High bacteria levels forced swimmers out of the ocean at Matunuck Town Beach in South Kingstown yesterday, and the beach will remain closed to bathing today pending the results of a second study of seawater drawn there this morning.

Matunuck Beach faces the open ocean and a closing there due to dirty water is rare, perhaps unprecedented.

"You just don't have that kind of problem here," said Andy Nota, South Kingstown's director of Parks and Recreation. "Unfortunately it had to happen heading into a Saturday instead of a Monday."

Nota said as many as 1,000 people will use Matunuck Town Beach on a summer Saturday. The beach's playground and picnic area will remain open today, but swimming will be off-limits until bacteria levels drop below 104 colonies per 100 milliliters of seawater.

Yesterday's test results showed a reading nearly 30 times greater than that threshold, and swimmers were ordered out of the water around 4 p.m. when test results came back.

The results don't seem to make a whole lot of sense, Nota said. "We're hoping it was just an anamoly." He theorized that hard rains from a thunderstorm that pounded South County on Thursday night may have washed pollutants into the sea, though that's never happened there before.

The state's Department of Health tests almost all of the 122 beaches it licenses. High-risk areas such as Warwick's Conimicut Beach in upper Narragansett Bay face weekly testing, while low-risk areas such as Matunuck are tested just once a month.

South Kingstown has hired a private contractor to draw samples early this morning; results from those samples should be available tomorrow morning. If bacteria readings are below the state threshold of 104 colonies then Matunuck will reopen to swimming tomorrow; if readings are still high then the town will mount an investigation to determine the pollution's source.

Enterococci bacteria are not only an indication that harmful bacteria may be present, but can also be harmful in themselves causing urinary-tract infections, diverticulitis, and meningitis. In 2004 it replaced fecal coliform as the new federal standard for water quality at public beaches.

While polluted water tainted traditionally clean Matunuck Beach, the often-tainted waters of upper Narragansett Bay turned up good news yesterday: teams of researchers found much better levels of dissolved oxygen mixed into the Bay's water column than they had found last week.

During last week's heat wave the 53 stations tested between Providence and Jamestown's northern tip found oxygen levels too low to support fish at some level of the water column.

Yesterday just 1 of nearly 50 sites tested showed dissolved oxygen levels that low, a spot in Greenwich Bay.

"The Bay was in really good shape today," said Warren Prell, a Brown University geology professor who helps conduct periodic testing of the Bay. "It was very well mixed and it's really kind of a combination three things:"

Cooler air temperatures which chill the oxygen-rich surface waters; cooler waters sink, carrying oxygen deeper into the water column.

Increased tidal pull near the full moon. A greater fluctuation between high and low tides creates a greater current, again mixing oxygen-rich layers deeper into the water column.

Stiff winds, which agitate the water.

"The winds were really cranking today," Prell said. "In fact we actually dropped the last couple of stations [from the survey] -- it got pretty rough farther down the Bay."

Chris Deacutis, chief scientist for the Narragansett Bay Estuary Program, found similar positive results in the dissolved oxygen readings he took yesterday.

"This was like a good deep breath for the Bay," Deacutis said.

gcarbone@projo.com / (401) 277-7434