A Comprehensive Water Quality Model of Boston's Drainage Systems

The Boston Water and Sewer Commission has developed a 4,000-node EPA SWMM computer model 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 TMDLs and water quality standards. The model simulates buildup and washoff across nine land uses and 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 sources in sediment bed load, and pollutant removal through natural and constructed detention-treatment systems. The model is uniquely complex in its combination of detailed hydraulic representation of a city's natural and constructed drainage systems with dynamic water quality process simulation.