Until 2011, combined sewer overflows (CSO) discharged about 20 times a year at six outfalls along the South Boston beaches.

The South Boston CSO Storage Tunnel a 17-foot diameter, 2.1-mile soft ground tunnel that eliminates combined sewer overflows and stormwater discharges to the beaches in South Boston. During wet weather, CSO and stormwater flows are stored in the tunnel until the storm subsides. The tunnel is then pumped out and the flows sent to Deer Island for treatment and discharge. After decades of sewer overflows and stormwater being released onto these beaches, the completion of this project ensured clean and safe beaches for South Boston residents. They are now some of the cleanest urban beaches in the country.

The South Boston CSO Storage Tunnel is the single largest project in MWRA’s 900 million program to control combined sewer overflows. The $225 million tunnel project received $9.09 million in federal stimulus funding. This project was mandated under the same federal court order that oversaw the Boston Harbor Clean-up program. The goal of the MWRA’s CSO control program is to protect swimming beaches, shell fishing beds and other sensitive waters from combined sewer overflows due to heavy rainfall.