Assuring the Water Supply in the DMV

 Since the mid 1800’s, the Potomac River has been the source of drinking water for Washington DC. Today the Potomac River supplies 78% of the Washington, DC, metropolitan area’s water, with public water supply intakes for WSSC, the Washington Aqueduct, Fairfax Water and Loudoun Water located in the river upstream of the city. (For security reasons the exact locations of the water intakes is not disclosed and is blocked on Google Maps).

The Potomac River belongs to Maryland. In 1632 King Charles I granted to Lord Baltimore land from the “first fountain” of the Potomac River and along its south shore to the bay. There have been numerous legal cases over the centuries tightening up that boundary, but the fundamental ownership of the Potomac has not changed. There is; however, a multijurisdictional agreement to share the waters of the Potomac.

This agreement established the Interstate Commission on the Potomac River Basin (ICPRB) to cooperatively preserve water quality and to conserve and share water and related land resources of the Potomac River Basin. ICPRB is a non-regulatory agency that promotes water quality through watershed-based approaches. In 1979, the Commission created the Cooperative Water Supply operations on the Potomac (CO OP), which provides water usage forecasts and coordinates water management of the Upper Potomac Reservoirs and with the operations of the water utilities in the Washington metropolitan area.

The ICPRB was one of the first organizations with a congressional mandate to consider water resources on a watershed basis, rather than along political boundaries. However, now, we have reached the point in population density and development that during times of drought, natural flows on the Potomac are not always sufficient to allow water withdrawals by the utilities (including power generation which takes an awesome amount of water) while still maintaining a minimum flow in the river for sustaining aquatic resources. 

Current threats to the region’s water supply include severe droughts due to climate change, population and demand growth from data centers; increasing salinization of the Potomac and the watershed, and potential spills intentional or accidental polluting the Potomac River.  The region’s water supply relies heavily on the Potomac River and would be faced with moderate to severe water shortages in the event of an interruption in the Potomac River water supply.  Washington DC has the least storage. A shutdown of Potomac River intakes is predicted to result in a critical loss of water in some areas of the region after just one day.

During drought, water from three upstream reservoirs can be released if necessary to increase river flow, but it would take nine days for the water to reach the river intakes for the water treatment plants and be treated. Use of these upstream reservoirs would not be feasible in the case of an accidental or intentional spill. Plus, drought response in made tricky because of the time delay and that during an extreme drought the Jennings Randolph Reservoir is unlikely to be refilled. The ICPEB needs to carefully pick their moments for release. 

The public water suppliers in the region utilize the Potomac River as a source of raw water and distribute treated water to homes, businesses, and critical government facilities. Combined, they serve five million residents and over three million workers in the District and surrounding Maryland and Virginia. DC Water distributes the water treated from the Washington Aqueduct to Washington DC and only has enough water storage to supply the District for 24 hours, if not less. Without water, every facet of the city and government is affected. This vulnerability, though well-known regionally was laid bare last summer, when an algal bloom on the Potomac River overwhelmed the Dalecarlia Water Treatment Facility run by the Washington Aqueduct.

Drought remains a significant threat. We have not had a severe drought in this region in over 30 years, but the water basin has been in a low level drought for two years. The ICPRB allocates and manages water resources of the river through the management of the jointly owned Jennings Randolph Reservoir (built in 1981), Potomac River Low Flow Allocation Agreement (1978) and the Water Supply Coordination Agreement (1982) which designated a section of the ICPRB as responsible for allocating water resources during times of low flow. These steps improved reliability of the water supply and ensured maintenance of in-stream flows to meet minimum aquatic habitat requirements as defined by the Maryland Potomac River study in 1981. The section of ICPRB responsible for all this is known as the Section for Cooperative Water Supply Operations on the Potomac (CO-OP), and is formally empowered in its duty by the Water Supply Coordination Agreement. 

Currently,  for a limited period of time Fairfax Water can supply their entire system from the Occoquan Reservoir and that is one of the most powerful tools available for supply management. Fairfax Water is engaged in a long term plan of expansion of their water storage. The Vulcan Quarry almost adjacent to the Occoquan Reservoir will be converted to a reservoir in phases and continue to operate during phase 1 which will convert a portion of the quarry to a reservoir with storage of of about 1.8 billion gallons by 2035. Quarry operations will end with Phase II which will convert the remaining area to Fairfax Water reservoir with storage capacity of up to 15 billion gallons by 2085. The Vulcan Reservoir will store water from the Occoquan Watershed.  Loudoun is adding the Luck Quarry as a reservoir to store water diverted from the Potomac River. 

The ICPRB’s 2017 study, Washington Metropolitan Area Water Supply Alternatives, evaluated a suite of options to address increased drought severity in the face of climate change and identified use of a local quarry to create a backup supply for WSSC and the Washington Aqueduct to complement the Occoquan. This could allow the entire DMV to step off the Potomac for a period of time.  Another study, the National Capital Region Water Supply and Distribution System Redundancy Study, completed in 2016 for the Metropolitan Washington Council of Governments, also concluded that raw water storage in a local quarry was an effective solution to the threat of water shortages or disruptions to use of the Potomac River.

An existing quarry, Travilah Quarry, located in Montgomery County, Maryland, could be converted to a regional raw water storage reservoir with tunnels to carry water by gravity to  the Washington Aqueduct and WWWC  water treatment plants, bypassing the Potomac River. The estimated project cost was $800 million. Black & Veatch was retained by the Interstate Commission on the Potomac River Basin to perform the feasibility study of potential prerequisites for use of the Travilah Quarry as a raw water supply storage facility, to supplement the existing water supply for the Washington Suburban Sanitary Commission and the Washington Aqueduct.

The assessment was divided into two phases. The first phase of the study focused on studying characteristics of the quarry for water storage potential and water quality aspects. The study found the quarry suitable for raw water storage:

• The current storage available in the Quarry is approximately 7.3 billion gallons (smaller than the Occoquan Reservoir).
• However, the ultimate storage in the Quarry could reach 17.4 BG, based on the mining plans and quarry reservoir pool elevation. This capacity is expected to be available sometime around 2060
• It is expected that stored water will be in the range of available water treatment technologies. The management of water quality can be accomplished through inlet/outlet design and pumping strategies.

The second phase assessed potential options for conveyance, pumping and presents life cycle costs for different alternatives. It includes a conceptual design and layout of the infrastructure necessary to convey water from the quarry and plant to the Washington Aqueduct and ultimately concluded that the Conveyance Tunnel connection to the Great Falls was the better solution in terms of constructability, operations, cost and community acceptance. The tunnel can be built without much interference with the existing utilities and surface features. Because of the existing infrastructure at Great Falls, the consultant expected that this alternative offers a relatively compact system in terms of ease of system integration and O&M.