What's Causing The Bacteria That Is Leading To Beach Closures On Nantucket's North Shore?

Dionis Beach was closed to swimming for five consecutive days late last month due to bacteria detected in the water. One day after it reopened, Washing Pond Beach, located just to the east of Dionis, was closed to swimming.

What's causing these issues on two of Nantucket's most popular north shore beaches?

We reached out to Nantucket's chief environmental health officer John Hedden for some answers.

"The beaches are sampled for Enterococci bacteria," Hedden explained. "We sample the beaches every Tuesday morning, and we re-sample the next day if a sample exceeds the single count for enterococci. Beaches are closed when they exceed their geomean standard. Geometric failure is calculated from the 5 most recent samples."

"The warmer water plays a factor in the elevated bacteria levels: bacteria are in a better environment to grow in the warmer temperatures," Hedden added. "Things seem to be much worse when we get a lot of wind out of the north, which could be blowing in organic matter towards the north shore. Dionis usually gets closed for a few days each summer, typically not too long in terms of days, however, in my experience, there are usually a few days here and there when it gets closed."

The specific threshold for geometric failure for a marine beach like Dionis is 35 cfu (colony forming units) per 100ml.

"As far as the cause of failure, there are many factors that could be the cause," Hedden said. "It could be fertilizer runoff, septic systems in the area, or wildlife waste, although I don’t think those are the main sources for the failure at Dionis. I think it’s seaweed (or wrack) that blows in after we’ve had wind come out of the north for a period of time and it hangs around slowing decomposing until it is eventually diluted and washed away. The inspectors taking the samples have indicated that there’s been an abundance of wrack in the water on the north shore. I think this is similar to the situations we’ve been seeing with Jetties Beach the past two summers."