TECHNICAL AND REGULATORY APPROACHES TO CSO CONTROL IN A CHANNELIZED URBAN STREAM

Alewife Brook is a channelized urban stream that runs along the borders of Cambridge, Somerville, and Arlington, Massachusetts. Currently, untreated CSO can discharge to Alewife Brook from six outfalls, all associated with community-owned combined sewer connections to a Massachusetts Water Resources Authority (MWRA) sewer interceptor system along the banks of the brook.MWRA has been responsible for developing and implementing a long-term CSO control plan for the metropolitan Boston area, including Alewife Brook. Two particular challenges have arisen in the development of the CSO control program for Alewife Brook:The water quality standard for Alewife Brook is Class B (fishable/swimmable). Receiving water modeling demonstrated that the Class B criteria for bacteria were not met in dry weather, and that the elimination of CSO would have little effect on peak bacteria concentrations in wet weather, due to the impacts of separate stormwater.Alewife Brook and developed areas along its banks are currently subject to flooding in large storms, and residents along Alewife Brook are concerned about the public health effects of CSO-laden flood waters. Eliminating CSOs by sewer separation, however, would significantly increase the stormwater flow to Alewife Brook.The regulatory approach taken by MWRA to address CSO issues in Alewife Brook incorporated the flexibility provided by the EPA National CSO Control Policy, and regulatory options available through the Massachusetts Water Quality Standards. The technical approach included both technology-based and water quality-based assessments, relying on a detailed system understanding developed through use of a collection system model, and a watershed approach supported by data collection and development of a receiving water model. Public input has been and will continue to be a key factor in shaping the final CSO control plan for Alewife Brook, as well as the extent of potential revisions to the existing water quality standard for the brook to account for wet weather impacts.