Heavy rainfall in St. Mary's County last week caused the Leonardtown sewage treatment plant to overflow into Breton Bay, forcing health officials to issue a swimming advisory.

Ebenezer Israel, the county's health director, said the advisory is voluntary.

"We're just advising people: Don't go swimming [in Breton Bay] until the bacterial levels are back to normal," Israel said.

Health officials said the swimming advisory will be in effect until after the state Department of Natural Resources completes testing. DNR workers will collect samples tomorrow and testing results are expected on Wednesday.

The sudden thunderstorms that hit the region Wednesday morning and continued through the afternoon brought more than seven inches of rain in Leonardtown and other parts of St. Mary's.

South of Leonardtown on Breton Bay, the area around the sewage plant received eight inches of rain in five hours. The water level in Town Run, the tributary where the plant empties discharges, rose by eight feet, said Robin Guyther, Leonardtown town administrator.

The water level rose within six inches of discharge pipes from the sewage treatment plant.

"We could have been in a lot of trouble if the water had kept going up. Because we would have had a backup problem," Guyther said.

Still, two large lagoons that normally take in overflow were suddenly full of rain runoff. Untreated sewage poured into the creek and into Breton Bay for 10 hours, Guyther said. Officials did not have an estimate of the volume of untreated sewage discharged.

Heavy rain started pouring about 10 a.m. on Wednesday in Leonardtown. By noon, town officials worried about the rising water level in Town Run had notified the Maryland Department of the Environment, Guyther said.

The Leonardtown sewage plant was built in 1983 and treats up to 350,000 gallons of sewage daily. The plant serves about 3,000 residents.