Coliform bacteria counts in Norwalk River sometimes exceed level set by state

Testing for coliform bacteria along the Norwalk River in Ridgefield hasn’t set off alarm bells, although there are occasional tests showing results that exceed levels “indicative of potential human health risk” as identified by the state. Concerns would be raised if a series of tests at a site showed high counts.

“They’re saying Ridgefield is fine,” First Selectman Rudy Marconi told fellow selectmen Aug. 3, as he passed out results of five tests done from late May to mid July at six sites along the river in town.

Of the 30 samples taken at the six Ridgefield sites, there were five that showed coliform counts above the state’s criteria for “potential human health risk” of 576 colony-forming units per 100 milliliters (576 CFU/100mL) set by the Connecticut Department of Energy and Environmental Protection (CT DEEP).

“When we see a single high count, it isn’t cause for concern,” said Dr. Sarah Crosby, director of Westport-based Harbor Watch, which monitors 12 river systems in Fairfield County. “It’s more a systemic situation where we have elevated counts for multiple sampling dates.

“A single water sample could be influenced by any number of environmental factors, so it’s really about having a number of replicates over a course of the season,” Crosby said.

Ridgefield’s two sewage treatment plants discharge effluent into the Norwalk River. District 1 into the Great Swamp off South Street, and District 2 off Route 7 near Little Pond.

One of the Harbor Watch testing sites is at the effluent discharge from the District 1 plant.

“From April to October, the plant has UV (ultraviolet) lights on to sanitize the effluent which results in no bacteria entering the Norwalk River from the treatment plant,” the Harbor Watch report says.

“They’d like us to run the UV lights in the winter,” Marconi told the selectmen.

He said it isn’t necessary then. “It’s so cold, the bacteria don’t survive,” Marconi said.

“We wouldn’t make any recommendation based on this data until we have the full season.” Coliform bacteria are commonly found in the intestines of warm blooded animals, so they can be used as a marker to detect leaky septic systems or poorly functioning sewage treatment plants. But the bacteria also occur naturally, and there could be a number of causes — including geese — behind a coliform count over the state’s 576 CFU/100mL on an isolated occasion in water at a given site.

“The way that we use that number, we take eight-to-10 samples over the course of the summer and we see what percentage of samples at each site are in exceedance of the 576,” Crosby said. “If it’s over 10% that means that site fails the state criteria.”

Of the five samples in which the testing found coliform numbers above the state’s 576 CFU/100mL criteria, four were taken on the same date — May 24.

“Bacteria levels in the Norwalk River have consistently met the CT DEEP criteria with the exception of the upper watershed on 5/24 which may be due to a rain event just prior to sample collection,” the Harbor Watch report said.

“Dissolved oxygen values for the Norwalk River have been meeting the CT DEEP minimum criteria … Overall, we are not overly concerned about bacteria levels in the river.”

The report given to the selectmen was just a “mid-season update” and not a complete report, Harbor Watch said.

In addition to the six sites on the river in Ridgefield, Harbor Watch tested three sites in Wilton and two in Norwalk — none of those was problematic.

“There weren’t any sites on the Norwalk River that jumped out as needing attention,” Crosby said. “We’ll continue to monitor the river and keep the towns we are working with informed.”

Harbor Watch is a non-profit water quality research program that is affiliated with Earthplace in Westport.

“We do water-quality monitoring and pollution trackdown in addition to running educational and citizens science programs for the public,” Crosby said.

“We do a lot of work in the Norwalk River and Norwalk Harbor, but we’re actually working in 12 different river systems this year,” she said.

“This is our 30th season.”


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