||Contaminant occurrence data collected under the first Unregulated Contaminant Monitoring Regulation (UCMR 1) are nationally representative public water system monitoring results. This UCMR 1 monitoring was conducted for select unregulated contaminants in drinking water under the authority of Safe Drinking Water Act. The UCMR 1 program specified that a statistically representative group of small public water systems (serving between 25 and 10,000 persons) and all large public water systems (serving more than 10,000 persons) were required to monitor and submit drinking water sample results for a list of specified unregulated contaminants from the second Contaminant Candidate List (CCL 2). This report presents the United States Environmental Protection Agency (EPA) analysis of the national occurrence of unregulated contaminants on the CCL 2 that were monitored in public water systems (PWSs) under the UCMR 1. Detailed occurrence analyses are presented for ten contaminants evaluated during EPAs CCL 2 Regulatory Determinations which include nine UCMR 1 contaminants, plus one additional contaminant: DCPA degradates, 1,3-dichloropropene,2 DDE, 2,4-dinitrotoluene, 2,6-dinitrotoluene, EPTC, fonofos, MTBE, perchlorate, and terbacil. Less detailed occurrence summaries are presented for all the other UCMR 1 contaminants as well. This report also describes the sources, quality, management, and characteristics of the UCMR 1 data.