National Grid is declaring success in a decadelong effort to clean up a massive toxic plume in downtown Bay Shore, but some residents aren't buying it.

At a public meeting in Bay Shore Wednesday night, officials from National Grid and the Department of Environmental Conservation detailed progress in the cleanup effort, saying some shallow groundwater was at "drinking water standards."

But the DEC stopped short of complying with homeowner requests for a letter that said their individual properties were free of contaminants.

One woman who lives near the source of the plume described her experience as nerve racking. "I don't want to be part of a science project," she said.

In a report released earlier this month, the DEC noted, "The cleanup of this site is not yet complete, but contaminant levels have decreased sharply throughout the area and this trend is expected to continue."

"We would definitely consider it a success," National Grid spokeswoman Wendy Ladd said. "We've made major progress in decreasing the plume."

The report notes that two of eight oxygenation treatment systems aimed at reducing the plume and its sources have done their job and been removed. Ladd said "major construction activities are completed," though monitoring and treatment will continue.

The DEC report also says shallow groundwater contamination has been reduced "to levels which meet drinking water standards" in most areas of the plume, with the exception of a "small area" at Union Boulevard and North Clinton Avenue.

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"Pockets of contaminated groundwater remain in the rest [of the plume] at depths of 13 to 60 feet below the ground surface," the DEC report says. "These pockets are responding more slowly to treatment, but they are responding."

The cleanup is taking place at a former Long Island Lighting Co. plant that made a form of natural gas by heating coal and fuel. Though closed for decades, the site left a legacy of toxic coal tar and other contaminants in and around the original site at Clinton and Fifth avenues and Union Boulevard. A toxic plume was found to extend 4,000 feet to the south of the plant. The most recent map shows the shallow groundwater plume reduced to less than a quarter of that amount.

But some say the cleanup effort has left too much toxic material deep underground, and they question the methods used to test water and measure the cleanup's success.

"How do I know what they're saying is correct?" asked resident Bill Sullivan, whose home sits on the plume. "There's no third-party verification."

Sullivan is one of more than 100 residents who have sued National Grid, saying the plume has hurt home values and compromised their health. A state Supreme Court judge last year dismissed the claims, citing a statute of limitations. Residents have filed for an appeal, and briefing papers will be filed by Nov. 5, said their attorney, Irving Like.

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Like noted the deepwater contamination that remains and testimony from an expert hired by his firm that offers different results than the state and National Grid's findings of reduced toxins. Like said he didn't view the DEC report "as a statement of victory. They're trying to spin their version of the cleanup and it's not true."