Community Based GSI in a CSO Abatement Program

The Narragansett Bay Commission (NBC) is a non-profit public corporation providing wastewater collection and treatment services to over 350,000 Rhode Islanders in Providence and surrounding communities. Created in 1980 under statute of the Rhode Island General Assembly, it remains a leader in protecting and enhancing water quality in Narragansett Bay and its tributaries. NBC is implementing Phase III of the Combined Sewer Overflow (CSO) Program, the final stage in their three-phase CSO abatement program that began in 1998. The Program aims to lower annual CSO volumes and reduce annual shellfish bed closures in accordance with a Consent Agreement with the Rhode Island Department of Environmental Management (RIDEM). Phases I and II, completed in 2008 and 2015 respectively, have diverted over 15 billion gallons of untreated stormwater and sewage from the bay. Phase III of the Program targets CSO abatement in sewersheds that have yet to be addressed. Like previous phases, a combination of strategies is planned including a large CSO storage tunnel and pump station, sewer separation, regulator modifications, and green stormwater infrastructure (GSI). At an estimated $750 million, Phase III of the CSO Program is the largest public infrastructure project in Rhode Island's history. Upon the completion of Phases I and II of the CSO Program, NBC underwent a comprehensive re-evaluation of the 1998 plan for Phase III. The re-evaluation process included an extensive stakeholder engagement process that included regulatory agencies, municipalities, non-profit organizations, and community groups. This group of stakeholders overwhelmingly asked that NBC incorporate GSI into their plans for CSO abatement. Although GSI was not found to be suitable as a stand-alone solution for CSO control, it was found to be a beneficial and integral part of Phase III with the potential to cost-effectively reduce volumes and the scale and scope of necessary grey infrastructure. A desktop level evaluation was performed to identify opportunities for GSI. A review of sewersheds served by combined sewer, supplemented with site reconnaissance, was performed to identify many sites that met specific criteria for successful GSI projects. Property size and use, subsurface condition (e.g., soil type, depth to groundwater), and regulatory (e.g., proximity to resource areas) were among the criteria evaluated. Only State or municipal properties and public rights-of-way were considered to maximize benefits to the community at large. NBC has partnered with the City of Central Falls, Rhode Island to advance several GSI projects in their community. Central Falls is a densely urban, approximately 1-square mile municipality with large populations of underserved residents. It is also an Environmental Justice (EJ) Focus Area. Of particular interest are locations where a large volume of runoff can be removed from the collection system and where other benefits to the community are maximized. While NBC is performing design and construction, the City of Central Falls has agreed to maintain the constructed facilities. One GSI project converted an abandoned, blighted former commercial property into the new Louis C. Yip Soccer Field. This project, envisioned as a 'GSI Demonstration Project', incorporated several different GSI strategies around the outside of the soccer field to demonstrate to the public the benefits of stormwater management strategies that embrace low impact development. It also established design criteria and standards for future projects to follow and capped a contaminated site, making it safe for recreational use. A second GSI project redeveloped the Macomber Stadium athletic complex into a new state-of-the-art synthetic turf athletic field with a subsurface infiltration system and sand filter with enough capacity to treat and infiltrate a 25-year design storm. The system captures runoff from the site as well as over 6 acres of surrounding impervious surfaces in the drainage area, including the roof of a large industrial building that had previously been tied directly into the CSO interceptor. The site is used for regulation interscholastic football, soccer, and baseball. Additionally, the project remediated the site which had been closed to play since 2017 following the discovery of contaminated soils within the playing fields. Recently completed projects include another nearby recreational facility, Pierce Park, where underground infiltration systems were incorporated beneath a new pervious pavement basketball court and a closed ballfield was restored. This project was completed in 2023. NBC's GSI projects are being designed to capture stormwater and divert it from the combined sewer collection system. These GSI systems will reduce the volume and frequency of overflows to the Blackstone and Seekonk Rivers, major tributaries to Narragansett Bay. Stormwater that is diverted from the catchment area is collected and infiltrated or attenuated through storage. In keeping with the requirements of the Consent Agreement, impervious surface runoff from the 3-month 6-hour precipitation event is the minimum basis of design. Many of the GSI systems capture stormwater from much larger storm events. This presentation will review NBC's use of GSI as a tool for CSO abatement and the many community benefits these projects provide. The Phase III CSO Program is an introduction to GSI for the Narragansett Bay Commission, and many valuable lessons have been learned in the 8 years of planning, design, and construction completed to date. These projects have been supported by State Revolving Fund loans administered through the Rhode Island Infrastructure Bank and Recreation Grants from the State of Rhode Island. Several more GSI projects are planned in Central Falls and Pawtucket, RI in upcoming years. NBC plans to invest over $40 million on GSI under the Phase III CSO Program.