CLINTON — City workers discovered a discharge of partially treated wastewater on Thursday about 3:30 p.m.

The discharge of about five gallons per minute reached Mill Creek, which drains into Beaver Slough before flowing into the Mississippi River. City workers traced the discharge to a previously repaired crack in the primary clarifier. They sealed the crack Thursday, but found it was still leaking on Friday morning. Workers contained about 1,000 gallons of wastewater behind a temporary dam and are pumping it back to the clarifier.

The concrete tank is 60 years old and has leaked and been repaired in the past. Water Pollution Control Superintendent Gary Schellhorn said the tank is one of three settling tanks and the leak was located in a junction between two areas built decades apart. The tank was shut down and drained completely on Friday. Schellhorn said shutting down the tank will not affect flow capacity as the plant can treat 15 million gallons per day and current flow is approximately 4 million gallons per day.

An unknown amount of wastewater reached the creek. Workers will monitor the area around the clock to prevent additional discharges. Permanent repairs to the tank will begin today. It is estimated it will take a week to complete repairs.

Facilities are required to report bypasses caused by mechanical failures to the Department of Natural Resources within 12 hours of onset or discovery. Schellhorn said the delay in reporting the incident was due to workers being busy immediately addressing the problem. He added he believed the report time to be 24 hours. The DNR will send the facility a notice of violation for failing to report the discharge within 12 hours.