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Expanding the Dalecarlia Reservoir could give the Washington region roughly 12 additional hours of stored water during a Potomac River emergency, but the U.S. Army Corps of Engineers acknowledges the project would not solve the area’s larger drinking water vulnerability.
Trevor Cyran, chief of the Civil Works project management office for the Corps’ Baltimore District, said the agency is pursuing the expansion as a near-term improvement while continuing to examine more ambitious regional options.
“We’re trying to find a quick win that addresses some of the near-term issues, because they are quite, quite dire,” Cyran said. While the extra storage “doesn’t fix the problem,” he said, it would improve the situation.
The Corps is conducting a three-year feasibility study funded by Congress and the Metropolitan Washington Council of Governments. Lawmakers questioned the agency’s progress during a House Transportation and Infrastructure Committee hearing last week, pressing officials for dependable backup options if the Potomac becomes unusable.
D.C. Del. Eleanor Holmes Norton criticized the apparent narrowing of the study toward an expansion of Dalecarlia, arguing that more reservoir capacity is not the secondary water source Congress sought.
The Washington Aqueduct remains the sole drinking water source for the District, Arlington County and parts of Fairfax County. Norton warned that the system has only about one day of backup supply, leaving residents, government operations and the regional economy exposed to natural disasters or human-caused contamination.
The Corps does not dispute that concern, Cyran said. But it considers Dalecarlia the most likely recommendation that could deliver meaningful help relatively quickly.
Sewer collapse underscores the risk
The region recently received a reminder of its dependence on the Potomac when part of the aging Potomac Interceptor sewer line collapsed in January. Drinking water in D.C., Northern Virginia and Maryland remained safe because the failure occurred downstream from the primary Potomac intake serving the Washington Aqueduct.
Still, Cyran said the spill demonstrated that a serious threat is not merely hypothetical.
The public has been advised to avoid contact with the Potomac in the District and Montgomery County near the spill site along Clara Barton Parkway. The D.C. Department of Health was expected to lift the advisory Monday after E. coli readings returned to levels typical for the District’s rivers.
The District Department of Energy and Environment is now testing the Potomac and Anacostia rivers daily.
Dalecarlia could double its capacity
The Corps estimates that expanding Dalecarlia across 54 available acres could add capacity equivalent to about 70 million gallons per day, roughly doubling the reservoir’s current capacity.
Because the Washington Aqueduct already owns the property, the project would not require a land purchase.
Officials have not determined whether the added capacity would hold raw Potomac water or finished drinking water that has passed through the Aqueduct’s purification system. Either approach would give the system more water to draw upon if the river suddenly became unsafe.
Another near-term possibility is adding advanced treatment capable of handling a broader range of contaminants. Cyran cautioned that the Potomac Interceptor failure, which discharged approximately 240 million gallons of raw sewage into the river, might not be a good example of the type of incident that such technology could address.
Water reuse also remains on the Corps’ list of possibilities. In November 2025, DC Water described plans to recycle water from its Blue Plains Advanced Wastewater Treatment Plant, the largest facility of its kind in the world.
Quarry storage remains decades away
Potential long-term regional solutions include using the Travilah Quarry in Montgomery County and two Loudoun County quarries owned by Luck Stone.
Travilah, on Piney Meetinghouse Road in Rockville, was already under quiet consideration in 2016 as an emergency source for DC Water, WSSC Water and Fairfax Water. Those utilities, together with the Interstate Commission on the Potomac River Basin and the Metropolitan Washington Council of Governments, have spent years examining ways to make the regional system more interconnected and resilient.
A decade later, major logistical, financial and real estate obstacles remain.
Cyran said the quarry concepts are not impossible, but they cannot be implemented quickly. Travilah is still operating, and storage may not be possible there until mining ends—potentially 30 years from now.
Dalecarlia would also fall short of a true regional solution because its benefits would be limited to customers directly connected to the Washington Aqueduct. The Corps nevertheless plans to keep examining alternatives that could eventually strengthen water supplies across multiple utility systems.
Study limits constrain broader options
An interconnected network capable of sharing water among DC Water, WSSC Water and Fairfax Water would provide greater resilience. But Cyran said the Corps must operate within congressional rules that generally limit a feasibility study to four years and $5 million.
“We can’t hand half-baked ideas to Congress,” he said.
For now, the agency is concentrating on improvements that can be delivered sooner while continuing to explore a larger regional strategy. Cyran said expanded congressional authority, along with additional time and funding, could allow the Corps to pursue a more comprehensive solution.