MI HB6164

Establishes a microplastics monitoring program running from July 1, 2025 through July 1, 2028 to establish baseline conditions in Michigan public water supplies.

Requires the Department of Natural Resources to conduct quarterly testing of public water supplies using sampling and analytical methods consistent with standards from the International Joint Commission, EPA, American Society for Testing and Materials, or California state methods.

Program must monitor shoreline Great Lakes sources, major tributaries, inland surface water lakes, groundwater near agricultural operations and landfills, and consider water supply size and environmental justice impacts.

Mandates submission of a comprehensive report to the legislature by January 1, 2029 that includes findings from quarterly tests, microplastic quantities, toxicity limits based on peer-reviewed research, preliminary risk assessment, and recommendations for continued monitoring and public protection.

Defines "microplastics" as solid polymeric material less than 5 millimeters in size, excluding naturally-derived polymers that are not chemically modified.