Occurrence Estimation Methodology and Occurrence Findings Report for the Six-year Review of Existing National Primary Drinking Water Regulations - DRAFT

This report presents a detailed review of the contaminant occurrence data used and the statistical methods developed to estimate regulated contaminant occurrence in public drinking water systems. These regulated contaminant occurrence estimates are generated in support of the Environmental Protection Agency's Six-Year Review of National Primary Drinking Water Regulations (NPDWRs). This "Six-Year Review" assesses the potential revision of regulations for regulated contaminants. The contaminant occurrence data used for the Six-Year Review's statistical estimations are Safe Drinking Water Act (SDWA) compliance monitoring data (documenting occurrence of regulated contaminats in public drinking water systems). Using the data and statistical methods described, estimations of national occurrence and preliminary assessments of exposure are derived, evaluated, described and presented. The contaminant occurrence estimations conducted for the Six-Year Review represent a long-term, detailed, and comprehensive undertaking of data acquisition, quality analysis, editing and formatting. This extensive data management work was conducted concurrently with the development of a two-stage contaminant occurrence estimation approach. The first stage of analysis comprises a simple, straight-forward assessment that provides a broad overview of contaminant occurrence. The second stage of analysis is a more rigorous, statistical approach that provides detailed estimates of occurrence with quantified error of estimation (enabling measures of the certainty of the estimates made). The detailed descriptions of these data and estimation methods are presented in the following sections and appendices of this report.