septic systems will flow via groundwater into Occoquan watershed streams, so as population increases the trend line shows increasing salinity in the Occoquan Reservoir.11
salt is increasing in the Occoquan Reservoir
Source: Fairfax Water, Why So Salty? Freshwater Salinization - Trends and Regional Impacts (Prince William Conservation Alliance webinar on March 5, 2024)
The Upper Occoquan Service Authority wastewater treatment plant was not designed to extract salt. The Frederick P. Griffith Jr. Water Treatment Plant had no process for desalinization. The level of salt in the raw water supply (Occoquan Reservoir) is the level of salt in the finished product delivered to homes. Standard water filters placed on faucets do not remove salt; a reverse osmosis system is required for that process.
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)