Discovered Monday, February 12, the sinkhole at DC Water’s Manhole #31 appeared small.
  

The DC Water and Sewer Authority's Potomac Interceptor sewer line at Manhole #31, approximately 400 feet (just more than a football field) from the Potomac River, and varying proximity to 18 private wells in Great Falls, 158 feet to 2,755 feet, failed on or before Feb. 12. 

As part of its emergency response plan, DC Water immediately activated an on-site bypass pumping contractor. On Tuesday, Feb. 13, "the site experienced a substantial amount of rain," and on Feb. 14, the pipeline again failed. "The eroded area expanded substantially," DC Water reported. The utility company provided the words as captions for slides presented by the Fairfax County Health Department at the Feb. 27 virtual Great Falls Community Meeting.

"I recognize that DC Water was handling certain things, but they don't report to you, our county staff, and I do," said Fairfax County Supervisor Jimmy Bierman (Dranesville) at the meeting. "I'm not very happy with DC Water right now for a number of reasons… . I felt like we needed to have a follow-up community meeting and that we needed to bring in the experts that we have in the county to try and give you as much information as possible."

The utility company collects sewage for several adjoining localities in Maryland and Virginia, including Fairfax County, transporting about 60 million gallons of wastewater daily via the Potomac Interceptor to the Potomac Pumping Station in Washington, DC. Flows from the pump station are then sent to the Blue Plains Advanced Wastewater Treatment Plant for treatment before discharge into the Potomac River. DC Water provided the updated statements in the captions of photographs shown as slides at the Fairfax County Health Department's community meeting.

The broken pipe was part of a larger project to upgrade and rehabilitate the Potomac Interceptor.

Concerns and speculations had spread among the close-knit Great Falls community, particularly after the Fairfax County Health Department issued a boil-water advisory to potentially affected households. The county conducted bacteriological tests from Saturday, Feb. 17, to Sunday, Feb. 25, excluding testing on Feb. 20. Coliform bacteria were detected in 11 of the 77 test results of the 18 impacted wells.

Breaking it down further by well occurrence, Fairfax County reported that four of the 18 private wells tested positive for coliform organisms, with one positive for E. coli. Wells #3 and #8 tested positive for coliform bacteria once each; well #4 tested positive three times; and well #9, the furthest away from Manhole #13, tested positive six times.

Questions arose about evaluations of well water quality, safety precautions, well disinfection with response options, how the contamination occurred, and whether there was an impact on the Potomac.

Seamus Mooney, coordinator for Fairfax County's Department of Emergency Management and Security, provided an overview of the repairs from the DC Water perspective. Mooney said DC Water installed four pumps and was bypassing pumping with a fifth pump installed and pumping water back into Manhole #31. The process had been ongoing for five days, with DC Water moving slowly "to ensure the pumps can handle the capacity." According to Mooney, DC Water was to decide when to insert the plug into the line at the end of the week to begin the dewatering process.

Christopher Herrington, P.E., director of Fairfax County's Department of Public Works and Environmental Services, presented findings from the hydrogeologist’s report and an overview of what the lab reports on the well testing indicated. Other issues might be with the private wells or homes' plumbing.

Herrington stated that while the county's Department of Public Works does not directly regulate or monitor groundwater, he was there to assist the Health Department because, as a professional, he had over three decades of experience with water quality and investigation across multiple aquifer systems.

Going through several slides, Herrington described Manhole 31 as being north of the wells and having a higher elevation gradient than the Potomac interceptor. "Water flows downhill unless it is under pressure," Herrington explained. He pointed to the 18 well locations and stated there was no reason to believe any sewage traveled over land.

Local patterns indicate that the groundwater gradient was flowing towards the excavation rather than from it to the wells. Regarding what could be going on underground — bedding planes, faults, or fractures in the bedrock that could potentially convey water — Herrington stated that the pressure gradient in the bedrock beneath the excavation site is believed to be going downhill toward the excavation.

According to Herrington, E. coli in water samples is a good indicator of bacteria because it indicates fecal contamination. The sewage that would have filled Manhole 31 contained hundreds of thousands of colony-forming units of E. coli bacteria per deciliter. When they examined the data, they found only one well had a few samples with E. coli indicator bacteria. Furthermore, given that the groundwater flow is heading north toward the Potomac, some of the intervening wells should have detected E. coli, but they did not.

"I can say that there is no conclusive evidence that Manhole # 31 sewage contaminated these wells," Herrington said.

Dr. Parham Jaberi, deputy director for Medical Services, Fairfax County Health Department, discussed coliforms and explained that they are found in soil and indicate fecal contamination. E. coli is a specific type of coliform that can cause serious illness. The Health Department advises homes with positive coliform and E. coli tests to boil water and disinfect their wells. The department will issue new letters outlining these recommendations. 

Jaberi emphasized the importance of retesting two weeks later to ensure that the biological contamination had been removed. He also said that pipes could have biofilm buildup. Jaberi considered that it is possible that in homes with coliform, the septic system is not functioning optimally and may add additional contaminants to the environment. "It's a good idea also to check your septic system."

Fairfax County reports more than  15,000 homes and businesses rely on groundwater wells for their domestic water supply.