State and county environmental officials are investigating a Manorville neighborhood after tests of a private water well detected levels of a banned gasoline additive recorded at more than 10 times what is allowed by health regulations.

The Bennett family had their private well tested recently and Suffolk County Department of Health Services told them Wednesday not to drink or use their water for cooking because of high levels of methyl tertiary butyl ether, or MTBE.

The gas-blending compound is not regulated by the U.S. Environmental Protection Agency, but New York banned the use of MTBE in 2004 and set the safe-drinking-water concentration at 10 parts per billion.

The Bennetts’ well had MTBE levels of 110 parts per billion.

“I’ve always felt comfortable drinking my water,” Clare Bennett said Thursday. “I have a filtration system. I’m in the middle of the pine barrens. Who would have thought this was a problem?”

At room temperature, MTBE is volatile, flammable and colorless. It dissolves easily in water, according to the EPA.

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In a note to the Bennetts, Suffolk Health Sanitarian Anthony J. Condos said exposure to MTBE can irritate the eyes and respiratory tract. Effects on the central nervous system include “headaches, lightheadedness, stupor, dizziness, nausea and feelings of disorientation or confusion,” he wrote.

The Bennetts said they had no idea their water was contaminated. “I can’t say that I smelled anything,” Clare Bennett said. “I can’t say the water looked any differently.”

The family built their 1,000-square-foot home 20 years ago along Oakwood Drive, close to the Peconic River. Because they have private water, which is not regulated by drinking water rules, they have theirs tested every few years to be safe, Bennett said. This is the first time there has been a problem.

“We have a full house filtration system but it does not take out MTBE,” Bennett said.

She has been washing dishes in the dishwasher and then rinsing them with bottled water. It’s unclear what to do with a recently washed load of laundry.

“It’s concerning, without a doubt,” Bennett said. “I’ve called all my neighbors around me.”

Suffolk County Department of Health Services spokeswoman Grace Kelly-McGovern said the agency would sample more wells in the neighborhood. She said the state Department of Environmental Conservation was leading the probe. “There’s not much known at this point,” Kelly-McGovern said. “We’re going to be looking in the neighborhood to see if anyone else has this.”

MTBE “can enter groundwater when there are leaking storage tanks from gas stations or gasoline storage facilities near wells,” said Rick Andrew, director of global business water program for NSF International, an organization that writes standards, tests and certifies products for water and other industries.

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Removal systems are offered by 28 manufacturers and include sets that are plumbed into systems or linked to faucets, he said.

DEC spokeswoman Erica Ringewald said in a statement that the source of the contamination was unclear and the agency was investigating to track that down and determine the extent of contamination.

She said the agency would take samples at the Bennetts’ property and deliver bottled water. “DEC will then arrange to have treatment installed for the well or connect the home to public water, if available,” Ringewald said.

About 45,000 homes serving 200,000 people use private wells rather than public water supply systems in Suffolk County. The county will sample private wells for a $100 fee if they have previously been approved. The fee can be waived for households earning less than $25,000, according to the county website.