National Water-Quality Assessment
National Analysis of Trace Elements - Publications

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

Citation:
Focazio, M.J., Welch, A.H., Watkins, S.A., Helsel, D.R., and Horn, M.A., 1999, A retrospective analysis on the occurrence of arsenic in ground-water resources of the United States and limitations in drinking-water-supply characterizations: U.S. Geological Survey Water-Resources Investigation Report 99-4279, 21 p.

View the full report in PDF (2,243 Kb)
Download arsenic data (1,130 Kb, Tab-delimited text)

The full report is available in printed form for $4.00, plus a $5.00 handling charge, from:
  USGS Information Services,
  P.O. Box 25286,
  Denver, CO, 80225
or by fax request to: 303-202-4693, or call 1-888-ASK-USGS.
Orders should specify USGS report WRIR 99-4279.

Abstract

The Safe Drinking Water Act, as amended in 1996, requires the U.S. Environmental Protection Agency (USEPA) to review current drinking-water standards for arsenic, propose a maximum contaminant level for arsenic by January 1, 2000, and issue a final regulation by January, 2001. Quantification of the national occurrence of targeted ranges in arsenic concentration in ground water used for public drinking-water supplies is an important component of USEPA's regulatory process. Data from the U.S. Geological Survey (USGS) National Water Information System (NWIS) were used in a retrospective analysis of arsenic in the ground-water resources of the United States. The analysis augments other existing sources of data on the occurrence of arsenic collected in ground water at public water-supply systems.

The USGS, through its District offices and national programs, has been compiling data for many years on arsenic concentrations collected from wells used for public water supply, research, agriculture, industry, and domestic water supply throughout the United States. These data have been collected for a variety of purposes ranging from simple descriptions of the occurrence of arsenic in local or regional ground-water resources to detailed studies on arsenic geochemistry associated with contamination sites. A total of 18,864 sample locations were selected from the USGS NWIS data base regardless of well type, of which 2,262 were taken from public water-supply sources. Samples with non-potable water (dissolved-solids concentration greater than 2,000 milligrams per liter and water temperature greater than 50^o Celsius) were not selected for the retrospective analysis and other criteria for selection included the amount and type of ancillary data available for each sample. The 1,528 counties with sufficient data included 76 percent of all large public water-supply systems (serving more than 10,000 people) and 61 percent of all small public water-supply systems (serving more than 1,000 and less than 10,000 people) in the United States. The arsenic data were summarized for the selected counties by associating the arsenic concentrations measured in the ground-water resource with the numbers and sizes of public water-supply systems using ground water in those counties. Targeted arsenic concentrations of 1, 2, 5, 10, 20, and 50 ug/L were exceeded in the ground-water resource associated with 36, 25, 14, 8, 3, and 1 percent respectively of all public water-supply systems accounted for in the analysis.

Contributions to uncertainty such as changes in sampling methods and changes in laboratory reporting appear to be less important to the national occurrence estimates than other factors such as temporal variability in arsenic concentrations at a given well, the types of wells sampled, and density and types of sampling locations. In addition, no attempt was made to quantify arsenic concentrations in relation to depth within aquifers. With these qualifications, the USGS data represent the ground-water resource in general and are not restricted to wells currently used for public drinking-water sources. In this way, the broad spatial extent, large number of water samples, and low detection limits used for the USGS data provide a unique source of information to determine where targeted concentrations of arsenic are likely to occur in the ground-water resources within much of the United States.

These results indicate USGS data can be effectively used to augment national estimates of arsenic occurrence in the nation's ground-water resources if limitations are recognized. Existing estimates of the occurrence of arsenic in ground water that are used as a source of drinking water can be supplemented with the USGS arsenic concentration data when associated with the public water-supply data base. One such supplementary application is the additional insight gained by establishing relations between arsenic concentration data in the ground-water resource and small public water-supply systems that serve less than 1,000 people on a national scale.

Contents

• Abstract 8
• Introduction 12
• Purpose and Scope 14
• Acknowledgments 14
• Description of Data Bases 14
• The public water-supply data base from the U.S. Environmental Protection Agency Safe Drinking Water Information System 16
• Arsenic point data base from the U.S. Geological Survey National Water Information System 17
• Arsenic data in selected counties of the United States 18
• Relation of arsenic concentration data with public water-supply system information 20
• Median arsenic concentrations in all counties of the United States 20
• National Arsenic Occurrence Estimates 21
• Occurrence of arsenic in ground-water resources of selected counties in the United States 22
• Median arsenic concentrations in ground-water resources of all counties in the United States 22
• Limitations of the U.S. Geological Survey Arsenic Data and Relation to Public Water-Supply Information Used for Drinking-Water Characterizations 24
• Arsenic concentration data 25
• Source of sampled water 25
• Sample-collection protocols 30
• Laboratory reporting 31
• Temporal variability 31
• Density and location of sampling sites 34
• Association of the arsenic point data base with public water system information 35
• The public water-supply data base 36
• Description of the National Arsenic Occurrence Survey 37
• National comparative 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 37
• Regional comparative 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 38
• Comparative analysis of targeted arsenic concentrations in raw and finished water 38
• Conclusions 39
• References Cited 42

List of Figures

1. Flow diagram of the data bases.
2. Locations and concentration ranges of samples in the arsenic point data base.
3. National distribution of counties based on the number of arsenic concentrations (actual and estimated) selected for the arsenic occurrence analysis.
4. Comparison of arsenic concentrations in ground water from public water-supply wells and all other types of wells in the arsenic point data base.
5. Major physiographic provinces of the conterminous United States and arsenic data collected from wells used for various purposes.
6. Relation between (a) arsenic concentrations in filtered and unfiltered samples and (b) the number of samples with selected ratios of filtered to unfiltered arsenic concentrations.
7. Coefficient of determination for the relation between arsenic concentration and time.
8. Coefficient of variation (a) and standard deviation (b) for each of 355 wells with 10 or more arsenic samples collected over time and the relation to mean arsenic concentrations.
9. Standard deviation for each of 355 wells with10 or more arsenic samples collected over time and the relation to well depth.
10. Percentage of public water systems exceeding targeted arsenic concentrations calculated with USGS (raw water) data and NAOS (finished water projections) for (a) large and (b) small public water systems. Percentage values calculated with USGS raw-water data and NAOS finished-water projections.
11. Percentage of public water systems that exceed arsenic concentrations in raw water of 5 µg/L by region.
12. Percentage of public water systems that exceed targeted arsenic concentrations in raw water from the Western (a) and Mid-Atlantic (b) regions.
13. Percentage of public water systems that exceed targeted arsenic concentrations in the Western region (National Arsenic Occurrence Survey finished water projections).

List of Tables

1. Data elements retrieved or derived from U.S. Environmental Protection Agency Safe Drinking Water Information System.
2. The estimated number of public water-supply systems in selected counties that exceed targeted arsenic concentrations in the associated ground-water resources for various public water-supply system sizes categorized by population served.
3. Percentage of all public water-supply systems in the selected counties estimated to exceed targeted arsenic concentrations in the associated ground-water resource.
4. Statistical summary of median arsenic concentrations estimated for all counties.
5. Summary of factors considered in making median arsenic concentration estimates for counties with no arsenic data.
6. Statistical results of differences in arsenic concentrations in water collected from public water-supply wells and other types of wells, by physiographic provinces of the United States.
7. Regions and associated states used in the National Arsenic Occurrence Survey
8. Safe Drinking Water Information System data for populations served by ground water and surface water, by state; 1998.

The full report is available in printed form from:
  USGS Information Services,
  P.O. Box 25286,
  Denver, CO, 80225
or by fax request to: 303-202-4693, or call 1-888-ASK-USGS.
Orders should specify USGS report WRIR 99-4279.

Citation:
Focazio, M.J., Welch, A.H., Watkins, S.A., Helsel, D.R., and Horn, M.A., 1999, A retrospective analysis on the occurrence of arsenic in ground-water resources of the United States and limitations in drinking-water-supply characterizations: U.S. Geological Survey Water-Resources Investigation Report 99-4279, 21 p.

National Analysis of Trace Elements - Publications
National Water-Quality Assessment