SEATTLE — A new 29 million gallon storage tunnel is being built in Ballard to help reduce sewage overflows that can hit our waterways when it rains, Seattle and King County officials said.

At issue is the combined sewer system, where stormwater and sewage share the same pipes in parts of Seattle. That can be overwhelmed during rain storms, sending excess water – and raw sewage – outflows into waterways.

Now, crews have excavated a 122-foot deep pit in Ballard to begin constructing a 29 million gallon storage tunnel, which is expected to come online in 2025. It will be positioned to capture and store excess runoff during storms, until it can be pumped out to the West Point treatment plant and processed.

The tunnel, officials say, is more efficient to store rainwater than tanks, and reduces the need to acquire land.

“With this project, we can solve both our problems, while having the least impact on the neighbors,” said Bruce Kessler, Deputy Director of King County’s wastewater treatment division.

Seattle Public Utilities (SPU) says in 2018, 84% of the city's overflows came from the combined sewer systems in Crown Hill, Ballard, Fremont, Wallingford, Queen Anne, Downtown and Capitol Hill.