A News 12/Newsday investigation has uncovered a new Suffolk County health policy that some say could threaten water quality.

Experts say small, on-site commercial sewage treatment plants - which are designed to reduce nitrogen and kill bacteria - fail to filter out drugs, personal care products and household chemicals, which then seep into the groundwater.

Researchers say they do not yet know what happens when barely detectable amounts of these chemicals get mixed in with drinking water.

In 2010, a draft of Suffolk's water management plan said future sewage plants should be placed far away from public water wells. Far enough so that it would take at least 50 years for contaminated groundwater to reach the well's intake.

The county's Health Department didn't follow that recommendation, according to the News 12/Newsday investigation.

In fact, this year's final version says sewage plants can be much closer to public water wells - meaning it would take as little as two years for wastewater to get to the nearest well.

Critics say the new guidelines put public health at risk. They accuse the Health Department of caving to pressure from developers.

"We would like to see any discharge to the groundwater as far away as possible from our wellfield," says Joseph Pokorny, deputy CEO of the Suffolk County Water Authority.

The Water Authority wasn't even consulted before the new policy came out.

The Health Department insists its new siting policy for projects with sewage plants is safe.

Long Island's largest developer group - Long Island Builders Institute - denies pressuring the Health Department and says the public has nothing to worry about.

Suffolk officials turned down News 12's request for an on-camera interview on this story, citing scheduling conflicts.