State drinking water programs managed the original unregulated contaminant monitoring program that applied to public water systems (PWSs) serving more than 500 people.

The Round 1 dataset contains PWS monitoring results for 62 then-unregulated contaminants. These data were collected from 40 states and primacy entities between 1988 and 1992.

The Round 2 dataset contains PWS monitoring data for 48 then-unregulated contaminants. These data were collected from 35 states and primacy entities between 1993 and 1997.