Prioritizing Green Infrastructure for Phosphorus Reduction within Boston’s MS4
- Matthew Jones, Chuck Wilson - Hazen and Sawyer
The Boston Water and Sewer Commission (BWSC) is underway with efforts to reduce phosphorus loading to the Charles River. Among the reduction strategies being considered is the implementation of green infrastructure (GI) to better manage stormwater runoff throughout separately sewered areas of the City. BWSC is piloting this approach through three study areas, each about 1,000 acres in size and served by the City’s MS4. Within each of these areas, BWSC’s consultants are analyzing opportunities for green infrastructure and developing conceptual designs.
Hazen and Sawyer is conducting an evaluation of the Lower Stony Brook area, which is located southwest of the City center. The Lower Stony Brook area includes the neighborhoods of Roxbury and Jamaica Plain and is primarily residential, with some intermixed commercial properties. The approach to GI implementation included a review of existing information, area assessment of drainage areas and GI opportunities, site suitability investigations, conceptual design development, and reporting. As an area-wide approach to green infrastructure implementation within Boston’s MS4 area, establishing protocols for analysis and design efforts were an important part of the overall initiative. Maximizing the magnitude and cost-effectiveness of phosphorus removal was a key objective of the analysis and informed protocol development.
An adaptive and flexible area-wide assessment approach was adopted for Lower Stony Brook in order to ensure all potential opportunities were identified, support the compilation of statistics regarding GI feasibility and limiting constraints, and extrapolate total potential load reductions that may be possible if conceptual designs were replicated throughout the entire area. Desktop analyses and field investigations informed GI feasibility for each delineated drainage area, considering factors such as the presence of trees, narrow sidewalks, utilities, parking areas, transportation structures, building entrances, street furniture, and steep topography. Analyses revealed that nearly 50% of right-of-way (ROW) drainage areas could be managed through a combination of GI within the ROW or adjacent parcels.
Following the identification of feasible GI opportunities, Hazen prioritized a subset of sites for site suitability investigations. Site suitability was assessed through soil investigations and stormwater sampling. The soil investigations were conducted to characterize underlying soils and test infiltration rates in order to inform GI sizing and design. Stormwater samples were collected at prospective GI locations in order to characterize existing nutrient concentrations. These concentrations were assessed to prioritize GI implementation in areas with the highest nutrient concentrations, as well as develop a refined understanding of potential phosphorus load reductions.
Conceptual designs were developed for a set of GI opportunities that provided the best phosphorus load reductions, as well as those that could be broadly replicated throughout the study area. In total, this green infrastructure analysis and the resultant conceptual designs have provided a characterization of the feasibility and cost-effectiveness of using green infrastructure to reduce phosphorus loading to the Charles River. The results of these efforts have established a framework for BWSC to advance phosphorus load reduction efforts, as well as tools for other MS4s to consider how green infrastructure can improve water quality.
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