06- 096 C.M.R. ch. 580, § 3
The assessment of whether classified waters are attaining the bacteria standards ascribed to their classification shall be based on the geometric mean of at least twelve samples collected at regular time intervals between May 15th and September 30th and shall be analyzed through use of upper 90% confidence limits incorporating a 0.5 log standard deviation, except that a lesser number of samples may be used for assessment purposes when collected in close proximity to a known point source of residential/municipal wastewater. Local Health Officers, in cooperation with the Department of Human Services, may use an analysis of any number of samples, including a single instantaneous bacteria level, as the basis for determining whether waters under their jurisdiction should be closed for purposes of recreation in the water.
06- 096 C.M.R. ch. 580, § 3