A million gallons of sewage has discharged into the Cannon River from a wastewater treatment plant in Northfield.
The City of Northfield says the leak happened at 8 p.m. Monday, when a PVC pipe broke.
Officials then found that this line break had caused a million gallons of treated and untreated wastewater to enter the nearby Cannon River between 1 a.m. and 4 a.m. on Tuesday.
The leak is causing concern for cities downstream, with Northfield advising the public consider avoiding direct contact with the water on the Cannon River until 6 a.m. on Friday.
The leak has already prompted the closure of the Lake Byllesby Regional Park beach, with Dakota County saying the public should have no contact with the waters in Lake Byllesby until it's safe to do so.
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The Cannon River flows into Lake Byllesby, the city says, though other park amenities including the campground remain open.
Boating and fishing is still allowed, it says, though swimming, tubing, water-skiing and paddle-boarding are also out.
The advisory will only be lifted after the water has been tested as safe for two days in a row.