Completed: 2008

Phase I was the first major stage of NBC’s long-term CSO abatement program and established the foundation for the region’s deep-tunnel system.

The Providence Tunnel

The centerpiece of Phase I is the Providence Tunnel, a 3.1-mile-long, 26-foot-diameter deep-rock tunnel located approximately 300 feet below Providence. The tunnel can store approximately 65 million gallons of combined stormwater and sewage during major rain events.

How It Works

During heavy rainfall, stormwater can overwhelm the region’s combined sewer system, causing untreated sewage and stormwater to discharge into nearby waterways. The Providence Tunnel captures flows from 12 major CSO outfalls in the Field’s Point service area and temporarily stores the water underground.

Once capacity is available at the Field’s Point Wastewater Treatment Facility, the stored water is pumped back to the facility for full secondary treatment.

Phase I Benefits

The Providence Tunnel significantly reduced the amount of untreated pollution entering the Providence and Seekonk Rivers and Narragansett Bay.

Key benefits include:

• Approximately 39%–50% reduction in CSO volume
• Approximately 40% reduction in fecal coliform loading
• Approximately 30% reduction in total suspended solids (TSS)
• Approximately 31% reduction in biochemical oxygen demand (BOD)
• More days when shellfishing areas could remain open—up to 65 additional days per year in some areas
• Fewer beach closures
• Significant reductions in bacteria, solids, and other pollutants entering Narragansett Bay

Phase I marked the beginning of NBC’s three-phase CSO program and demonstrated the effectiveness of deep-tunnel storage in protecting Rhode Island’s waterways.

Completed: 2014–2015

Phase II expanded the CSO system established during Phase I by connecting additional overflow points, upgrading key interceptors, and increasing the system’s ability to treat wet-weather flows.

Unlike Phase I, Phase II did not construct another major deep tunnel. Instead, it expanded the existing system and increased its treatment capacity.

What Phase II Added

Major components included:

• Connections from additional CSO outfalls to the existing Providence Tunnel system
• Improvements to key sewer interceptors
• Expanded wet-weather treatment capabilities
• Upgrades to the Bucklin Point Wet Weather Treatment Facility, with capacity of approximately 116 million gallons per day

How It Works

By connecting more CSO sources to the existing tunnel system and increasing wet-weather treatment capacity, Phase II allowed more storm-driven flows to be captured and treated rather than discharged untreated into the Providence and Seekonk Rivers.

Phase II Benefits

• Further reduced untreated CSO discharges
• Significantly reduced overflows from the North Diversion Structure
• Achieved approximately a 95% reduction in North Diversion Structure overflows following wet-weather facility improvements
• Increased the volume of stormwater and combined sewage that could be captured and treated
• Advanced NBC toward its long-term goal of substantially reducing CSO pollution

Phase II built upon the success of the Providence Tunnel by expanding the network of connected CSOs and increasing the system’s ability to manage heavy rainfall.

Timeline: 2021–2028

Phase III is the final phase of NBC’s CSO Program, completing the infrastructure needed to capture and treat nearly all remaining combined sewer overflows.

The phase represents a major expansion of the deep-tunnel system into communities east of Providence and is designed to bring the overall program toward its long-term goal of reducing CSO overflow volume by up to 98%.

The Phase III Tunnel

The centerpiece of Phase III is a new 2.2-mile-long, 30-foot-diameter deep-rock tunnel that will travel through Pawtucket, Central Falls, and East Providence at a depth of approximately 125 feet below ground.

The tunnel will have a storage capacity of more than 58 million gallons of stormwater.

Additional Infrastructure

Phase III also includes:

• Four drop shafts
• One launch shaft
• One receiving shaft
• Additional interceptors
• Connections to remaining CSO outfalls
• Improvements to treatment and conveyance infrastructure

How It Works

The Phase III system will capture storm-driven combined sewage and stormwater that would otherwise overflow into local waterways. The captured flows will be stored within the tunnel system and directed to treatment facilities when capacity becomes available.

Long-Term Benefits

When fully implemented, Phase III will:

• Complete the region’s long-term CSO abatement system
• Capture and manage nearly all remaining combined sewer overflows
• Reduce CSO overflow volume by up to 98%
• Further reduce bacteria, solids, and other pollutants entering Narragansett Bay
• Improve water quality in the Providence River, Seekonk River, and Narragansett Bay
• Support long-term recreational and shellfishing opportunities
• Help Rhode Island meet its Clean Water Act water-quality obligations

Phase III completes the three-phase CSO program, building on more than two decades of investment in tunnels, treatment facilities, interceptors, and other infrastructure designed to protect Narragansett Bay for generations to come.

Together, the three phases of the Narragansett Bay Commission’s CSO Program create an integrated system for managing stormwater and combined sewage:

Phase I built the Providence Tunnel, establishing large-scale underground storage capacity.

Phase II expanded the system by connecting additional CSOs and increasing wet-weather treatment capacity.

Phase III is completing the system with a new deep-rock tunnel, additional infrastructure, and final connections designed to address the remaining overflows.

The result is a comprehensive CSO control system designed to dramatically reduce untreated discharges and protect the health and water quality of Narragansett Bay and Rhode Island’s rivers, beaches, and coastal communities.

• 4.3 miles of new light-rail service
• Two new branches serving Union Square and Medford/Tufts
• Seven new stations
• Relocated and upgraded Lechmere Station
• New vehicle maintenance facility
• Expanded connections to the existing MBTA Green Line network

Community Impact

Completed in 2022, the GLX has transformed regional mobility by providing residents, students, employees, and visitors with a convenient alternative to driving.

The project:

• Supports more than 50,000 daily trips
• Reduces roadway congestion
• Expands access to reliable public transportation
• Connects Somerville, Medford, Cambridge, and surrounding communities
• Provides greater access to employment, education, healthcare, and other regional destinations
• Advances more sustainable transportation options and helps reduce reliance on automobiles

The Green Line Extension represents a significant investment in the region’s transportation infrastructure, connecting communities to the broader Boston-area transit network while creating safer, more accessible, and more sustainable mobility options for generations to come.

• Highway reconstruction and roadway improvements
• Bridge replacement and rehabilitation
• Construction of a new flyover bridge
• Interchange redesign and improvements
• Drainage and roadway infrastructure upgrades
• Erosion and sediment control measures throughout construction

The project is improving safety, mobility, drainage, and long-term resiliency along Route 146, a critical regional transportation corridor serving more than 170,000 vehicles each day.

The Green Line Extension represents a significant investment in the region’s transportation infrastructure, connecting communities to the broader Boston-area transit network while creating safer, more accessible, and more sustainable mobility options for generations to come.