Purple Line construction in December in downtown Bethesda, Md. (Doug Kapustin for The Washington Post)

Suburban Maryland’s water utility has asked state transit officials to “cease and desist” construction on part of the Purple Line that it says is perilously close to a major pipe that provides drinking water to Prince George’s County and would explode if broken.

The Washington Suburban Sanitary Commission (WSSC) says that, despite its objections for several years, the Maryland Transit Administration (MTA) has allowed its contractor building a train maintenance and storage facility in Glenridge to move soil atop an underground, 66-inch water main.

If the main broke under the state’s current Purple Line design, the WSSC says, the force of the water could wipe out train tracks, harm passengers, and limit drinking-water and fire hydrant operations in southern Prince George’s. WSSC officials say regrading also would put the pipe up to 30 feet underground — too deep to reach easily for routine maintenance or emergency repairs.

“I am writing to demand that you cease unauthorized construction work at the Glenridge Rail Storage Yard until further notice,” the WSSC’s general manager, Carla A. Reid, wrote Friday to MTA head Kevin Quinn.

Jeff Ensor, the MTA’s director of project delivery and finance, said he and other Purple Line project members showed WSSC officials during a site visit Tuesday that nothing has been built atop the pipe. Moreover, he said, state transit officials told the WSSC last week that they had agreed to move the water main farther from the tracks and facility.

Purple Line work underway in February in Silver Spring, Md. (Evelyn Hockstein for The Washington Post)

Under current plans, Ensor said, the Purple Line’s maintenance and operations building would have been 65 to 85 feet from the pipe, and train tracks would have been 50 to 70 feet away. He said the WSSC has been objecting to the pipe’s proximity for more than a year.

“Last week, we said we understood their concerns and offered to relocate the pipe, and we’re working through the details of that with WSSC,” Ensor said. “Both parties, I think, are optimistic that we’ll have a good resolution.”

WSSC officials have long worried about new construction within 80 feet of their massive, highly pressurized distribution mains. A 2012 utility analysis found that the pipes can explode with the force of 20 to 200 pounds of dynamite and spew water at up to 90 mph.

A 66-inch water main that ruptured in Bethesda in 2008 flooded River Road, leaving stranded motorists to be rescued by boat and helicopter. A torrent of water from a 54-inch pipe that broke in 2011 near the Capital Beltway in Prince George’s blew off doors and ripped out walls in nearby businesses and threw parked vehicles upside down.

Those pipes were made of concrete. The one buried near the Purple Line construction is made of steel and was installed in 1991, WSSC spokesman Chuck Brown said. However, he said, it is pressurized in the same way and, if it broke, “would cause a considerable amount of damage.”

Ensor said the state has agreed to move the main to make it easier for the WSSC to access for repairs, not out of any safety concerns.

“Safety is [the state’s] number-one priority,” Ensor said. “We feel confident that even if the pipe stayed in place, nobody would be at risk, even with a pipe blowout.”

He said water pipes lie safely beneath rail lines all over the country, including Metro tracks. This water pipe is in “very good condition,” he said.

“Structurally, it could handle everything the contractor planned to put on top of it,” he said.

He added that the contractor is not doing any work atop the water main and will not until the WSSC approves it.

But Brown said Tuesday’s site visit supported the utility’s “cease and desist” letter because utility officials photographed “large amounts of dirt piled on top of the main,” which makes the “critical” pipe too difficult to reach in an emergency.

Per an agreement, Brown said, the MTA is supposed to seek the WSSC’s approval for any construction work that affects the water main or “continuous access” to it. He said the state’s 2014 Purple Line plan showed the water pipe being relocated, but the design apparently changed.

“We are glad the state has agreed to pay for the relocation of the pipe,” Brown said. “We now need them to agree to stop work impacting access to this crucial main. We stand by our cease-and-desist letter.”

A spokesman for Prince George’s County Executive Angela D. Alsobrooks (D) said county leaders are concerned about potential damage to a pipe that provides drinking water to so much of the county. He said county officials have been in touch with the MTA and its contractor, Purple Line Transit Partners.

“We made sure they understand we want them to work together to remedy this issue,” spokesman John Erzen said. “We’ll continue to monitor the situation as we go forward to see what solution they come up with.”

Maryland transit officials have said the pace of the Glenridge facility’s construction on Veterans Parkway (Route 410) is key to the 16-mile Purple Line’s overall timeline because it must be completed before trains can be tested between it and the New Carrollton station. It will be the primary storage and maintenance yard for Purple Line trains, as well as the main operations center.

Ensor said he did not know how much it will cost to move the pipe or who — the state or contractor — will pay for it.

That is likely to further complicate a long-running debate over when the Purple Line, now in its second year of construction, will begin carrying passengers and how much its $2.4 billion construction budget might grow.

The line is scheduled to open to passengers in December 2022. However, the contractor has said it will not be done until March 2023, and then only if the state pays an additional $300 million to offset delays and accelerate work.

Purple Line officials have said they have not agreed to pay any cost overruns. Ensor said that he did not know how long it would take to move the Glenridge pipe but that it would not add to any construction delays.