Worcester officials told people to avoid the Blackstone River for 48 hours after hundreds of thousands of gallons of sewage flowed into the river early Thursday morning.
The city’s Department of Public Works and Parks sent out an email notification stating that the overflow occurred at 4:23 a.m. behind the Walmart on Tobias Boland Way, and 879,000 gallons of untreated or partially treated sewage and waste was discharged into the river.
The areas of the river potentially affected by the overflow are in Worcester, Millbury, Sutton and Grafton, officials said.
“Avoid contact with the affected water bodies for 48 hours after the overflow ceases due to increased health risks from bacteria and other pollutants,” the notification said.
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The overflow ended at 5:03 a.m., the notification said.
The overflow that happened Thursday morning came from the city’s combined sewer system, which collects both sewage and storm water that gets treated before it’s discharged to the Blackstone River. Combined sewer overflows can happen during heavy or prolonged rains, according to the city’s Department of Public Works and Parks website.
“Overflows such as this can happen during rainfall events as many as five to 25 times per year,” Robert Burgess, a spokesperson for the Worcester City Manager’s office, said in an email. “This event has increased visibility because of the new notification process to the public which is the result of a state law that went into effect this month.”
People can sign up to receive sewer system overflow notifications on the city of Worcester’s website.
Whenever a sewer overflow happens, the city sends out an email within two hours advising people of the date, time and location of the incident, and follow-up notifications are emailed every eight hours until the overflow stops. The notifications are provided in English, Spanish and Vietnamese, officials said.
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Worcester’s combined sewer system covers about four square miles of the city, including downtown, Shrewsbury Street, Green Island and parts of Main South, according to the city’s Department of Public Works and Parks website. Some of the oldest pipes in the city are combined sewers that are made of brick and date back to the 19th century. They can be as large as 8 feet in diameter, the website states.
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Emily Sweeney can be reached at emily.sweeney@globe.com. Follow her on Twitter @emilysweeney and on Instagram @emilysweeney22.