MWRA monitors bacteria at locations surrounding the MWRA outfall and elsewhere in Massachusetts Bay (see station map below). Water samples are analyzed for sewage indicator bacteria—fecal coliform and Enterococcus. Fecal coliform is used by the Massachusetts Division of Marine Fisheries (MADMF) to monitor water quality in shellfish areas. An "S" following the station ID means the sample was collected at the surface; a "P" means the sample was collected below the pycnocline (collected only if a pycnocline is present). Results are in bacterial count per 100 milliliters.

MADMF's standard for unrestricted shellfishing is <= 14 colonies per 100 ml water. Another applicable water quality standard is the Massachusetts Department of Environmental Protection (MADEP) geometric mean limit of <= 200 colonies per 100 ml water, with no more than 10% of samples exceeding 400 colonies per 100 ml.

While not used currently by MADEP or MADMF, Enterococcus is now recognized by the EPA as a better indicator of public health risk to swimmers than fecal coliform in seawater. The Massachusetts Department of Public Health adopted the Enterococcus standard for swimming areas. This standard includes a geometric mean of 35 colonies per 100 ml and a single sample maximum of 104 colonies per 100 ml Enterococcus.

See previous bay monitoring results