Combined sewers

From Great Lakes Wiki

Combined sewer systems carry multiple kinds of waste, including sewage, industrial waste and storm water to the waste treatment plant. During large rain storms the sewers reach capacity and the combined waste flows into nearby bodies of water contaminanting them with industrial by-products and bacteria. Many local government have spent mucho dinero to separate their sewer systems (which are typically found in urban areas) to prevent these overflows.

Total Daily Maximum Load or TDML is a term used to described how much pollution a body of water can handle as determined by the Department of Water Quality.

Bodies of water get a TDML when they do not meet federal water quality standards.

The state is required to list bodies of water that do not meet the Clean Water Act standards and will not do so with technologic interventions or bodies of water that are at risk of not meeting the standards. See the Michigan list here.