The rehabilitation work also included draining the 1.2-mile-long, 12ft diameter tunnel and inspecting and repairing its brick-lined interior. The activation of three submersible turbine pumps is bringing up to 252 million gallons of oxygen-rich water to the head of the Canal each day, or roughly 30 per cent more than was possible before the upgrade. The infusion of fresher water provided through the Flushing Tunnel has already increased the dissolved oxygen content of the water in the Canal which will dramatically improve its aesthetics and provide a more suitable habitat for plant and aquatic life.

In addition to the Flushing Tunnel, DEP is completing a rehabilitation and upgrade of a wastewater pumping station at the head of the Canal that will reduce sewer overflows during heavy rain storms. In June, DEP reactivated the pump station which will allow it to send up to 30 million gallons of wastewater to the Red Hook Wastewater Treatment plant each day, a 50 per cent upgrade over its previous capacity. DEP will continue site and building improvements over the summer, including the removal of the temporary oxygenation system, with work on the $177 million projects expected to be completed this fall.

"The USD 177M upgrade of the Flushing Tunnel is a significant milestone in the City’s efforts to improve the health and cleanliness of the Gowanus Canal," said DEP commissioner Emily Lloyd. "Over the summer we will also complete an upgrade to an important wastewater pump station and begin the installation of hundreds of curbside gardens in the surrounding neighborhoods, all of which will help to improve water quality in the Canal."

After Hurricane Sandy, construction plans for the Flushing Tunnel, wastewater pump station and the support buildings were altered to include resiliency measures such as raising the control room floor and its critical electrical equipment, flood-proofing the service building and installing a dike wall and mechanical flood gate. As part of the upgrade, the Flushing Tunnel will now operate around the clock, including at low-tide, when the Canal water is at its most stagnant. The system of three pumps also provides redundancy that will ensure that the tunnel remains operational during future maintenance and repairs.

The Gowanus Canal is a nearly two mile long man made waterway that was built in the 1860s to facilitate commerce in western Brooklyn. The Canal runs from New York Harbor northeast through the Red Hook, Carroll Gardens, and Park Slope neighborhoods where it dead ends at Butler Street. Over time, commerce in the area grew but the Canal and its environs were left with a legacy of industrial contamination. In addition, with limited natural movement of water, the Canal became a stagnant and polluted waterway.