Sen. Charles Schumer demanded Monday that Northrop Grumman and the U.S. Navy allow the Massapequa Water District access to wells so the agency can test if contaminated ground water is linked to a Bethpage plume.

The district asked the Navy and Northrop Grumman for permission in November to perform an isotope analysis at five to seven monitoring wells within four and a half miles of the plume.

Water district officials want to see if a correlation exists between substances found where the plume began and contaminants showing up in the wells.

Schumer accused Northrop Grumman and the Navy of stonewalling, which he said should stop.

“They want to know how quickly the plume is spreading and what carcinogens are there,” Schumer said of the Massapequa agency.

The senator said the water disrict “should get any information they want. Their water is in peril.”

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District officials said it would cost about $15,000 to have a lab do a compound specific isotope analysis — a forensic technique to essentially identify a specific signature.

The Navy responded in December and Northrop Grumman this month. Both questioned the agency’s analysis method and suggested tests at more wells would be needed to get an accurate picture. Northrop Grumman said more than 30 tests may be needed.

“The response is really more red tape for us,” Massapequa Water District Superintendent Stan Carey said. “They want us to expand the wells we requested, almost double the size of the amount of wells, and we don’t think it’s fair to place that burden on a small water district.”

The Navy did not respond to requests for comment. Northrop Grumman would not comment on specific questions about the testing, their response or Schumer’s claims of stonewalling.

In a statement, Northrop Grumman spokesman Vic Beck said the defense contractor is working closely with all regulatory agencies involved.

“As part of our effort, we work with the Departments of Environmental Conservation and Health to determine what data to collect and all such data is publicly available,” he said in a statement.

For decades, the state Department of Environmental Conservation, the Navy, Northrop Grumman and area water districts have struggled with soil and groundwater contamination emanating from a 609-acre site in Bethpage once used by the Navy and what was then the Grumman Aircraft Engineering Corp. Manufacturing at the site lasted from the 1940s to 1998.

In 1983, the Navy and Northrop Grumman sites were added to the state’s Superfund list. Several plume cleanup programs are underway. Water suppliers and local politicians said the plume puts the drinking water supplies for 250,000 people at risk.

State Assem. Joseph Saladino, R-Massapequa, said the Navy and Northrop Grumman have an ethical responsibility to contain and clean up the plume. Under a plan he supports, it would cost $70 million and 35 years to complete.

“The foot-dragging on this has been going on for decades,” Saladino said. “The ultimate goal is to get Navy and Grumman to do what they’re responsible for.”