A broken sewer main that has been leaking since Monday is scheduled to be repaired Saturday night, Lexington officials said Friday.
The work will begin at 11 p.m. and could take eight hours, said Charles Martin, director of the city's Division of Water Quality.
During that time, sewage that has only minimal treatment will flow into Bryan Station Creek, then North Elkhorn Creek.
The North Elkhorn Pump Station, which sends water through the 24-inch main to the Town Branch Wastewater Treatment Plant, will be shut down.
The station, on Elkhorn Road near Winchester Road and Interstate 75, pumps about 1,000 to 1,500 gallons a minute during the late night and early morning.
A tank system will be set up to allow solids to settle and to pump air into the sewage before it reaches the creek. That is to minimize the impact on fish in the creek, because untreated sewage can cause dissolved oxygen levels to drop, killing fish.
To dilute the sewage, several fire hydrants in the area will be opened. Water will flow into storm sewers and then into the creek.
Water coming from the hydrants will be dechlorinated, because too much chlorine also can harm fish.
"We think we can get about a 9-1 ratio of fresh water to sewer water, which should mitigate the impact downstream," Martin said.
Officials think that the underground main is not completely ruptured, because only about 300 to 500 gallons a minute has been leaking.
The repair has been delayed because a custom-made part was needed. A stainless steel sleeve will be slipped inside the existing pipe to stop the leak.
The force main carries sewage under pressure from the North Elkhorn Pump Station east of town to the Town Branch Wastewater Treatment Plant west of downtown.
When it is turned off, workers at the point where the main is leaking will quickly dig down to the main and make the repair.
The leak was discovered when people at a private swimming pool at a development near Star Shoot Parkway and Flying Ebony Drive noticed untreated sewage bubbling from the ground and running downhill toward an unnamed tributary of Bryan Station Creek.
Workers tried various means to keep the sewage out of the creek, with varying degrees of success. By Thursday, two 8-inch pipes were laid over land to a sewer line that runs to Lexington's other wastewater treatment plant.
The leaking main was installed in 1980. It has broken eight times in the last dozen years.
The city is in the process of building a new force main as part of its settlement of a lawsuit brought by the federal Environmental Protection Agency. The EPA charged that Lexington's old, leaky storm and sanitary sewer systems were harming streams in violation of the Clean Water Act.
The fire hydrants to be opened are on Call Drive, Thunderstick Drive and Elkhorn Road. A police officer will be in the area while the hydrants are flowing.
Residents in the area might notice lower-than-normal water pressure, said Ray Golden, a spokesman for Kentucky American Water. Anyone concerned about water pressure should call the utility at 1-800-678-6301.
Should a fire create a need for water in the area, the hydrants sending water to the creek would be turned off, Martin said.
Residents should not notice an impact to sanitary sewer systems, he said.