Nassau County officials ordered six North Shore beaches closed Friday evening after a treatment plant released 125,000 gallons of partially treated wastewater and sludge into Oyster Bay Harbor.
County health department spokeswoman Mary Ellen Laurain said the closures were being undertaken "out of an abundance of caution."
The department should have results from testing on Sunday, at which time the county will decide whether to reopen the beaches, she said.
The closed beaches include Centre Island Bay Beach, Centre Island Sound Beach, West Harbor Beach, Laurel Hollow Beach, Soundside Beach, and Theodore Roosevelt Memorial Beach.
The Centre Island beaches and Roosevelt beach had been scheduled to open Saturday for the first time this year, Oyster Bay spokeswoman Marta Kane said.
The county acted after New York State officials ordered the halt of shellfishing Friday afternoon in Oyster Bay Harbor and other areas. The shellfishing closure is in effect until further notice.
The discharge started at 7:15 a.m. Friday and ended at 1 p.m., state Department of Environmental Conservation spokeswoman Lori Severino said.
The source of the discharge was a wastewater treatment plant run by the Oyster Bay Sewer District. The discharge was caused by a malfunction of a mixer, said Steve Hearl, project manager for Melville-based H2M Architects + Engineers, which designed the system and serves as consulting engineers to the district. The solid material in the sewage is mixed and aerated and then allowed to settle before the water is discharged. But Friday morning the mixer continued to churn after it was supposed to stop, Hearl said. By the time operators discovered the problem, it was too late to stop the discharge, he said. Hearl said the computer-operated system was restarted and subsequently operated without problems.
"We're still trying to figure out what the glitch was," Hearl said. He added, "It shouldn't happen; that's why we're trying ... to prevent so it doesn't happen again."
The DEC said the closure of shellfishing affects about 8,150 acres of shellfishing beds. Areas closed include Oyster Bay Harbor, Cold Spring Harbor in the towns of Oyster Bay and Huntington.
The DEC warned that partially treated sewage and sludge carry pathogenic bacteria and viruses that can make shellfish hazardous to eat.