A Retrospective Analysis on the Occurrence of Arsenic in Ground-Water Resources of the United States and Limitations in Drinking-Water-Supply Characterizations

7 Introduction 7 Purpose and Scope 9 Acknowledgments 9 Description of Data Bases 9 The public water-supply data base from the U.S. Environmental Protection Agency Safe Drinking Water Information System 9 Arsenic point data base from the U.S. Geological Survey National Water Information System 11 Arsenic data in selected counties of the United States 11 Relation of arsenic concentration data with public water-supply system information 12 Median arsenic concentrations in all counties of the United States 12 National Arsenic Occurrence Estimates 13 Occurrence of arsenic in ground-water resources of selected counties in the United States 14 Median arsenic concentrations in ground-water resources of all counties in the United States 14 Limitations of the U.S. Geological Survey Arsenic Point Data Base and Associations with Public Water-Supply Information Used for Drinking-Water Characterizations 14 Arsenic concentration data 15 Source of sampled water 15 Sample-collection protocols 18 Laboratory reporting 20 Temporal variability 20 Density and location of sampling sites 21 Association of the arsenic point data base with public water system information 22 The public water-supply data base 22 Description of the National Arsenic Occurrence Survey 23 National comparitive analysis of percentages of public water-supply systems with water that exceed targeted arsenic concentrations estimated from the National Arsenic Occurrence Survey and the U.S. Geological Survey Retrospective Analysis 23 Regional comparitive analysis of percentages of public water-supply systems with water that exceed targeted arsenic concentrations estimated from the National Arsenic Occurrence Survey and the U.S. Geological Survey Retrospective Analysis 24 Comparitive analysis of arsenic concentrations in raw and finished water 24 Conclusions 25 References Cited 26