Heavy rains temporarily closing many Mass. beaches

The number of late spring and early summer beach closings is the highest in at least eight years, and represents more for the period than in 2002 through 2005 combined, according to the state Department of Public Health.

Of course, the recent weeks of persistent clouds and rain have kept most people away from beaches anyway. But that is likely to change as heavy thunderstorms are expected to give way to sunnier skies today through at least Saturday. Beachgoers in some places might be greeted by red flags warning them not to swim because bacteria levels in the water could make them ill.

A temporary closure does not actually mean a beach is closed - lifeguards are still on duty, but they spend much of their time strongly urging people not to go in the water.

Beaches that have experienced the most closures tend to be in urban areas such as Boston or areas with an influx of visitors, such as Provincetown.

When it rains, so much water can pour into the region’s network of storm water and sewage pipes that they can overflow and send untreated human and animal waste into waterways.

Health officials often test beach water daily.

“Once rainfall stops for a period of 24 to 48 hours, we see a recovery of beach water quality,’’ said Suzanne Condon, director of the Bureau of Environmental Health at the state Department of Public Health. “You’ve got this elimination of the source of the contamination and then you have the natural flushing of the tidal changes.’’

Some beaches, such as Nantasket in Hull, Salisbury, and Horseneck in Westport tend to stay open after heavy rains because they face the open ocean.

Despite this year’s numbers, Massachusetts beaches are markedly cleaner than they were 30 years ago, when joggers stepped in raw sewage on the beaches that ring Boston Harbor. The neglect won the harbor the dubious honor of being called the filthiest in the nation by George H.W. Bush during his 1988 presidential campaign against thenGovernor Michael Dukakis.

After a two-decade, $4.5 billion cleanup, the harbor is swimmable most days. More work is being done to make it even cleaner: A giant 2-mile tunnel is being constructed under South Boston to hold enormous amounts of untreated storm water until the Deer Island Sewage Treatment Plant can process it. It is expected to come online in 2011.

Bruce Berman of Save the Harbor, a Boston Harbor advocacy group, said the closing of beaches “underscores we have more work to do.’’

But, he added, “No one has seen a June like this in the recent past, and hopefully we won’t see a June like this anytime soon.’’

Beth Daley can be reached at bdaley@globe.com, and Stephen Smith can be reached at stsmith@globe.com.