Ice House was reopened to swimming at 10:30 Friday morning after test results from a Thursday sample showed that levels were safe, land bank officials confirmed. The pond had been closed since Tuesday afternoon when water testing revealed unusually high levels of bacteria.

West Tisbury health agent Omar Johnson said the popular West Tisbury swimming hole is tested weekly for enterococci and e coli. State law mandates ponds be closed to swimming if levels are above 61 colonies per 100ml. Ice House pond levels were at 399.8 colonies per 100ml on Monday.

Mr. Johnson said enterococci is a bacteria found in the intestines of warm-blooded animals. He could not account for the sudden increase in contaminants.

“If it hasn’t exceeded all summer and then you get a hit like that yesterday, what can you attribute that to and how can you know if it will continue?” Mr. Johnson said. “I’m as puzzled as anyone.”

The pond is located off Lambert’s Cove Road in West Tisbury and is owned and managed by the Martha’s Vineyard Land Bank. It is accessible from two different entrance points and walking trails.

Samples are tested at the Wampanoag Environmental Laboratory in Aquinnah.