The Water Impacts from PW Digital Gateway

The Occoquan Reservoir contains 8.33 billion gallons of water (calculated in 2010 when the volume of the reservoir was last calculated) that provides 40% of the daily water supply for Fairfax Water which supplies about 2 million people including 350,000 in Prince William County. The Occoquan Reservoir is already the most urbanized watershed in the nation. Prince William County seems determined to make it industrialized.

Development in the Rural Area threatens the health of the Occoquan watershed and the very sustainability and affordability of the drinking water supply for Northern Virginia. When generally open rural area is developed stormwater runoff increases in quantity and velocity washing away stream banks, flooding roads and buildings carrying fertilizers, oil and grease, and road salt to the Occoquan Reservoir.

Increased development in the Bull Run and Occoquan watershed as outlined in the PW Digital Gateway CPA will increase paved and compacted surfaces and runoff and decreased forested and agricultural land. The result will decrease groundwater recharge, reduce stream and river flow and increase salinity and chemical and sediment contamination in the watershed.

Increased stormwater volumes, sediment loads and pollutant loads are a direct result of clearing the land of trees and vegetation, construction activities for buildings and roads. Increases in sediment would serve to reduce the water storage capacity of the Occoquan Reservoir beyond the 18% that has been lost over time; and increase the Prince William County discharges to the Chesapeake Bay. These discharges would result in non-compliance with theU.S. EPA mandated Total Maximum Daily Load (TMDL) standards required by the Chesapeake Bay agreement.

According to Odhimbo et al (see citing below), the sediment accumulation rate estimates in Occoquan Reservoir range from 0.26 g cm-2 year-1 in the upper reaches to 0.37 g cm-2 year- in the lower reaches at this time. Lake Manassas also had comparable accumulation values ranging from 0.22 to 0.40 g cm-2 year. However, the reservoirs' sediment accumulation rates and basin soil losses reflects the current land use and cover, basin slopes, and erosion mitigation efforts within the watershed. The lower reaches, though more urbanized, have well-developed storm water management systems with an MS4 permit limiting run-off and related soil losses. The forested riparian zones surrounding both reservoirs also has served to limit sediment fluxes, producing the relatively low sediment accumulation rates. That  would change with the development of the PW Digital Gateway.

In addition, salinity in the Occoquan Reservoir would increase. There are two ways that data center will increase the salt in the Occoquan. The first is increased pavement.  In winter all that pavement in parking lots around backup generators and roads will be sprayed with brine solution or salted and increase the salt content in the runoff.

The second source of salt from data centers is the cooling system. Data centers use water (lots of water) for cooling and that evaporates some of the water concentrating the salt that is already in the water. In addition to increase the efficiency and life of cooling equipment they soften it (which means they add brine, salt water). The minerals and salt build up in the cooling tower and are blown out. The cooling tower blow down contains high salt levels and is sent to the wastewater treatment plant which has no ability to remove salt.

Data centers are also a potential source of diesel contamination to the watershed. The data centers need to operate 24/7 so they maintain a backup power system consisting of banks of giant generators. Data centers need immense amounts of power (2.6 gigawatts capacity and growing fast) they require about 40 - 2 Mw generators each to keep operating.  Backup generators at a data center usually run on diesel; not one of the cleanest fuels. Banks of backup generators are lined up at each and every data center. To power these generators, diesel fuel needs to be stored onsite, so each site contains large fuel storage tanks with pipes and valves to feed each generator- all potential spots for leaks and spills. Despite their size and the amount of fuel on-site backup generators are not subject to the same secondary containment and monitoring requirements as USTs. The environmental control considerations and requirements are not part of the PW Digital Gateway CPA.

Despite available tools, Prince William County did not study the impact of the proposed changes to the quality, availability and sustainability water supply as they are required to do under the Comprehensive Plan law. Nor did Prince William County study the impact of the proposed changes of the 2040 Comprehensive Plan and PW Digital Gateway to compliance with the WIPIII developed by Virginia Department of Environmental Quality and approved bythe U.S. EPA. Moving ahead without examining the consequences of building more than 27.5 million square feet of additional data centers, $5 billion in roads and the massive rezoning throughout the county is denial and irresponsible. We do not have the luxury of acting without thought or concern for the consequences when we see a shiny pile of gold. Before we do irreversible harm to the ecology and our regional drinking water supply, the county needs to look at what the impacts of planned changes will be to the water supply.  

Fairfax Water has taken the unusual step to ask that Prince William County convene the Occoquan Basin Policy Board and oversee a Comprehensive Study of the proposed PW Digital Gateway CPA and the 2040 Comprehensive Plan Update to evaluate their impact on water quality and quantity in the Occoquan Reservoir before any action is taken. The cost to restore the basin and treat the water is in the billions of dollars (if it can be even done) that will be borne by us, the residents who remain- not those who get the windfall from the sale of their land and dash off with their millions. We are sacrificing the ecology of Prince William County, the reasonable cost of and availability of adequate drinking water supplies for all and even the relatively low cost, availability and reliability of power.

1. Odhiambo BK, Rihl G, Hood-Recant S. Historic land use and sedimentation in two urban reservoirs, Occoquan Reservoir and Lake Manassas, Virginia, USA. Environ Sci Pollut Res Int. 2022 Feb;29(8):11481-11492. doi: 10.1007/s11356-021-16461-2. Epub 2021 Sep 18. PMID: 34535864.