Floating Vegetation Islands Pitched As Latest Solution For Mill Pond

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Frustrated by misfires in two recent, ambitious and expensive plans to address water quality problems in Mill Pond, residents who live near the body of water are proposing a new and less costly idea for reducing the amount of nutrients flowing into the pond.

Water Mill residents and a North Carolina botanist on Monday pitched an idea of creating a floating island of vegetation along the pond’s northeastern shoreline, where streams of polluted stormwater now flow into the water. The plants, woven into floating foam platforms, would sprout roots into the water and sop up nutrients before they have a chance to feed the choking algal blooms that have plagued the pond for decades.

On Monday, botanist Rob Crook of Floating Islands International, told Southampton Town Trustees that a 200- to 300-foot-long by 10-foot-wide floating island—effectively an artificial ribbon of wetlands vegetation—could potentially capture all of the nutrients being carried into Mill Pond by runoff from Deerfield Road.

The floating vegetation, which Mr. Crook recommended be planted with locally indigenous grasses, would cost approximately $90,000 and could be expected to last for at least 10 years, and possibly 20 to 30 years if maintained correctly.

“It does take time—they’re not magic,” Mr. Crook said. “You have to work with them and manage them like any other tool. It’s a capital investment.”

Planted through holes in the foam floats, the roots of the plants would soak up nutrients from the stormwater and feed them into the fronds of the plants above water. Each year, the fronds, laden with nutrients, would be trimmed and carted away, and they could be used as fertilizer elsewhere.

Plants’ fondness for nutrients like phosphorous and nitrogen has been the root, so to speak, of the problems in Mill Pond for years. High levels of nutrients carried into the pond in groundwater tainted with fertilizers from centuries of farming in its watershed, and rainwater runoff from fields and roadways along Deerfield Road, feed algal blooms that stain the pond’s waters a pea soup green each summer. Mr. Crook noted that this week, even with water temperatures still relatively cold, algal blooms had already started popping up in Mill Pond.

The pond has been the focus of two recent attempts by the town to treat or stanch the impact of nutrients in the pond. Last spring, the town and Town Trustees conducted a mineral treatment of the pond intended to sap phosphorous from the water column. That effort’s success has been debated in the year since, its less-than-dramatic results blamed on unusually high rainfalls in both the spring and fall, following the application. A second application of the mineral, known as Phoslock, has been put off by the town for at least a year while it awaits more detailed assessments of the experiment’s effects.

This summer the town had been planning to embark on a two-phased stormwater runoff abatement project. The first phase, which will establish more concentrated drainage of runoff alongside the roadway, is due to begin this month. But the second phase, a more ambitious effort to capture runoff and reduce the amount of nutrients it carries to the pond, has been shelved because of logistical concerns about its cost effectiveness.

Mr. Crook explained that a barrier of carefully designed vegetation islands, anchored off the outflow of the road’s drainage into the pond, could strain out the nutrients before they have time to disperse into the pond’s main body and feed algal blooms. Floating vegetation islands have been used around the United States and overseas to address nutrient loads in water bodies.

To be effective, a floating island usually needs to measure about 5 percent of the size of a water body: in this case, an estimated 4 acres would normally be needed to assist the 92-acre Mill Pond. However, by siting the island at the northeastern corner of the water body, near Muller’s Corner on Deerfield Road, Mr. Crook said he thought a much smaller island could be effective.

Steve Abramson, a Water Mill resident who has led a variety of efforts by local residents to find innovative ways to address the pond’s water quality problems, said he hoped town officials and the Town Trustees can find a way to foot the cost of the plantings, pointing to the cancellation of the other two projects.

“The other thing is to cap the [nutrient] input at its source,” Mr. Abramson said. “This is one natural way to cap it. It looks like it would be able to capture all of that nutrient matter, and more.”

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