The Mystic River flows from the Mystic Lakes in Winchester and Arlington, east through Medford, Somerville, and Chelsea to Boston Harbor. Only five miles long, much of the river passes through dense residential and industrial areas, and shows the impacts of pollution from surrounding development. Like the Charles, the Mystic River is dammed at its mouth where it enters Boston Harbor.

Water quality problems in the Mystic Basin include toxic pollutants in industrialized areas, eutrophication, and bacterial contamination.

Water quality in the Mystic Basin varies greatly depending on location. Problems include contamination from sanitary sewer overflows, stormwater, and combined sewer overflows (CSOs).

Alewife Brook, a tributary to the Mystic in Cambridge and Arlington, is one of the most contaminated water bodies in the Boston area, due mainly to stormwater. Here, fecal coliform counts routinely fail to meet the state standard. Bacterial counts are fairly good downstream, where MWRA’s Somerville Marginal CSO Treatment Facility is located, but counts increase near Medford Square.

Download Mystic River sampling data