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For all public water systems tested under the Unregulated Contaminant Monitoring Rule that had one or more detection of PFOS, PFOA or another contaminant, an average was calculated using the all samples collected. Samples with no detections were treated as zero when calculating the average, which in turn may skew the value lower when the contaminant is more common.
All locations represented on the map are approximate, and meant to visualize the general area of a contamination site or public water detection. Locations were mapped using the best available data preserved in official records.
Data on contaminated industrial and military sites was current as of July 2018.