A Riverhead treatment plant discharged 2.5 million gallons of partly treated sewage into Peconic Bay over three days in November, according to Suffolk County records, leading the state Department of Environmental Conservation to issue a notice of violation to the plant last week.
The notice, issued Thursday, was related to spills on Nov. 5, 19 and 26, and one in August, DEC spokeswoman Aphrodite Montalvo said.
The Riverhead Sewer District plant reported emitting wastewater on those dates that exceeded allowable levels of fecal coliform bacteria.
The town must draft a "corrective action plan" by Jan. 5, Montalvo said.
Sewer District Superintendent Michael Reichel said Friday that the discharges posed no danger to the public and were contained within an "exclusion zone" of the bay where shellfishing is not allowed.
He attributed the November incidents to a mechanical failure at the plant in late October that killed some of the bacteria that treats waste.
The mechanical problem has since been fixed and the beneficial bacteria have regrown, Reichel said.
Kevin McAllister, founder of the East End environmental advocacy group Defend H20, said he was looking into the discharges.
"Three times in one month is cause for some concern and greater scrutiny," he said Friday.
The Riverhead plant, which was built in 1937 and most recently upgraded in 2000, received passing marks in a 2011 sewage "report card" by the Citizens Campaign for the Environment.
The environmental group noted that the plant uses ultraviolet light to disinfect sewage and had a relatively low rate of violations.
The plant is undergoing another upgrade that is targeted to be complete in 2016, Reichel said.