Changes in wet weather bacterial water quality in Boston Harbor 1987 to 2003, monitoring data collected by MWRA's Central Lab and Environmental Quality monitoring program in Boston Harbor and tributary rivers, and from MDC's beach monitoring. Enterococcus is the sewage indicator bacteria recommended by EPA for monitoring marine waters. Contours show the geometric means of Enterococcus data collected when more than 0.2 inches of rain had fallen in the previous 24 hours; blue areas meet the EPA standard and red-purple areas exceeded the standard.

1987-1991 shows data from before the Boston Harbor project and CSO plans began up through the last year that sludge was discharged (1991). In wet weather most of the Inner Harbor, the tributary rivers, and areas affected by discharges of sewage and sludge from the DITP and NITP failed the standard, some with extremely high counts.
1992-1998 reflects impacts of CSO upgrades, the end of sludge discharges, full pumping at DITP, improved primary and beginning secondary treatment at DITP. Most of the harbor meets standards, except tributary rivers, Fort Point Channel, and along Wollaston Beach.
1999-2003 shows continued improvement due to closure of 22 CSO outfalls, upgrades of CSO facilities, ending of harbor treatment plant effluent discharges as the new outfall began operating, and local efforts to abate stormwater pollution.