Seven private water wells south of Gabreski airport have tested positive for a hazardous chemical, with results from four of those wells testing higher than the federal heath advisory level, Suffolk County said Thursday.

The wells were tested for PFOS, a fluorinated organic chemical contained in the type of firefighting foam used at the airport by Suffolk County Air National Guard.

Officials moved to test the private water sources after elevated levels of PFOS were found in public wells near the airport two years ago.

County and state officials said the National Guard Bureau – which oversees the Air National Guard – has agreed in principle to pay for hooking up all of the homes on private water wells in the affected area to the public water supply, which is treated to remove PFOS.

Deputy County Executive for Administration Peter Scully said once agreements are in place, the 85 parcels on private wells in the priority areas south of Gabreski airport in Westhampton Beach could be connected to Suffolk County Water Authority lines within four weeks.

He said it was unclear how much it would cost to hook up the parcels to the public water supply.

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Suffolk County Water Authority spokesman Tim Motz said it would cost $1,850 per parcel just to start water service, not including the cost of installing a service line between the home and the water main, or the cost of extending the main itself, in cases where the homes on private wells aren’t close to one.

“I think that’s a good first step and I think the county has been extremely responsible in notifying the public and being very transparent and coming up quickly with an emergency response,” said state Assemb. Fred Thiele (I-Sag Harbor), who said his office has begun receiving calls from the public about the PFOS issue at Gabreski. “There’s still a lot of work to be done.”

The PFOS testing was done after the state Department of Environmental Conservation sent a letter to the county this month, informing it of an investigation the state began into the presence of PFOS in groundwater near the airport.

Last week, the county began going door to door to the first of the 85 targeted homes, handing out bottled water to be used for cooking and drinking as a precaution and requesting permission to test their wells for PFOS and other contaminants.

By Thursday, Scully said, the county had tested 34 private wells and had results from 17. One homeowner so far has refused testing.

Four of the wells had PFOS levels above the U.S. Environmental Protection Agency’s health advisory level of 0.07 parts per billion — the level above which action should be taken to avoid consuming the water.

One of the private wells had a combined level of PFOS and PFOA – a related compound – at 0.56 parts per billion.

The four wells with the highest detections were located south and southeast of Gabreski, Scully said.

Three of the wells had detections of PFOS, but below the health-advisory level, said Scully, who added the county would retest the wells to make sure the results were accurate.

The county also will continue providing bottled water to the residents as long as they still are relying on private wells, he said.

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Elevated levels of PFOS were first detected in public-supply wells south of Gabreski airport two years ago, according to the DEC.

The Suffolk County Water Authority this year installed treatment systems to remove the chemical from the water that is delivered to the public.

While the PFOS is thought to come from the firefighting foam used at the airport, according to the state, the DEC investigation will determine the exact source.

DEC Chief of Staff Peter Walke said his agency began installing monitoring wells this week.

“The aim is to get an initial sense of where the contamination is coming from,” Walke said.

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The Air National Guard also is conducting its own investigation into the contamination, according to the DEC.

While there is no written agreement yet, the National Guard Bureau is “ready, willing and able to step up and help the county move forward with getting people hooked up to the public drinking water supply,” Walke said.

The National Guard Bureau did not provide comment Thursday.

“Our top priority is to get as many homes that rely on private wells connected to public water as soon as possible while DEC’s investigation of the site moves forward,” Suffolk County Executive Steve Bellone said in a statement released Thursday. “We have been moving quickly to scope out the project and the costs involved, but the big question until this morning was how the connections would be paid for. We learned this morning that the Air Force has agreed to do so, which is great news. Now we can focus on finalizing a plan and schedule to get the hookups completed.”

PFOS is considered an emerging contaminant of concern, and its exact health effects are not yet fully understood. The chemical is listed as hazardous by New York State and is thought to potentially cause blood, immune system, thyroid and fetal growth effects.

Roger Sokol, director of the state Department of Health’s bureau of water supply protection, said the EPA’s health advisory level has “a large margin of protection.”

“There’s no direct cause and effect that I’m aware of in any of the literature that shows if you have been exposed to this, you will have a health effect,” he said. “The purpose of these health advisories is to be overly protective.”

Sokol also said the state is offering assistance to the county in the form of advice, guidance and laboratory testing.