U.S. Sen. Charles Schumer is asking the head of Northrop Grumman to allow groundwater testing under its property in the Bethpage area where an elevated level of radium has been found in some drinking water samples.

"I am asking you to simply grant immediate and unfettered access to the water district so that mystery can be cleared up," Schumer (D-N.Y.) said in a letter yesterday to company chairman and CEO Wesley Bush.

A spokesman for Northrop Grumman, Mark Root, said the company doesn't comment "on correspondence from public officials."

Untreated water drawn from a Bethpage Water District well in August 2012 had a radium level of 4.72 picocuries per liter, a measure of radiation. The level was 5.87 in January and 4.82 in April, officials said.

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District officials have told Newsday that the amount of radium does not exceed federal or state drinking water standards, but the levels are rising and they want to run tests to try to determine where the radioactive element is coming from.

The district has shut down that well, but has assured residents that overall water quality is not affected and a half-dozen other wells are operating.