The Bethpage Water District Monday plans to sue Northrop Grumman Corp., claiming the company's actions contaminated groundwater and cost the district millions of dollars.

The lawsuit, to be filed in U.S. District Court in Brooklyn, seeks unspecified damages to cover treatment programs, water monitoring and steps to remove contaminants from groundwater before delivering it to customers.

District officials decided to sue "to forestall a water quality emergency and the attendant irreparable harm created by contamination of soil and groundwater in Bethpage," according to the lawsuit.

The district serves more than 33,000 people in Bethpage, Old Bethpage, Farmingdale, Levittown and Plainview. It has for decades battled groundwater plumes of toxic chemicals migrating from property where the Navy and Grumman created, tested and manufactured military and space equipment including the Apollo Lunar Module.

"Time and time again, Northrop Grumman has refused to do the right thing in assisting the residents of Bethpage in cleaning up a mess they created," Bethpage Water District chairman William J. Ellinger said. "We must hold them accountable for what they did."

Northrop Grumman representatives did not respond to telephone and email messages seeking comment.

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Relations with Northrop Grumman have soured through the years over the cost of treatment, whether it is necessary and where the financial responsibility lies. The company had been cooperative, but talks in the past three years have failed, Bethpage Water superintendent Mike Boufis said. "They've left us no choice at this junction," he said.

While Bethpage wells were the first to be affected, the plumes could impact wells serving 250,000 people in southern Nassau County, elected officials and water suppliers say.

The Nassau County Department of Health discovered a shallow plume in 1986 and traced it to the Navy and Grumman sites, as well as a nearby Hooker Chemical/Ruco Polymer plant. In 2009, a second, deeper and more contaminated plume was found moving from Bethpage Community Park, which had been used by Grumman as a legal dumping area for solvents, paints, cadmium, arsenic and chromium-tainted sludge. Both plumes are part of state Superfund cleanups.

New contamination concerns surfaced in May when the district announced it had found elevated levels of radium, a radioactive element, at its Sophia Street plant and closed one well. A month later, the district began digging a new well outside the known plume boundaries.

"It's really been three-plus decades that they've been dealing with this," said Rich Humann, president and CEO of H2M, a Melville engineering firm that consults for Bethpage and other districts.

The lawsuit had the support of County Executive Edward Mangano, a Bethpage resident who has pushed for a more stringent cleanup than what the state Department of Environmental Conservation ordered.

"I fully support the water district's efforts to recapture mitigation costs and reimbursements for necessary capital improvements to protect the public drinking water," Mangano said in a statement Sunday.

Sen. Charles Schumer (D-N.Y.) said Sunday, "Hopefully this lawsuit will force them [Northrop Grumman] to come to their senses and engage in a constructive way with the water district."

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Since the late 1980s, the district has spent more than $14 million on remediation. Each of the eight wells it operates require some form of treatment. The new well is expected to cost $3.5 million.

Bethpage reached a $2.3 million agreement with the Navy for upgrades at one of its plants.

Most volatile organic chemicals are limited by state and federal water standards to 5 parts per billion. Bethpage treats to zero.

That level of treatment could present challenges to the lawsuit, said environmental attorney Larry Schnapf, an adjunct professor at New York Law School. "All their costs can be deemed to be non-necessary," he said.

Weitz & Luxemberg, the Manhattan firm representing Bethpage, has a history of settling toxic tort cases. In 2008, the firm brokered a $423 million settlement with oil companies over contamination from the gasoline additive methyl tertiary butyl ether -- MTBE -- that affected more than 150 public water systems in 17 states.