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. WEF/CWEA Collection Systems 2002 Conference
The Massachusetts Water Resources Authority (MWRA) completed its Long-Term Control Plan for CSO control (LTCP) in 1997. The plan used a watershed approach to assess the impacts of CSO in the context of other sources of pollution in the watershed, including stormwater and upstream flow. The recommended plan proposed to eliminate CSO to critical use areas, and cost-effectively minimize CSO in non-critical use areas. Regulatory acceptance of the plan was contingent upon changing existing water quality standards in areas where CSO would remain. At the time that the LTCP was being developed, the state water quality standards were being revised to establish a category under which minimized CSO discharges could remain, provided that certain specific conditions were met. Based on the MWRA's LTCP, the United States Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (DEP) agreed to revise the water quality standards for certain waters within Boston Harbor, but the two agencies did not agree on the methodology for meeting the criterion for "substantial and widespread social and economic impact". For the Charles River, a temporary water quality variance was granted to allow further study of the impacts of non-CSO sources of pollution, as well as for additional CSO control alternatives, prior to a final determination of the water quality standard. The conditions of the variance defined the activities to be undertaken to provide the information needed to make a final water quality standards determination. These activities included: conducting an assessment of providing additional CSO storage; conducting a performance evaluation of the MWRA's Cottage Farm CSO Facility, which discharges to the Charles River; developing a more detailed stormwater runoff model for areas tributary to the Charles River; and upgrading the receiving water model for the Charles River. A report summarizing these activities was submitted to the DEP in January 2004, and the report is currently under review by that agency. KEY WORDS Water quality standards revision, CSO LTCP FIGURE 2. DRY WEATHER FECAL COLIFORM BACTERIA PROFILE FOR THE CHARLES RIVER, FROM THE 1997 LTCPAt the time that the LTCP was being developed, the water quality standard for the Charles River was Class B ("fishable/swimmable"; 200 counts/100ml fecal coliform bacteria). The modeling output indicated that even complete elimination of CSO to the Charles River would not result in attainment of the Class B standard, due to the non-CSO pollutant loads. Figure 4 presents the relative flow, fecal coliform bacteria, and biochemical oxygen demand (BOD) loadings of CSO, stormwater and upstream sources to the Upper Charles River for the 3-month, 24-hour and 1-year, 24-hour design storms. The data in Figure 4 represented existing conditions at the time of the 1997 LTCP. With these conditions as a starting point, the approach for developing CSO controls for the Charles River evolved into a plan to cost-effectively minimize CSO, consistent with the state...
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