The City of Cambridge’s sewer infrastructure system consists of separated systems and combined systems. During significant rain storms combined sewer systems fill up with a mixture of sanitary waste and rain water. When it rains for extended periods of time or when the rain is very heavy a Combined Sewer Overflow (CSO) acts like a relief valve allowing untreated sewerage to discharge into waterways instead of backing up into homes, businesses or into the streets.

Both the Charles River and Alewife Brook receive discharges from CSOs. There are seven (7) permitted CSOs on the Charles River: two (2) owned by the MWRA and five (5) owned by the City of Cambridge. There are eight (8) permitted CSOs on the Alewife Brook: one (1) is owned by the MWRA, one (1) owned by the City of Somerville and six (6) owned by the City of Cambridge. Cambridge is working to reduce and eliminate CSO discharges through a sewer separation and stormwater management program. The long term plans for CSO control will reduce the bacteria and other pollutants associated with CSO discharges to both the Alewife Brook and the Charles River.

Public health officials recommend avoiding contact with CSO contaminated waters and wearing protective clothing such as gloves and boots, as there may be increased health risks during these periods. Contact with floodwaters should also be avoided as they may contain similar contaminants and pose associated health risks. A Storm Fact Sheet is available on the Massachusetts Department of Public Health web site and a Home Care Guide from Massachusetts Department of Environmental Protection web site. In addition, for information on water quality in the Alewife/Mystic River you can consult Mystic River Watershed Real -Time Water Quality Monitoring Site and for information on water quality within the Charles River Basin you can consult the Charles River Watershed Association’s (CRWA’s) Water Quality Monitoring web site. The Mystic River web site is designed to collect remote data every 15 minutes and water quality samples daily within the Mystic River Watershed. The CRWA runs a daily flagging program in the lower reaches of the Charles River during the summer months, and a monthly testing program year-round throughout the Charles River watershed.