Northrop Grumman and the state Department of Environmental Conservation are one step closer to agreeing to a decades-old cleanup plan for part of a Bethpage plume that has spread more than four miles.

Environmental Protection Agency Regional Administrator Judith Enck sent a letter Friday to Sen. Charles Schumer, saying an agreement on the cleanup plan should be final by the end of April.

In February, Schumer asked the EPA to intervene in the cleanup process, which is part of a Superfund program managed by the state.

StorySchumer: EPA must pressure Grumman on Bethpage toxinsstoryDistrict: Include new toxic discoveries in suitStoryToxic plume spreads, new wells to monitor

Encompassing more than 600 acres, the Bethpage site was home to aviation manufacturing facilities between 1936 and 1996 run by the Navy and what is now Northrop Grumman. A shallow plume coming from the site was discovered in 1986 and another, deeper one was verified in 2005.

Northrop Grumman had refused to sign a 2001 cleanup plan. But after the Navy revealed high levels of the degreaser and carcinogen trichloroethylene, or TCE, at the site, the DEC in November requested the defense contractor sign on or risk EPA intervention.

"We agree that this is a matter of great importance, and that progress in addressing the full scope of contamination has been slow," Enck wrote to Schumer. "It is essential that this toxic plume be addressed in a timely and effective fashion to ensure that the people of Long Island can continue to have clean drinking water."

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Schumer (D-N.Y.) Tuesday said he's pleased by the progress.

"Bringing EPA in tells Grumman how serious we are and we expect they will sign a serious agreement," he said. "We will stay on Grumman until they do the right thing."

Northrop Grumman spokeswoman Christine Restani confirmed an agreement was in the works but referred questions to the DEC. The DEC said it is pressing for aggressive action to address the plumes and the negotiations are ongoing but declined to comment further.

Also last week, the Bethpage Water District amended a lawsuit filed against Northrop Grumman in 2013 seeking damages to cover the cost of past and future water treatment involving the groundwater plume.

A federal judge ruled Feb. 27 that the district could update its lawsuit to include details about TCE found in 2014 by the Navy. Those tests found levels of TCE as high as 4,600 parts per billion about 2,100 feet from a Bethpage well. The drinking water standard is 5 parts per billion.

"This data indicates that the containment systems on-site are inadequate to prevent migration of additional contaminants off-site," the amended lawsuit states. "Moreover, the substantial increase in TCE levels . . . may indicate a new plume from another [Northrop Grumman] source within the site."

Tuesday, the district announced that construction of a $3 million well outside the plume boundaries was completed. The district began drilling the well off South Park Drive in Bethpage State Park in 2013.

The site is two miles from the district's other eight wells and should not require costly treatment to remove contaminants, the district said.