The National Park Service has approved DC Water’s proposal to move forward with construction of the Potomac River Tunnel project following an environmental assessment, the agency announced Tuesday.

The Potomac River Tunnel project — part of DCW’s Clean Rivers Project — is estimated to reduce combined sewer overflow volume into the Potomac River by 93%, according to a news release.

Combined sewers are designed to collect rainwater runoff, domestic sewage and industrial wastewater in the same pipe and transport it to a treatment plant. During severe weather, like heavy rainfall or snowmelt, those systems can get overwhelmed and the overflow ends up in nearby streams or rivers.