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The Boston Water and Sewer Commission (the Commission) has conducted a comprehensive assessment of pollutant loadings from its closed conduit and open channel drainage systems to the receiving waters of the Charles River, Neponset River, and Boston Harbor. A uniquely detailed Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) computer model was adapted from an existing system-wide 4,000 node hydraulic model and used to assess sources, loads, and mitigation alternatives for 13 pollutants discharging from the city's drain systems to its receiving waters. The model is calibrated to baseflow and stormwater runoff observed throughout a five-month flow monitoring program, water quality data obtained at 20 sites over six rainstorms and six dry-weather sampling events during the flow monitoring program, and historic data. The model has been used to assess the potential of improved best management practices, green stormwater infrastructure implementation, and low-impact development practices for meeting total maximum daily loads (TMDLs) and water quality standards. The model simulates pollutant buildup and washoff across nine land uses as well as contributions from groundwater, illicit flow, and rainfall. It simulates pollutant conveyance through open and closed conduit systems, first-order decay of oxygen demand, bacteria die-off, bacteria resuspension from sources in sediment bed load and pollutant removal through natural and constructed detention/treatment systems.

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