Contaminated site cleanup in areas affected by Potomac Interceptor overflows has initially completed but more environmental remediation will be conducted. Photo courtesy of DC Water

“What we see here has been herculean,” said DC Water CEO and general manager David Gadis at a March 17 briefing. The hastily constructed channel bypass had prevented nearly two billion gallons of sewage from reaching the river since late January, he said, allowing the waterway to recover from the extensive discharge of contaminants. The event is one of the largest ever sewage spills in the U.S.

“Bacteria tests are trending in the right direction,” Gadis said, noting that most downstream areas are close to being within safety limits for recreational use.

DC Water has yet to officially state the expected cost of the spill response and repair effort—with reports it is a $20-million to $30-million effort—or how the incident will affect scope and cost of a planned $625-million rehabilitation program for the agency's entire 54-mile pipeline network.

President Donald Trump’s Feb. 21 disaster declaration, requested by Washington Mayor Murial Bowser (D), authorized the Federal Emergency Management Agency to coordinate all disaster relief efforts, which have also involved the U.S. Army Corps of Engineers’ Baltimore District, U.S. Environmental Protection Agency and Maryland Dept. of the Environment. FEMA-administered public assistance funding, which must be approved by Congress, will cover 75% of approved costs.

A Rapid Response

The Potomac Interceptor carries up to 60 million gallons of wastewater daily from Washington, D.C.’s Maryland and Virginia suburbs to the Blue Plains wastewater plant for treatment. DC Water says the rupture, which remains under investigation, sent sewage flowing into an adjacent natural culvert where it was then channeled beneath the canal and into an unnamed tributary of the river.

The first days after the rupture’s discovery saw DC Water and its contracting team, led by Washington, D.C.-based Anchor Construction Corp. and Fort Meyer Construction Corp., contend with major weather events, flow debris and other challenges to keep the crippled pipeline operating while limiting environmental damage around the collapse site and drainage areas tainted by the overflow.

Within a week, the team had implemented an eight-unit, 60-million-gal-per-day bypass pumping system that prevented subsequent large sewage spills and sufficiently lowered pipeline flows to allow inspectors to safely access the interior. The discovery of a large interior blockage of large rocks and boulders at the collapse site presented a literal and figurative obstacle to the repair effort.

Moussa Wone, DC Water vice president of engineering, told ENR that the boulders likely originated from backfill during the underground pipeline’s original construction. “It was essentially a cave-in, although we’re still unsure what caused it,” he said.

With the addition of another seven upstream pumps and a steel bulkhead that fully diverted interceptor flow around the collapse area, contractors lifted the pipeline crown to safely remove the obstructions and install geopolymer lining to reinforce 320 ft of adjacent upstream and downstream sections. Tests confirmed the repair area’s stability, allowing DC Water to gradually deactivate pumps and restore full flow through the section on March 14.

DC Water is evaluating other sections of the Potomac Interceptor and other system infrastructure to ensure no similar ruptures are imminent, DC Water CEO David Gadis said at a March 17 site briefing.
Photo by Jim Parsons for ENR

Comprehensive Cleanup

As emergency repair progressed, stormwater from a mid-February rainstorm overwhelmed the area’s drainage system and inundated DC Water’s pumping site. To protect the area and prevent more contaminated soil from entering adjacent waterways, the U.S. Army Corps of Engineers and Grunley Construction Co., Rockville, Md., established an 11-pump stormwater diversion system with a capacity of nearly 7,900 gallons per minute.  

Maj. Charles Martin, the Corps project manager, said soil and water quality tests will determine how much longer the system is needed as environmental restoration of contaminated areas continues. Two to three inches of soil and accumulated materials are being removed, followed by installation of organic matting and a new soil cover. Restoration finished of a natural tributary where the sewage spill entered the river, as well as of its adjacent shoreline

“Currently, our contract completion date is March 31, which includes the days of demobilization,” Martin noted. “Once successful remediation of the area is completed, and we receive favorable soil test results, the diversion system will no longer be needed.”

He said that a downstream water intake for the Corps-operated drinking water system serving metropolitan Washington was not in use when the spill occurred, and should require no special maintenance prior to its planned reactivation this summer.

Two sections of the Potomac Interceptor where the crown was removed and encased with 20-ft-deep trench boxes for the emergency repairs will remain as open-flow channels for the permanent repair effort, which includes sliplining a 2,700-ft section to encompass the collapse area. Material has been ordered for what is expected to be a nine-month process, with installation to begin soon of mechanical systems to push new pipe sections into place, DC Water said.

To restore the canal, a temporary upstream dam constructed by the National Park Service will provide fresh water to flush the bypass area into the interceptor. Debris clearing, mucking and removal of contaminated soil will follow, a process that DC Water said will likely take up to two months.

Wone said the Park Service “bought into this plan from Day One, asking only [to] be kept informed and the area repaired once we were done. The cooperation was remarkable.”

Meanwhile, the U.S. Environmental Protection Agency will now lead region-wide water quality testing in the river, while environmental advocates and experts continue independent testing and water and sediment quality monitoring, including the Potomac Riverkeeper Network, University of Maryland and George Mason University.

DC Water is also evaluating other sections of the Potomac Interceptor to ensure that no similar ruptures are imminent, “We’re looking at the rest of the pipeline and what’s needed to keep this from happening again," Gadis added.  

The utility now is fast-tracking an aerial drone assessment, with LIDAR scanning, of the Interceptor alignment "to evaluate current site conditions and better understand the changing landscape ... including areas where there may be rocks or boulders that need further on the ground inspection." The technology, in its first use by DC Water, will create high-resolution 3D maps of buildings, vegetation and ground surfaces, with data to support the long term system upgrade, it said, also noting ongoing inspections inside the pipe.

"This episode is far from over, as DC Water and the state and federal authorities must coordinate on repairs, response and restoration for months, even years, into the future," said Betsy Nicholas, Potomac Riverkeeper Network president.