Initial findings from a study of Quartermaster Harbor show that streams — not nearshore septic systems — dump the largest amounts of harmful nitrogen and other nutrients into the ailing inland bay.

Public and university scientists have taken regular readings of Judd, Fisher and Mileta creeks over the past four years. According to a recent analysis of those samples, part of a study funded by a grant from the U.S. Environmental Protection Agency and other agencies, streams carried almost four times as much nitrogen into the shallow bay as near-shore septic systems discharged.

But scientists say the results don’t get people — or even failing septic systems — off the hook. The nitrogen in Judd Creek, the Island’s largest watershed, could come from a variety of sources, including failing septics within the watershed, lawn and garden fertilizers and horse and cow manure.

“You can’t presume that all of that (nitrogen) from the streams is exclusively from humans, but a big chunk of the land-derived nutrients are potentially from human activity,” said Curtis DeGasperi, a hydrologist in King County’s Department of Natural Resources and Parks and the lead scientist on the Quartermaster Harbor Nitrogen Manage-ment Study.

Beach houses lining the bay could be part of the problem; many have septics that are failing, DeGasperi noted. But those failing systems are the source of other harmful substances — fecal coliform, for instance, which is a public health threat — not just nitrogen, a substance that some believe is suffocating Quartermaster Harbor.

The bigger question now, he added, is determining the sources of nitrogen in the watersheds that empty into Quartermaster Harbor.

“We’ll have to dig deeper,” he said.

“Without some better information about what those potential upland sources are,” he added, “we won’t be able to make good policy decisions.”

The study is part of a four-year effort to determine whether nitrogen-loading is leading to the bay’s depleted oxygen levels — levels so low they could be lethal for some fish species. The $893,000 study, funded by a $625,000 EPA grant as well as funds from King County, the University of Washington-Tacoma and the state Department of Ecology, started in 2009 but builds on years’ worth of data collected by scientists and field researchers.

It comes at a time of mounting concern about the health of Puget Sound. Over the past few years, water samples taken from Quartermaster Harbor show very low levels of dissolved oxygen, a potentially serious situation, DeGasperi said. Low oxygen levels have been implicated in the massive fish kills that have plagued Hood Canal and other water bodies in recent years.

Excess nitrogen has been identified as a major source of pollution in parts of Puget Sound. The naturally occurring element creates a cycle that can suffocate a water body: It triggers the rapid growth of algae, which eventually decompose; as they do so, bacteria that break down the algae suck oxygen out of the water.

Susie Kalhorn, an environmental educator who has paid a lot of attention to the issue of septic systems and nitrogen-loading in Vashon’s water bodies, said she wasn’t surprised by the study’s findings.

“It’s always been my contention that we should be looking upland,” she said.

Shorefront homes, many of which have inadequate septic systems, are part of the problem, “no question about it,” Kalhorn said.

But a big watershed such as Judd Creek’s drains a much larger amount of land than those small parcels along the bay, she noted. “All of us contribute to what’s going on,” she said.

Particularly noteworthy, DeGasperi said, is the amount of nitrogen scientists found in Mileta Creek, a tiny tributary that flows from the top of Maury Island near the Vashon Island Golf & Country Club west into Quartermaster Harbor.

Nitrogen levels in the stream — the only one on Maury that scientists monitor regularly — spike in the winter, a pattern they’ve not seen anywhere else in the region, DeGasperi said. Not even streams near dairy farms have been found to carry so much nitrogen, he said.

Scientists are trying to determine the source of Mileta’s nitrogen. Some, for instance, have speculated that an old chicken farm that once thrived in the area could be the source. Others have suggested the fertilizers used on the golf course could be a contributor. DeGasperi, however, said golf courses near other streams haven’t been a huge problem.

It also could be a problem that affects much of Maury, he said, an issue “we may have missed because we only had the resources to sample the one stream.”

More work needs to be done to understand the impact of stream water on Quartermaster Harbor, he added. The next step, he said, is for scientists to find a way to quantify the potential sources upstream from the bay — the number of homes, horses and other pets and the kinds of agricultural or landscape practices that could be contributing to the problem.

“It’ll take a lot of effort to tease it all out,” he added.

Findings from the study will be reported at an open house at 6:30 p.m. followed by a public presentation at 7:30 p.m. on Wednesday, Oct. 6, in the Vashon High School auditorium.