PITTSFIELD — A mediator's attempt to find a path forward on the Housatonic River cleanup inches forward, with cautious participation by environmental groups — and one notable absence.

Many of the parties that faced off at the nation's top environmental court in 2017 have agreed to continue discussions outside court. They are looking for a sort of wormhole that can resolve a decadeslong dispute over the General Electric Co.'s tainted legacy in the Berkshires.

Missing was the state of Massachusetts itself. The Department of Environmental Protection is focusing instead on defending an order that soils and sediment contaminated with polychlorinated biphenyls, or PCBs, be taken to disposal sites outside Massachusetts.

When the Environmental Appeals Board returned a decision this year, its three judges asked federal Environmental Protection Agency officials to revisit their order that toxins be shipped out of state.

That decision has been estimated to represent $250 million of the roughly $630 million cost of continuing the EPA's prescribed Rest of River cleanup of the river south of downtown Pittsfield.

Attorney John G. Bickerman met with mediation parties Tuesday in the Lee Town Hall.

"It's hard to predict what's going to happen," he said later in the week. "It's just not possible right now to know. These are difficult issues."

Missing from the proceedings is the state DEP — a key party to the cleanup plan.

Instead of joining mediation, the agency is helping staff of the EPA respond to January's request from the environmental court for more information on the disposal-site issue.

In taking that stand, the DEP, joined by the Department of Fish and Game, is working to support the state's opposition to burying PCB-laden materials locally.

"The Housatonic River is a valuable natural resource that provides a great environmental and economic benefit to the western portion of our state," said Ed Coletta, the agency's spokesman. "The Baker-Polito administration remains committed to its cleanup and restoration."

The agency's apparent goal is to see that the federal EPA's order on the Rest of River cleanup, issued in late 2016, is upheld.

Pursuing momentum

Bickerman, who runs a Washington-based dispute resolution firm, said he cannot predict what the DEP's absence will mean.

But he is hopeful that the talks now underway will create momentum toward resolution.

"If everybody else wants to do a deal, the state will probably assist the parties," he said. "We'll just have to wait and see."

A spokesman for the General Electric Co., which was responsible for polluting the river during years of making transformers in Pittsfield, said the firm remains engaged in the mediation effort. GE used PCBs until the substance, listed as a probable carcinogen, was banned in 1979. A consent decree in 2000 set down principles for future environmental remedies; the fight today is over the details for a continued cleanup from Pittsfield south, into Connecticut.

Environmental officials from Connecticut agreed to join Bickerman's effort, as did the Boston staff of the federal EPA. That office, which oversees the Housatonic project, is holding off recommendations on the PCBs disposal issue, pending progress of the mediation, officials say.

Tim Gray, executive director of the Housatonic River Initiative, said his group signed on as well.

"Right now, people are talking, which is a good thing," Gray said. But he said the path ahead for mediation has not been spelled out.

"There's still a lot of process that we're going to be discussing," Gray said.

The towns that lie along the section of river in Berkshire County south of Pittsfield have agreed to participate in the mediation, according to Thomas Matuszko, executive director of the Berkshire Regional Planning Commission.

The towns — Great Barrington, Lee, Lenox, Sheffield and Stockbridge — are represented by the Housatonic Rest of River Municipal Committee, which was scheduled to discuss the mediation effort Friday.

Matuszko said the agreement spelling out the group's participation has been undergoing a legal review. The commission serves as an adviser to the municipal committee.

Jane Winn, executive director of the Berkshire Environmental Action Team in Pittsfield, said that at her request, a sentence was added to the agreement governing participation in the talks. That sentence allows parties to speak publicly about their own positions on the cleanup.

"We can't repeat anything anyone says, but can state our views," Winn said.

While Winn said she learned later that the agreement did not forbid such comment, she was concerned that the document imposed a gag on public discussion and understanding.

"We feel much more comfortable now," she said. Her group signed the agreement Tuesday.

Kelsey Dumville, who works in the EPA's public affairs office in Boston, said the agency joined other parties in signing an agreement "that calls for confidentiality for discussions in the mediation."

Public comment

Winn said Bickerman spoke at Tuesday's session in Lee about ensuring that members of the public have an opportunity to comment on the process at some point.

Bickerman confirmed that.

"I believe that everybody is committed to getting public input at the appropriate time," he said.

But the mediation process, by design, allows for private exchanges.

Bickerman said that helps the parties reach out, tentatively, to one another, looking for common ground.

"We're living in such a polarized society right now," Bickerman said. "You have to get away from the choices that parties retreat to. You can't do good brainstorming in a public forum."

The mediation occurs in group sessions and in smaller exchanges among the parties, with Bickerman playing the role of matchmaker.

Even as her group takes part, Winn said she remains somewhat skeptical.

"My concerns [about the value of mediation] are still there. But I felt Tuesday's meeting was very positive, and I felt very pleased to be engaged," she said.

Michael O'Connor, public relations manager for Massachusetts Audubon, said the nonprofit also decided to opt in.

"Massachusetts Audubon is a full mediation partner in this process now," he said. That involvement extends to a desire to let the mediation advance without comment. "We are pretty much maintaining radio silence for the time being."

Larry Parnass can be reached at lparnass@berkshireeagle.com, at @larryparnass on Twitter and 413-496-6214.