A sewer backup at the King County’s Lake Ballinger pump station caused the discharge of an unknown amount of untreated sewage into the lake’s south end Thursday. The lake will be closed to all recreational uses until the water has tested at safe bacterial levels for two consecutive days.

Testing the samples takes a day, so park users should assume that the boat launch and beach will be closed through the weekend of March 16-17, King County’s Wastewater Treatment Division said in an announcement.

King County crews are onsite investigating the sewer backup, which affected approximately 12 properties adjacent to Lake Ballinger. A cleaning contractor is on location.

The pump station is located at 2205 N. 205th St. in Shoreline, on the King/Snohomish County border.

Wastewater treatment division spokesman Norm Mah said Friday afternoon that the cause of the spill and the amount of sewage discharged has not yet been determined.

There are no reports of sewage backing up into homes, the said. Water quality samples are being taken from Lake Ballinger and the county has notified regulatory agencies.

Sewage flows out of the unstaffed Lake Ballinger pump station are part of a “flow swap arrangement” with King County and the City of Edmonds, which also treats sewage from the City of Mountlake Terrace. Under that arrangement, Edmonds treats wastewater from Richmond Beach in exchange for an equal amount of Lake Ballinger area flow that is sent to the King County treatment system.