The groundwater below Suffolk that supplies the county's drinking water is still in pretty good shape, but quality is slipping. Creeping levels of nitrogen and other contaminants demonstrate that more must be done to protect the resource, according to a new report.

Five years and $700,000 in the making, Suffolk's draft water management plan found "a continued gradual decline in water quality" since 1987. It recommended more groundwater monitoring, and said the county should put a priority on open space preservation near drinking water wells and limit housing density in areas without sewers.

"When you consider that 1.5 million people live on top of the water supply, it's amazing how good our water quality is," said Mary Anne Taylor, a consultant with Camp Dresser & McKee, the Cambridge, Mass.-based firm that produced the plan with Suffolk's health department.

Still, more work lies ahead, she told county officials, environmental advocates and building industry representatives at a recent meeting in Yaphank. "I don't mean to imply that the sky is falling," Taylor said, "but there are some concerning trends."

Suffolk officials said the 419-page report, an update of a 1987 plan, provides a blueprint for protecting the county's water supply.

"We have a very fragile environment here in Suffolk County that must be protected," County Executive Steve Levy said in a statement.

The county will now seek public comment on the draft plan through March 1 before a final water management plan is issued. Then, county officials and legislators will evaluate the recommended options and decide which ones to go ahead with.

Excessive nitrogen levels

The worrisome trends include rising levels of nitrogen, which leaches into groundwater from septic systems and fertilizer use. While 98 percent of raw water tested met health standards, nitrogen levels have increased by about 1 milligram per liter over the past two decades, the report found.

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The standard is 10 milligrams per liter or less. Overall, average concentrations remained less than or equal to 6 milligrams per liter in nearly 87 percent of public supply wells.

Excessive nitrogen can cause health problems in infants. Nitrogen-laden groundwater seeps into streams and bays, triggering algae blooms that suck oxygen from the water and harm marine life.

Pesticides and industrial chemicals - many of them ingredients in common household products - continue to turn up in groundwater. Average concentrations of the dry-cleaning solvent tetrachloroethylene, also known as PCE, doubled between 1987 and 2005 in the same set of drinking water wells, the report found. Trace levels of drug and personal care products, such as shampoo, also turned up in supply wells and private wells; little is known about their cumulative health impact.

Water suppliers test raw water for contaminants and treat it to meet drinking water standards before it reaches consumers' faucets.

The plan also forecast rising demand. It highlighted summer spikes in water use for landscaping, which could require costly new infrastructure. Future development on the East End will likely require dozens of new wells, and the report advised conservation measures - such as seasonal rate hikes - to prevent further stresses on the system.

Irrigation system troubles

"It's all sprinklers," said Steve Jones, the former head of the Suffolk County Water Authority. He said automatic irrigation systems that water lawns, rain or shine, have spurred record-breaking use and made it harder for suppliers to provide adequate fire hydrant pressure.

Another focus was septic systems, which serve three-quarters of Suffolk households and produce much of the nitrogen fouling groundwater and local streams and bays. The report recommended a countywide study to identify places where sewage treatment plants could help reduce septic pollution.

Environmental advocate Kevin McAllister of Peconic Baykeeper, a clean water group, said Suffolk needed to overhaul its sanitary regulations because they only address human drinking water standards. Those allow higher nitrogen levels than aquatic life can tolerate, he said.

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The plan also suggested mandating bigger residential lots in unsewered areas to limit the septic burden - something that irked Robert Wieboldt, a lobbyist for the Long Island Builders Institute.

He said Suffolk should focus more on reducing pollution from existing development - for example, by encouraging wastewater improvement districts, which could provide low-cost loans to help homeowners upgrade aging septic systems. "That's where the real gains can be made," Wieboldt said.