Aim to ‘Treat That First Flush’

As part of an effort to remediate the harmful algae that have regularly bloomed in Three Mile Harbor since the 1980s, the chairman of the town’s water quality technical advisory committee recommended on Tuesday that the town board pursue several mitigation measures. 

Excessive nutrients in waterways, for which failing septic systems and stormwater runoff are blamed, promote the algal blooms that choke off oxygen, restricting the growth of eelgrass, killing marine life, and rendering shellfish unsafe for consumption. 

At the town board’s meeting on Tuesday, Chris Clapp of the water quality advisory committee proposed that projects aimed at eliminating the flow of nutrients and pollutants before they reach the harbor be moved from a conceptual to a design and engineering stage. 

At present, stormwater drains directly into the head of Three Mile Harbor via collection and discharge pipes at Fairway Drive, Gardiner’s Cove Road, and the Soak Hides dreen, Mr. Clapp told the board. He recommended the design and engineering of bioswales, vegetated depressions in the landscape that help filter stormwater — such as those created at the Town Pond green and behind the Methodist church in East Hampton Village — at each of these locations. 

Depressions would be designed to intercept and treat the initial inch to inch and a half of stormwater flow. “Imagine rinsing off your car after pollen is on it,” Mr. Clapp told the board. “That first inch and a half of water is what gets 90 percent off.” The same thing happens on roads in a rain event, he said. “That conveys the majority of pollutants, the nitrogen, phosphorous, and pathogens which are impairing our waters. These swales are designed to treat that first flush, as it’s known.” 

Also recommended was a project with Cornell Cooperative Extension that would see the installation of a permeable reactive barrier behind the bulkhead at the town dock. A device comprising trench boxes filled with ground woodchips or another reactive substance that intercepts groundwater as it seeps into a water body, such a barrier has demonstrated an 85-percent reduction in nitrogen seepage at Pussy’s Pond in Springs. “It’s able to remediate pollutants that are already in the groundwater stream before they enter into the surface waters,” Mr. Clapp said. 

A permeable reactive barrier at Three Mile Harbor would provide a mitigation measure “while you work on the long-term fix of treating the source of those nutrients,” leaching of nitrogen and phosphorous from aging and failing septic systems, he said.

“Many of the existing septic systems in that location are in groundwater at least part of the time, if not all of the time,” Supervisor Peter Van Scoyoc said. “And we’re seeing harmful algal blooms, some very concerning,” such as those that can accumulate in bivalves and crabs and, if consumed, cause paralytic shellfish poisoning. To remediate waters at the head of the harbor and improve the overall ecology of the entire waterway, “it’s critical that we take some steps,” he said.