Dakota County closed the Lake Byllesby Regional Park beach Tuesday after the city of Northfield dumped 1 million gallons of treated and untreated wastewater into the Cannon River.

A PVC pipe break at Northfield’s treatment plant forced the city to discharge the wastewater between 1 and 4 a.m. Tuesday, according to a news release. Northfield warned the public to avoid direct contact with the Cannon River downstream of the city until 6 a.m. Friday.

The discharge was less than 1 percent of the total flow of the river at the time, according to the release, but officials are continuing to monitor for public health risks.

Because the Cannon River flows into Lake Byllesby, Dakota County shut down the regional park’s beach until samples show the water has returned to safe levels. Swimming, tubing, water skiing, paddleboarding and other recreational activities that involve direct contact with the water are prohibited, but fishing and boating are allowed. The Lake Byllesby campground and other park amenities remain open.

Goodhue County, which operates a park on the south side of Lake Byllesby, also closed its beach until sample results indicate safe waters.

Northfield officials are working on plant maintenance repairs and monitoring operations, the release said. The cause of the breakdown is still being evaluated.

The Minnesota Pollution Control Agency has been notified.