Putting Stormwater into Context in the Merrimack River Watershed: Modeling of Bacteria Standards Attainment in the Future with CSO Reductions and Potential Basin Scale Stormwater BMP Implementation

摘要

The Merrimack River Watershed Assessment used a comprehensive water quality monitoring and modeling approach to evaluate current and potential future water quality in the mainstem Merrimack and Pemigewasset Rivers in New Hampshire and Massachusetts using an integrated planning approach that considered regional wastewater, stormwater, and CSO pollutant sources on instream water quality. Baseline water quality conditions were evaluated by conducting 16 independent water quality surveys over nearly 15 years. These surveys included wet- and dry-weather longitudinal surveys, focused impoundment surveys, and sediment flux measurements to develop a comprehensive understanding of fecal bacteria and nutrient-related impairments. The water quality monitoring results informed the calibration of a linked hydrologic, hydraulic, and receiving water quality model. Based on the model, impacts of implementing additional combined sewer overflow (CSO) controls, beyond the current controls, and impacts of additional stormwater management practices were evaluated. The monitoring data and baseline modeling conducted over the 15-year study have demonstrated significant improvements in water quality, especially relative to dry weather bacteria concentrations, but high bacteria concentrations remain during wet weather. Sensitivity analyses illustrated that even with full elimination of CSOs, the Merrimack River would not consistently meet bacteria standards. Scenarios were evaluated assuming long-term control plans (LTCP) CSO controls to achieve a 3-month level of control (four overflows per year) for all communities discharging to the Merrimack River and deploying green infrastructure best management practices (BMPs) for stormwater controls targeting highly impervious areas in the watershed (amounting to ～5% of the impervious area within the New Hampshire and Massachusetts small Municipal Separate Storm Sewer System [MS4] area). Implementation of each management option is estimated to be costly, with a 3-month level of CSO controls likely to cost nearly $400 million and up to $700 million for the limited implementation of green infrastructure BMP controls.