Ballard is one of the older neighborhoods in Seattle that are served by a combined sewer system. This system was designed in the early 20th Century to carry sewage from buildings as well as stormwater runoff from streets, rooftops, and parking lots in a single pipe to a wastewater treatment plant. The system’s purpose was to keep polluted water and sewage away from people in order to protect public health. During dry weather, polluted water flows to a wastewater treatment plant. When it rains, the pipes can become overloaded. A mixture of stormwater (about 90 percent) and raw sewage may overflow into lakes, streams, and the Puget Sound, which can harm people, fish, wildlife, and the environment.