The Mystic River Watershed, located a few miles north of Boston, is a 76-square mile area that drains into Boston Harbor. Encompassing all or portions of 22 urban and suburban communities, the watershed is highly developed and faces multiple water quality impairments. The Massachusetts Department of Environmental Protection’s (MassDEP’s) water quality assessment indicates that nutrients and pathogens are the primary causes of “use impairment” in the freshwater portion of Mystic River watershed, the focus of this report. EPA is supporting MassDEP in piloting an alternative TMDL designed to address nonattainment of nutrient related water quality standardsover a period of time. The approach, based on rigorous data gathering, scientific analysis, and modeling, provides guidance to communities based on a scientific understanding of conditions.