South Boston CSO Storage Tunnel

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The South Boston CSO Storage Tunnel is an underground facility designed to reduce untreated sewage that would otherwise flow into Boston Harbor from the city’s combined sewer and stormwater system. Also called the North Dorchester Bay CSO Storage Tunnel, it opened on July 23, 2011 as part of the Boston Harbor Cleanup project. CSO stands for Combined Sewer Overflow.

The main tunnel is 17 feet (5.2 m) in diameter and runs about 2.5 miles (4.0 km) along the harbor front. It starts at an Odor Control Building, runs along the harbor, and ends at a pumping station. The tunnel provides a buffer during heavy storms, storing mixed sewage and rainwater so fewer untreated discharges reach the harbor and nearby beaches (reducing events from about 20 times per year). After storms, the stored water is released back into the wastewater network for treatment at the Deer Island plant.

In addition to the tunnel, the MWRA is carrying out sewer separation and other drain improvements in parts of South Boston to further cut down overflows.