Due to poor water quality, visitors at Great Rock Bight in Chilmark were advised to swim at their own risk on July 27. The Martha’s Vineyard Land Bank Commission issued the advisory after finding water conditions testing below acceptable levels per the State of Massachusetts.

“The trail system and the beach is technically open, but the swimming is not,” Land Bank land superintendent Harrison Kisiel told the Gazette by phone Thursday morning. “We will test the beach again Monday, if that comes back with a good result, we’ll be able to reopen as of Monday afternoon.”

Great Rock Bight is state-tested weekly from June through August, and on July 25 tests found enterococci bacteria levels of 166 cfu/100 ml, exceeding the state standard of 104 cfu/100 ml. Two days later, levels had jumped to 216 cfu/100 ml on July 27. Levels had been at just 20 cfu/100 ml on July 18.

According to the Massachusetts Department of Public Health, enterococci are a group of bacterial species within the Streptococcus genus, some of which (e.g. Streptococcus faecalis) are typically found in human and animal intestines and are therefore present in sewage. These bacteria are also referred to as indicator organisms.

Chilmark public health agent Marina Lent told the Gazette that back-to-back tests exceeding state safety limits suggest that “there is fecal contamination or fecal contamination of that water.”

Mr. Kisiel called the high tests an “anomaly” and provided a number of potential explanations, including a high avian presence on the beach, which can flood the waters with fecal matter. He further suggested that an overgrowth of seaweed may be trapping the enterococci close to the shore.

Both conditions, Mr. Kisiel stressed, are “not indicative of the Vineyard Sound.” Other Island beaches have not experienced similar elevated bacteria tests.

Mr. Kisiel is hopeful that the beach will reopen next week.

“Because it’s such an anomaly, my hunch is that it will come back as a clean test on Monday,” Mr. Kisiel said.

State records show that July 2015 was the last time Great Rock Bight test results exceeded state standards.