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Heavy rain could cause spike in fecal bacteria on swimming beaches, officials say

A no swimming sign sits at the entrance of Chapoquoit Beach in Falmouth on August 3rd.
Liz Lerner
A no swimming sign sits at the entrance of Chapoquoit Beach in Falmouth on August 3rd.

Intense rains this week could flush more fecal bacteria into local waters, county officials say.

High levels of the bacteria forced closures at five Falmouth beaches — New Silver Beach, Megansett Beach Woodneck Beach, Chapoquoit Beach, and Chapoquoit Associates Front Beach,— on the same day in mid-July.

The beaches are among 30 in Falmouth that are regularly monitored for bacteria levels, and the closure of five was the largest such action in several years, said the town’s health agent, Scott McGann. Exposure to fecal bacteria can lead to gastrointestinal issues, vomiting, skin rashes, diarrhea, and other ailments.

Despite the closures, the situation is not yet a cause of worsening concern to McGann or Barnstable County health communications coordinator Bethany Traverse, who ran the county's beach monitoring program for more than 14 years. They said there’s no reason to suspect a pattern at this point.

“I can say with confidence after doing this job for so many years that we do have great water quality on the Cape,” Traverse said. “And the testing that we do, I think, is really effective at pointing out trouble spots.”

She theorizes that droughts that have plagued the region this summer are at fault for the latest failures in Falmouth.

“Nine times out of ten we can point to stormwater runoff as a culprit for high bacteria levels. In this case, there hasn't been any rain. And it can have the opposite effect,” she said.

“My theory is that what's going on in Falmouth is that this lack of rain has given the opportunity for this bacterial contamination to sort of build up. And then it eventually gets pushed out by the tide,” she said. “When there was a high tide, the water came in and took all that gunk out with it.”

There’s rarely a smoking gun, or a single route to the beach, but fecal bacteria often makes its way into local waters when stormwater runoff carries mammal and bird feces into marshes and harbors. Once there, the bacteria gets stored in the sediment.

“It just sits there in the hot sun and it's a great environment for growing bacteria,” she said. “And then the tide eventually pushes it out into the water body.”

Under certain circumstances, the combination of rain, wind, and outgoing tides can come in and flush it all out into the ocean.

“When you have a heavy rain, you're washing off the land [and] into the ocean and then that gets picked up and it easily triggers a positive result,” McGann said.

Before all of the bacteria can disperse, the water can be unsafe to swim in. But the problems don’t last long.

“Ocean beaches … clear themselves out really quickly and efficiently because of the tide,” Traverse said.

For those reasons, it's far more likely to see water quality problems at beaches adjacent to harbors and salt marshes, than at beaches without any backwater, such as Falmouth Heights Beach.

Water quality results for beaches all across the Cape will be complete tomorrow. But some towns tested before the rain, so it's unclear whether all the results will accurately reflect the risk.

Towns will post warning signs at beach entrances and parking lots if closures are deemed necessary.

Eve Zuckoff covers the environment and human impacts of climate change for CAI.