Mystic River Watershed Alternative TMDL Development for Phosphorus Management - Final Report January 2020 Prepared for: Prepared by: U.S. Environmental Protection Agency Eastern Research Group, Inc. Region 1 - Office of Ecosystem Protection 100 Hartwell Ave. 5 Post Office Square Lexington, MA 02421 Boston, MA 02109 Subcontractors: Reviewer: Horsley Witten Group Dr. Jeff Walker Paradigm Environmental Walker Environmental Research, LLC Nigel Pickering, PhD PG Environmental EPA Contract No. EP-C-16-003 Cover Photo: The Tufts University sailing program practicing on Upper Mystic Lake in Medford with informal swimming at the Bacow Sailing Pavilion. Upper Mystic Lake is a popular destination with additional access at Massachusetts Department of Conservation and Recreation’s Shannon Beach, and the Medford and Winchester Boat Clubs. Photo credit unknown. Mystic River Watershed TMDL Alternative Development – Final Report EXECUTIVE SUMMARY 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. Cultural eutrophication–the degradation of aquatic environments by nutrient pollution caused by human activity and urban development–is a major cause of impairments in the watershed as evidenced by excessive algal and macrophyte growth and harmful cyanobacteria blooms. Regular occurrences of severe algal blooms during the summer months reduce water clarity and contribute to anoxic bottom waters that do not support aquatic life. Algal blooms and macrophyte growth degrade the aesthetic quality of the river, reduce water clarity, and impair designated uses such as fishing and boating. Photo Above: The Mystic River Run and Paddle (May 2016). This annual race celebrates the return of the river herring and draws the public to the Mystic River. This image looks downstream from Route 16 to the Blessing of the Bay Boathouse in Somerville with the Boston skyline in the background. Photo credit: Ram Subramanian. Clean Water Act Requirements Section 303(d) of the Clean Water Act and the U.S. Environmental Protection Agency’s (EPA’s) Water Quality Planning and Management Regulations (Title 40 of the Code of Federal Regulations [CFR] Part 130) require states to develop Total Maximum Daily Loads (TMDLs) for impaired water bodies. A TMDL establishes the amount of a pollutant that a water body can assimilate without exceeding the applicable water quality standard. A TMDL consists of the sum of individual waste 2 Mystic River Watershed TMDL Alternative Development – Final Report load allocations for point sources and load allocations for nonpoint sources and natural background conditions. The Massachusetts Water Quality Standards (WQS), codified at 314 CMR 4.00, identify the Mystic River as a Category 5 water body on the Massachusetts 303(d) “List of Impaired Waters” (2014) for phosphorus, arsenic, chlordane, chlorophyll, DDT (dichlorodiphenyltrichloroethane), dissolved oxygen, E. coli, PCBs (Polychlorinated biphenyls) [in fish tissue], Secchi depth, and sediment bio-chronic toxicity. This report addresses those impairments associated with excessive nutrient loading including phosphorus, chlorophyll, dissolved oxygen, and secchi depth (water clarity). Alternative TMDL Process In 2013, the EPA announced a new framework (Vision) for prioritizing and implementing TMDLs and pollution control strategies. The guidance for this Vision (found here: https://www.epa.gov/sites/production/files/2015-07/documents/vision_303d_program_dec_2013.pdf) allows states to adopt strategies tailored to their water quality program goals and priorities. The Vision acknowledges that alternative restoration approaches may be more immediately beneficial or practical in achieving water quality standards than a traditional TMDL. The Vision calls on states to strategically focus efforts and demonstrate progress over time. EPA is supporting MassDEP in piloting an alternative TMDL designed to address nonattainment of nutrient related water quality standards over 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. The agencies have already begun working with communities to develop stormwater management (SWM) strategies to begin progress on implementing effective stormwater control measures (SCMs) to restore the river and degraded lakes and ponds. This "adaptive management" approach for the Mystic will be an iterative process of implementing control actions over an extended period of time while progress is monitored, and new information is gathered to further inform management needs for attaining water quality standards. Study and Analysis The objectives of this technical analysis, conducted between 2017 and 2019, were to: estimate annual loadings of phosphorus; relate phosphorus loads to response variables in critical surface water reaches of the watershed; estimate the load reductions needed to improve water quality and attain water quality standards; and introduce a pilot Opti-Tool analysis that demonstrates cost-effective and opportunistic stormwater load reduction strategies that communities can consider adopting. Much of the scientific research needed to document existing conditions occurred prior to this project, during which time EPA Region 1 collaborated with the Mystic River Watershed Association (MyRWA), MassDEP, U.S. Geological Survey (USGS) and the Massachusetts Water Resources Authority (MWRA). Baseline water quality monitoring included the collection of composite samples linked to streamflow. This project builds on past analyses to develop target reductions in phosphorus inputs in order to improve water quality in the Mystic River. To assist with developing phosphorus budgets and recommending load reductions, EPA convened a Technical Steering Committee (TSC) to provide data, expertise and advice. The Technical Steering Committee met three times annually. In addition to the Technical Steering Committee, the consultant team also benefited from expert review provided by Dr. Jeff Walker (Walker Environmental Research LLC). Further refinements to analyses were conducted based on Dr. 3 Mystic River Watershed TMDL Alternative Development – Final Report Walker’s input following consultation with the Technical Steering Committee. This interactive approach resulting in the project team providing the most comprehensive and complete assessment possible at this time and one that the Technical Steering Committee determined is suitable to support an adaptive management process for the Mystic River watershed. Technical Steering Committee members and our expert reviewer are listed in the Acknowledgements. The components of the analytical approach, discussed in detail in the report, are listed below: • Develop conceptual model of hydrology and nutrient dynamics • Evaluate existing water quality monitoring data • Review modeling endpoint approaches • Estimate watershed phosphorus loading • Evaluate combined sewer overflow (CSO) and sanitary sewer overflow (SSO) data • Conduct BATHTUB modeling and calibrate results • Determine critical period of interest for phosphorus load reduction analysis • Evaluate watershed phosphorus load reduction analysis • Develop nutrient stormwater management strategies using Opti-Tool The pollutant of concern for this study is phosphorus because it is directly causing or contributing to the excessive algal biomass. Since there are no numeric criteria available for phosphorus (i.e., no specific concentration of phosphorous that represents a violation of standards), a surrogate water quality target was needed to calculate pollutant load reductions to the river. Chlorophyll-a was chosen as the surrogate water quality target. Chlorophyll-a is the photosynthetic pigment found in algae and is, therefore, a direct indicator of algal biomass. EPA and the Technical Steering Committee determined that a seasonal average chlorophyll-a concentration of <10 µg/L would be protective of narrative eutrophication standards in the watershed, from which associated total phosphorus reductions could be derived. Photo Left: Cyanobacteria bloom in the freshwater segment of the Mystic River between Arlington and Medford in June of 2017. Photo credit: Jack Bitney. 4 Mystic River Watershed TMDL Alternative Development – Final Report Results Watershed analyses conducted during this study demonstrate that inadequately controlled stormwater (SW) runoff from developed landscapes are the predominant source of nutrient loads– specifically phosphorus loads–to the surface waters of the Mystic River watershed. Under existing conditions, this study estimated that to meet the selected chlorophyll-a water quality target for attaining water quality standards in the most impacted segment, the lower Mystic River, will require a 67 percent reduction of stormwater phosphorus loadings from the watershed However, this estimate assumes all reduction would be achieved through stormwater control measures. Load reduction estimates were also modeled for future conditions to account for key variables: wet vs. dry years; future