Federal authorities have given approval to DC Water and the District of Columbia to modify their combined-sewer-overflow plan and incorporate large-scale green infrastructure.

Under a deal with the U.S. Environmental Protection Agency and the U.S. Dept. of Justice announced on May 20, the agency can eliminate a planned underground tunnel along Rock Creek. The agreement calls for green infrastructure—such as rain gardens, green roofs and porous-pavement installations— as well as targeted sewer separation. The system's design allocates 365 impervious acres of land along Rock Creek for management of up to 1.2 in. of rainfall. A planned tunnel along the Potomac River also is scheduled for modifications. The revised design will hold up to 30 million gallons of combined stormwater and sewage that will flow to DC Water's advanced wastewater treatment plant at Blue Plains. In addition, 133 impervious acres of land will be used to manage up to 1.2 in. of rainfall.

The new agreement will allow DC Water to push final completion of its entire program to 2030 from 2025. George Hawkins, general manager of DC Water, notes that, although the green infrastructure projects will be completed later, DC Water expects to see the benefits of them earlier than it would with the tunnel solutions. Construction of the first projects are expected to begin in 2017. "We won't build all at once," Hawkins says. "We'll build and evaluate, build and evaluate—always improving throughout. Our goal is to get a performance level that is equivalent to the tunnels or better." Hawkins says he doesn't expect the total cost of the agency's $2.6-billion Clean Rivers program, which includes the Rock Creek and Potomac work, to change significantly. DC Water already has invested $14 million investigating and evaluating green infrastructure, Hawkins says. Although there is no existing certification program for such green infrastructure, Hawkins says DC Water is "committed to creating the nation's first such program."