METHUEN — It will be safe to take another dip in Forest Lake before summer ends.

The lake — the only city-sanctioned swimming hole — will open today, thanks to reductions in bacteria. Testing showed bacteria levels have dropped and the water is safe.

"It looks like we're seeing some improvement," Matt Kraunelis, chief of staff to Mayor William Manzi, said yesterday.

Officials took water samples yesterday and sanitary inspector John Bonnano of the Health Department informed Kraunelis and other officials yesterday afternoon that the lake is fine to swim in, and he would notify them if anything changes.

The lake has been closed since July 22 because bacteria levels made it unsafe for swimming. The lake was closed July 9 through July 15 for the same reason.

Officials from the health and public works departments, as well as the state Department of Public Health, tried to figure out what caused bacteria levels to rise. Culprits could have included animal waste — possibly from geese — rainwater runoff and hot weather. The DPW does not believe sewer leaks are a factor, Kraunelis said.

"They dye-tested all the sewer lines and found no leaks," he said.

Crews cleaned up the area, and that appears to have helped, Kraunelis said.

"The DPW moved a Dumpster away from the lake. They took out a swale where runoff was leading into the lake — they replaced that with stone so there's a better filtration system," he said.

The Recreation Department moved its swimming lessons to Latitude Sports Club at 116 Pleasant Valley St. after the latest lake closing.

City officials post notices on their Web site, www.ci.methuen.ma.us, to tell people whether the lake is open. They also hang a red flag at the lake if it's unsafe to swim.

The lake is scheduled to close for the season around Labor Day.

Beaches must be shut down when bacteria levels indicating the presence of fecal contamination are too high, according to the state Department of Environmental Protection. The department's Web site said exposure to the bacteria can cause illnesses, from minor gastroenteritis to the more serious hepatitis.

In past summers at Forest Lake, people have complained of "swimmer's itch," a minor rash, when bacteria levels are high.