This visualization allows users to view contaminant occurrences for each large public water system (those serving more than 10,000 people) in the Great Lake region, organized by county. Click on any county to view available occurrence data. US data comes from reporting under the Unregulated Contaminant Monitoring Rule (UCMR) which requires public water systems serving more than 10,000 people to monitor for a list of specific contaminants. Note that the UCMR results are from finished water, reflecting contaminants from source water that remained after treatment.