Chilmark Pond has been ordered closed to swimming until further notice following a routine water sample taken July 31 that revealed high levels of bacteria.

According to the state Bureau of Environmental Health, the water sample revealed 399 colony forming units (CFU) per 100 milliliters of water of enterococci bacteria. A subsequent sample taken on August 2 showed 2,014 CFU per 100 milliliters of water. The sample taken on August 2 coincided with rainfall, which is known to skew data.

The state requires bodies of water closed to swimming when tests in marine waters exceed 104 CFU.

Enterococci are a group of bacterial indicator organisms that may indicate the presence of fecal contamination. According to the state Department of Public Health, animal waste, such as from dogs or birds, can contribute to elevated levels of the bacteria. Swimming in water with high levels of enterococci can be dangerous for young children, the elderly, and people with compromised immune systems.

Chilmark Pond will be tested daily and reopened when bacteria counts return to allowable levels.

In West Tisbury, Seth’s Pond has remained closed to swimming since July 3 due to high levels of enterococci. West Tisbury board of health agent John Powers previously told the Gazette that the pond has remained closed for an extended period due to his inability to collect five clean water samples on consecutive days without rainfall, which the state requires to reopen a body of water following a high bacteria count.