PORTLAND – Environmental officials say unusually high tides that flush pollutants from salt marshes may explain why many beaches in Maine and other New England states were posted with health advisories or closed last month.

The so-called spring tides that occur during a full or new moon are known to wash pollutants from salt marshes into the surf zones of beaches, said Matthew Liebman, a biologist with the Environmental Protection Agency’s New England office.

Spring tides, which occur when the Earth, sun and moon are in a line, occurred July 19-21.

During that period, health advisories were issued at beaches in Lincolnville, Kennebunk, Kennebunkport and South Portland because of high levels of a bacteria indicating the presence of fecal matter. Fecal matter may carry human pathogens, which can cause illnesses.

During the same period, New Hampshire’s Seabrook Harbor was closed, as were beaches in the Massachusetts towns of Scituate, Duxbury, Marshfield, Hull and Quincy.

Scientists plan to closely monitor beaches in southern Maine’s York County during the spring tide that occurs Aug. 19 to see if the high-tide theory, which is based on studies in California, holds.

Under normal circumstances, beach closures occur after heavy rain, which washes sewage and polluted runoff into the ocean.

But no rain had fallen prior to the closings and advisories at beaches, which also had little or no history of pollution.

Bacteria can live much longer on dry sand and the dead vegetation of a salt marsh than in salt water, said Michael Shiaris, a biology professor at the University of Massachusetts.

“High tides can wash them out,” said Shiaris, who studies the ecology of bacteria in coastal water and sediment.

July’s spring tide was no higher than other spring tides, raising a question as to why it could have caused so many beach closures.

But Esperanza Stancioff of the Maine Healthy Beaches program pointed out that this spring’s heavy rains and the resulting high water table may have caused unusually high levels of fecal matter to enter the marshes.

The bacteria in turn could have been washed out by the spring tides.