MARBLEHEAD — Grace Oliver Beach is being shut down, for the second time in less than a week, after a geometric-mean failure, the Board of Health announced through a post on the Police Department’s Facebook page Thursday evening.
The beach’s bathing water was deemed unacceptable based on standards set in the state Sanitary Code regarding indicator organisms. For marine water, the indicator organism is Enterococci, which cannot exceed 104 colonies per 100 milliliters. The geometric mean must not exceed 35 colonies per 100 milliliters for the five most recent Enterococci levels in the same bathing season.
The geometric mean of Grace Oliver’s last five tests came out to 42.87, including a high of 142 colonies per 100 milliliters on Wednesday, Aug. 2. Test results Thursday came back with 20 colonies per 100 milliliters.
The Board of Health closed the beach last Thursday after results from samples taken from the previous day contained unsafe bacterial levels. The beach was opened a day later after tests taken the day of the closure came back with lower bacterial counts, only to be shut down again six days later.
