U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

U.S. Environmental Protection Agency

Office of Ground Water and Drinking Water

A Retrospective Analysis on the Occurrence of Arsenic

in Ground-Water Resources of the United States and

Limitations in Drinking-Water-Supply Characterizations

Water-Resources Investigations Report 99–4279

Arsenic in water from

18,850 wells and springs

Greater than 10 μg/L

5 to 9.9 μg/L

3 to 4.9 μg/L

Less than 2.9 μg/L

EXPLANATION

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U.S. Department of the Interior 2

U.S. Geological Survey

Prepared in cooperation with the

U.S. Environmental Protection Agency

Office of Ground Water and Drinking Water

A Retrospective Analysis on the Occurrence of Arsenic

in Ground-Water Resources of the United States and

Limitations in Drinking-Water-Supply Characterizations

Water-Resources Investigations Report 99–4279

By Michael J. Focazio, Alan H. Welch, Sharon A. Watkins, Dennis R. Helsel, and Marilee A. Horn
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Reston, Virginia 2000

For additional information write to:

Office of Water Quality

U.S. Geological Survey

MS 412

12201 Sunrise Valley Dr.

Reston VA 20192

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Denver CO 80225-0286
Contents

Abstract.......................................................................................................................................................................................... 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
Figures

1. Flow diagram of the development of arsenic concentration and public water-supply system

data bases ............................................................................................................................................................................ 9

2. Locations and concentration ranges of samples in the arsenic point data base ................................................................ 13

3. National distribution of counties based on the number of arsenic concentrations

(actual and estimated) selected for the arsenic occurrence analysis................................................................................. 14

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 ...................................................................................................... 18

5. Major physiographic provinces of the conterminous United States ................................................................................. 19

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 ...................................... 20

7. Coefficients of determination of arsenic concentrations regressed with time as a function of mean arsenic

concentration for each of 355 wells with 10 or more arsenic samples collected over time ............................................. 21

8. Coefficient of variation (A) and standard deviation (B) for each of 355 wells with 10 or more arsenic samples

collected over time as a function of mean arsenic concentration ..................................................................................... 21

9. Standard deviation for each of 355 wells with10 or more arsenic samples collected over

time and the relation to well depth ................................................................................................................................... 22

10. Percentage of public water-supply systems that exceeded 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. .............................................................................................................................. 23

11. Percentage of public water-supply systems that exceed arsenic concentrations of 5 μg/L in raw-water

arsenic by region ............................................................................................................................................................... 23

12. Percentage of public water-supply systems that exceed targeted arsenic concentrations in

raw water from the Western (A) and Mid-Atlantic (B) regions......................................................................................... 24

13. Percentage of public water-supply systems that exceed targeted arsenic concentrations in the

western region (National Arsenic Occurrence Survey finished water projections) ........................................................... 24

Tables

1. Data elements retrieved or derived from U.S. Environmental Protection Agency Safe

Drinking Water Information System ................................................................................................................................. 10

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............................................................................................ 15

3. Percentage of all public water-supply systems in the selected counties estimated to exceed

targeted arsenic concentrations in the associated ground-water resource ......................................................................... 15

4. Statistical summary of median arsenic concentrations estimated for all counties ............................................................ 16

5. Summary of factors considered in making median arsenic concentration estimates for

counties with no arsenic data. ........................................................................................................................................... 17

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 ....................................... 19

7. Regions and associated states used in the National Arsenic Occurrence Survey .............................................................. 19

8. Safe Drinking Water Information System data for populations served by ground water and

surface water, by state, 1998 .............................................................................................................................................. 23
 Conversion Factors, Water-Quality Units,

and Abbreviations

Multiply By To Obtain

Length

millimeter (mm) 0.03937 inch (in.)

centimeter (cm) 0.3937

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Volume

liter (L) 0.2642 gallon (U.S.)

milliliter (mL) 0.001057 quart

Physical and Chemical Water-Quality Units

Temperature: Water and air temperature are given in degrees Celsius (°C), which

can be converted to degrees Fahrenheit (°F) by use of the following equation:

°F = 1.8(°C) + 32

Milligrams per liter (mg/L) or micrograms per liter (μg/L): Milligrams per liter is

a unit expressing the concentration of chemical constituents in solution as weight

(milligrams of solute per unit volume (liter) of water). One thousand micrograms per

liter is equivalent to one milligram per liter. For concentrations less than 7,000 mg/L,

the numerical value is the same as for concentrations in parts per million.
1

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 watersupply 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

18,850 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° 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 arsenic data were summarized 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 the same counties. 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. Targeted arsenic concentrations of 1, 2, 5, 10,

20, and 50 μg/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 drinkingwater 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 that 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.

INTRODUCTION

The U.S. Geological Survey’s Drinking Water Initiative provides information useful to regulatory agencies and

others who must balance water-supply protection with the

wise use of public funds (Patterson, 1997). The evaluation

of the distribution and vulnerability of public water-supply

systems to selected contaminants in drinking water, such as

arsenic, radionuclides, disinfectant by-products, microbes,

and volatile organic compounds, has been the major thrust

of the Drinking Water Initiative. The Safe Drinking Water

Act, as amended in 1996, requires the U.S. Environmental

Protection Agency (USEPA) to (1) review current drinkingwater standards for arsenic, (2) propose a maximum contaminant level for arsenic by January 1, 2000, and (3) issue

a final regulation by January 2001 [Public Law 1412 (b)

A Retrospective Analysis on the Occurrence of Arsenic

in Ground-Water Resources of the United States and

Limitations in Drinking-Water-Supply Characterizations

By Michael J. Focazio, Alan H. Welch, Sharon A. Watkins, Dennis R. Helsel, and Marilee A. Horn
2

(12) (A)] Sec. 109(a). In fulfilling the goals of the Drinking

Water Initiative and the needs of the USEPA, the U.S. Geological Survey (USGS) in cooperation with the USEPA

performed a retrospective analysis of USGS arsenic data

collected from ground-water sources.

The maximum contaminant level (MCL) for arsenic

in drinking water has been 50 μg/L since 1974. A recent

study by the National Academy of Sciences (1999) recommended that the USEPA lower the MCL for arsenic, citing

new human-health research that highlights the occurrence

of arsenic concentrations in water and the associated risk

of skin and internal cancers, such as bladder and lung cancer. Patterns of the occurrence of arsenic with concentrations less than 50 μg/L in drinking-water sources are useful

to assess the national costs and benefits associated with

various MCL options for arsenic concentrations that range

from 1 to 50 μg/L.

The two major existing sources of data for determining the occurrence of arsenic in drinking water nationwide

are the National Inorganic and Radionuclide Survey

(NIRS; Longtin, 1988) and the National Arsenic Occurrence Survey (NAOS; Frey and Edwards, 1997). The NIRS

which was completed in 1984, was based on stratified random sampling of 1,000 public water-supply systems, and

on a minimum detection level of 5 μg/L. The more recent

NAOS, which was completed in 1995, was based on a

stratified random sampling of 275 public water-supply systems and on a minimum detection level of 0.5 μg/L. Frey

and Edwards (1997) summarized the two most comprehensive national surveys for the occurrence of arsenic in drinking water and concluded that where national estimates

could be compared, the results indicated that NIRS and

NAOS were similar. The higher minimum detection level

used in NIRS limited its use for a range of arsenic concentrations of interest to USEPA.

Some of the possible uses of arsenic occurrence data

include (1) quantification of the occurrence of arsenic in

public water-supply systems nationally to be used as input

to decision-tree models used to assess costs associated with

various potential MCL’s nationwide, (2) detailed information on public exposure to arsenic contamination, and (3)

general information on geographic distributions of arsenic

in the ground-water resource likely to be used by public

water-supply systems serving various populations. Data

collected directly from public water-supply systems are

most appropriate for the first two examples, whereas data

used for the third example may include various types of

wells. Problems associated with potential misinterpretation

of occurrence data were detailed in an American Water

Works Association Research Foundation publication

(Raucher and others, 1994). National estimates of the occurrence of radon in ground water were used as an example

of considerations for developing and using data bases for

deriving occurrence estimates, including sample design,

sampling point, minimum levels of detection, and availability of data on important explanatory variables. Others

have also discussed the limitations and requirements of using existing data bases for occurrence estimates on national,

and other, scales (Alley, 1993; Hamilton and others, 1993;

Lapham and others, 1997). Recognizing some inherent limitations in the NIRS data base (for example, the reporting

limit was too high for the range of concentrations of interest) and the NAOS data base (for example, small sample

size; systems serving less than 1,000 people not represented), the USEPA determined that additional data on the

occurrence of arsenic was necessary to supplement existing

data bases. This report assesses the USGS National Water

Information System (NWIS) data base as one source of data

on arsenic in the Nation’s ground-water resources; discussions on the uncertainties and associated limitations of estimates on the occurrence of arsenic are provided. Among the

major limitations in using the NWIS data base is the fact

that the data were not collected as part of a random survey

design and the water samples were collected from a variety

of well types. Franke and others (1997) describe various

conceptual frameworks for ground-water-quality monitoring

and list advantages and disadvantages of large-capacity and

small-capacity wells for sampling ground-water quality.

Large-capacity wells typically include public water-supply

wells, industrial/commercial, and irrigation wells generally

yielding ground water at rates of hundreds of gallons or

more per minute. Small-capacity wells typically include domestic wells and monitoring/observation wells generally

yielding ground water at rates of tens of gallons or less per

minute. The NWIS data base includes data collected from

large- and small-capacity wells with the majority coming

from small-capacity wells. In this report, characterizations

of “the ground-water resource” includes data collected

from public water-supply wells and many other ground-water sources that are not used specifically for regulated drinking-water purposes. In this sense, the entire ground-water

resource is characterized and may or may not accurately

portray the quality of water at particular points in public water-supply distribution systems (that is, at the pump head or

entry points), or that is consumed by people. However, in

characterizing the entire ground-water resource, users can

gain insights such as where high arsenic concentrations occur in the United States and how frequently drinking-water

sources may be expected to exceed targeted concentrations

of arsenic.

Although a rigorous error analysis was not possible in

this study, most of the presumed major sources of uncertainty have been examined by various statistical, graphical,

and qualitative techniques. The principal question when

considering the integrated effects of these uncertainties is

whether the USGS arsenic concentration data, which were

collected for various purposes (for example, data collected

from public water-supply wells and data collected from

other types of wells), can be related to public water-supply

system information in a meaningful way for various applications that require national and regional estimates of arsenic

occurrence in drinking-water supplies.
3

Purpose and Scope

In response to the U.S. Environmental Protection

Agency’s need to supplement existing data on the occurrence of arsenic in the Nation’s ground water, a retrospective analysis of all data in the U.S. Geological Survey’s

NWIS data base was completed. Data for analysis were selected on the basis of several criteria, including the amount

of ancillary data associated with a given data point (sampled

well), the number of data points in a county, and the proximity of data points to the center of a given county (within

50 km of the center of the county). Concentrations of arsenic in water were then related to data on the numbers and

sizes of public water-supply systems in each county

throughout the country. Estimates of the percentage of public water-supply systems having water that exceeds targeted

concentrations of arsenic were calculated for the entire

United States on the basis of available USGS data. Uncertainties and the associated limitations of the USGS data are

discussed by use of statistical, graphical, and other qualitative measures.

Acknowledgments

The authors would like to thank Irene Dooley and Jennifer Wu of the U.S. Environmental Protection Agency for

their thoughtful input and review of this report and their patience in waiting for its publication. Zoltan Szabo, Glenn

Patterson, and Celso Puente of the U.S. Geological Survey

also provided invaluable review.

DESCRIPTION OF DATA BASES

This section includes (1) descriptions of source data

bases and the methods used to create and select data bases

and associated data used for this study, (2) the quality-assurance tests of the USGS data that were used to characterize

its uncertainties and associated limitations as a source of information on the occurrence of arsenic nationwide, and (3)

methods used to relate arsenic concentration data with public water-supply system data to estimate the occurrence of

arsenic in ground-water resources nationwide.

Two data bases were developed for this project: a public water-supply data base and the USGS arsenic point data

base. The data bases were used to test various approaches to

relate arsenic concentration data from the USGS NWIS data

base with data on the numbers and sizes of public watersupply systems in the Nation. The methods used to develop

the data bases are described below and ancillary information

is provided in tables. The data bases were derived from existing data bases and information (fig. 1).

THE PUBLIC WATER-SUPPLY DATA BASE FROM

THE U.S. ENVIRONMENTAL PROTECTION

AGENCY SAFE DRINKING WATER INFORMATION

SYSTEM

Data were retrieved from the USEPA Safe Drinking

Water Information System (SDWIS) for all community water suppliers and their sources of water such as surface,

ground, and purchased water, during late summer of 1997.

Data elements retrieved are listed in table 1.

Figure 1. Flow diagram of the development of arsenic concentration and public water-supply system data bases.

- includes potable and 

nonpotable water data 

U.S. Geological

Survey National

Water Information

System (NWIS)

Arsenic Point

Data Base

-includes potable 

ground-water 

data collected 

from various 

types of wells

Compilations 

of arsenic 

concentration 

data on a 

county basis

The National

Arsenic

Occurrence

Survey (NAOS)

-includes data 

collected from public 

water-supply systems

Estimates of 

percentage of public 

water-supply 

systems that exceed 

targeted arsenic 

concentrations

The Public Water-Supply 

Data Base

- derived from U.S. Environmental Protection Agency 

Safe Drinking Water 

Information System (SDWIS)

Association 

of arsenic 

concentration 

data with 

public watersupply 

system data

Estimates of percentage of 

public water-supply systems 

that exceed targeted arsenic 

concentrations

Comparisons 

of arsenic 

occurrence 

estimates
4

Not all public water-supply systems in the data base

are associated with a service area on a county level. This

complicates the accounting for the numbers and sizes of

public water-supply systems on a county-by-county basis.

The county associations were estimated for all public watersupply systems on the basis of one or more of the following:

(1) city in public water-supply system owner mailing address,

(2) zip code in public water-supply system owner mailing address, (3) and public water-supply system identification number; some states assign the identification number by county.

Some public water-supply systems, especially small ones like

mobile-home parks, may have mailing addresses in a different county than where the system is located.

The sources of water for each public water system

were examined to determine if they were supplied from

their own (1) wells and springs; (2) surface-water intakes;

Table 1. Data elements retrieved or derived from U.S. Environmental Protection Agency Safe Drinking Water Information System

[Abbreviations: PWSID, Public water-supply system identification number; PWS, public water-supply system;

SDWIS, Safe drinking-water information system; FIPS, Federal information-processing system; SRCE, source; ID,

identification number; USEPA, U.S. Environmental Protection Agency.]

System Name Name of PWS system SDWIS-Water Systems

Address Street address of PWS system owner SDWIS-Water Systems

City City of PWS system owner SDWIS-Water Systems

State State of PWS system owner SDWIS-Water Systems

Zip code Zip code of PWS system owner SDWIS-Water Systems

Phone Contact phone number SDWIS-Water Systems

County County of PWS system service area SDWIS-Water Systems

Data element name Definition Source of data

State code State FIPS code of PWS system service area Derived from county PWS system

service area

State/County code FIPS State and county code of PWS Derived from county and State

system service area FIPS codes

Population Served Population served by PWS system SDWIS-Water Systems

Direct Source G=Ground water; S=Surface water; Derived from data in

C=Combined ground and surface water SDWIS-Facility

Purchased Source G=Ground water; S=Surface water; Derived from data in

C=Combined ground and surface water SDWIS-Facility

PWS SRCE ID (Primary key) USEPA unique source id from SDWIS SDWIS-Facility

State State in which source is located SDWIS-Facility

Source Name Name of source SDWIS-Facility

Source Type IN=intake; WL=well; SDWIS-Facility

CC=Consecutive Connection; TP=Treatment Plant

Availability e=emergency; i=interim (peak); o=other; SDWIS-Facility

p=permanent; s=seasonal

Latitude Latitude SDWIS-Facility

Longitude Longitude SDWIS-Facility

PWSID (Foreign key) USEPA Public Water Supplier (PWS) ID number SDWIS-Facility

PWSID* (Primary key) USEPA Public Water Supplier (PWS) SDWIS-Water Systems

ID number

Source water type gu=ground water under the influence of SDWIS-Facility

surface water; gw=ground water; sw=surface water
5

(3) wells, springs, and surface-water intakes; (4) purchased

ground water; (5) purchased surface water; or (6) purchased

ground and surface water. This information was added to the

public water-supply data base. The SDWIS has information

on whether the system is primarily ground water or surface

water. For this study, however, it was important to know if

the system was totally or partially on ground water or totally

on surface water. The name of the source of water in the

Consecutive Connection facilities data element (public

water-supply system from which water was purchased) was

replaced by the identification number of the public watersupply system from which the water was purchased for better association of selected data elements between data bases.

The study included all public water-supply systems that

completely or partially relied on ground water and all systems that purchased at least some ground water. Only those

water-supply systems that were totally dependent on surface

water or purchased surface water were excluded from the

data base.

ARSENIC POINT DATA BASE FROM THE

U.S. GEOLOGICAL SURVEY NATIONAL WATER

INFORMATION SYSTEM

The arsenic point data base contains all the arsenic and

associated water-quality data selected from the USGS

NWIS data base. Each water sample represents a physical

sampling point, such as an individual well, spring, drive

point, piezometer, and so forth, that taps water from a

unique location in the aquifer, hence the name “arsenic

point data base.” The latitude and longitude of each sampling location are preserved in the data base, enabling the

data to be spatially displayed and associated with attributes

from other sources of information. Samples that were collected and analyzed to meet specific criteria were selected

for determining summary statistics for arsenic concentrations in water. The criteria are (1) analytical—analyses that

were made using hydride-generation and atomic-adsorption

spectrometry for arsenic determination from 1973 to 1998

(Fishman and Friedman, 1989; Fishman and others, 1994)

and (2) sample collection — samples that were filtered

(0.45 μm or finer) and acidified in the field. A single analysis was used for any particular data point (well or spring)

regardless of how many times the site was sampled. Although NWIS does not identify the filter pore size used for

field processing, a 0.45-μm pore-size filter was used for the

majority of the water samples. In addition to arsenic, ancillary physical data, such as site location, water use, and wellconstruction information, and ancillary water-quality data,

including pH, temperature, specific conductance, and other

major and minor inorganic constituents, were retrieved. Although the inorganic constituents were available, they were

not used in this study other than for locating sites and to

identify potable and nonpotable water.

The USGS has maintained fairly consistent sample

preservation, analytical methods, and field-sampling protocols for over 20 years, thereby ensuring the comparability of

the data. The arsenic point data base includes approximately

20,000 ground-water sites (springs and wells) that are used

for a variety of purposes, ranging from research to water

supply. Location information allows matching these point

data with information from public-water suppliers. In addition, ancillary data on the use of the water derived from the

wells or springs (domestic, industrial, or public supply, irrigation, monitoring, or other), well construction, aquifer, and

water levels are part of the USGS arsenic point data base

and can be useful for present, as well as future studies on

the occurrence and other characteristics of arsenic in ground

water. An additional designation was added for each arsenic

data point. Geothermal water that has a temperature greater

than 50°C and(or) water that is slightly saline (dissolved

solids concentration greater than 2,000 mg/L or specific

conductance greater than 4,000 μS/cm) were assigned the

designation “nonpotable.” Data points without both dissolved solids and specific-conductance data were designated

“unknown” and were not used in arsenic analyses; this enables the data to be easily categorized into potable and

nonpotable water sources. The potable-water data points

have the additional designations to indicate whether the water is from a public water-supply source or some other

source. The classification as public water supply is based on

primary use of water for bottling, commercial, medicinal,

public supply, or institutional purposes; all other water-use

categories were considered non-public water supply. No attempt to distinguish among transient and non-transient systems was made. Ground-water samples collected from public water-supply systems by USGS researchers typically

come from small systems (serving less than one or two hundred people), such as schools, small community supplies,

and domestic wells.

Arsenic data in selected counties of the United

States

Data from counties with five or more arsenic concentration data points from “potable” water sources were selected from the USGS arsenic point data base for use in

making subsequent national occurrence estimates and analyses. These data include all potable sources of ground water

regardless of whether they are public water-supply wells or

not. The counties that were included in this analysis were

selected on the basis of a spatial estimation approach that

uses measured data in nearby counties. In addition, this approach accounts for the large differences in the size of counties in the western United States as compared to those in the

eastern United States A radius distance of 50 km was selected to approximate the area of a circle equal to the median area of counties in the western United States (as determined by calculating all the county areas of the United

States). The selection process began with those counties that

had five or more arsenic concentration data points. Selection

of the remaining counties was determined by locating the
6

centroid of the county and searching for arsenic concentration data in nearby counties that fell within the 50-km

search radius. The five arsenic data points were assumed to

be representative of the ground water in the county into

which the search radius was extended. This process allows

one sampling location to be used for more than one county.

No attempt was made to analyze the arsenic concentration

data in relation to depth within aquifers.

Relation of arsenic concentration data with public

water-supply system information

A common procedure used to summarize the national

occurrence of contaminant data for use by regulatory agencies is to present the data in terms of numbers (or percent) of

public water-supply systems using water that exceeds specified concentrations of a contaminant. The public water-supply systems are divided into size classes, and the concentrations of contaminants are selected to correspond with the

ranges of potential drinking-water standards that are being

assessed as part of the regulatory process. Additional insights

on the occurrence of contaminants in ground water may be

obtained if the concentration data are grouped by geographic

regions, which are based on physiography or some other

physical or chemical characterization.

The percentage of public water-supply systems (and

number of systems) of various population-served size

classes were associated with the arsenic concentration data

selected from the arsenic point data base on a county basis

by use of the public water-supply data base. The percentage

of arsenic concentrations that were within specified concentrations (1, 2, 5, 10, 20, and 50 μg/L) were determined for

each county that was selected by the procedure described

above for the selected counties. Arsenic concentrations were

then associated with the numbers and sizes of public watersupply systems within that county. For example, suppose a

certain county contained ‘x’ small public water systems and

‘y’ large public water-supply systems. Also assume that no

arsenic concentrations in ground water in the county exceeded 5 μg/L, 10 percent of the arsenic concentrations in

the county exceeded 2 μg/L, and 90 percent exceeded 1 μg/

L. The estimating method would result in 0.1x small and

0.1y large systems in that county with arsenic concentrations that exceed 2 μg/L. Similarly, 0.9x small and 0.9y

large systems would exceed 1 μg/L; no systems in the

county would exceed 5 μg/L. This procedure was repeated

for all selected counties summed for determination of regional and national summaries of the occurrence of arsenic

concentrations in ground water.

Median arsenic concentrations in all counties of

the United States

The arsenic point data base provides measured arsenic

concentration data that are distributed throughout the Nation, however, there is little or no data in many parts of the

Nation. The arsenic concentration data are displayed and described in various ways and spatial scales in the subsequent

sections of this report; this section focuses on the development of a simple table that provides a general portrayal of

median arsenic concentrations nationwide. The table is only

intended to provide a preliminary guide for arsenic concentrations in ground-water resources of the Nation and is not

intended for any subsequent quantitative analyses or applications. The previous section (“Arsenic data in selected

counties of the United States”) describes the spatial estimation procedure that is used in this report for subsequent

analyses of arsenic occurrence in ground water on national

and regional scales.

All counties in the United States were assigned a representative median arsenic concentration that is based on

one of two criteria: (1) the median arsenic concentration of

ground water in that county (when the county contained

three or more arsenic data points), or (2) an estimated concentration that is based on the median concentration of arsenic in ground water from a nearby county that was underlain by the same principal aquifer. Determinations of the

principal aquifers underlying the counties and arsenic concentration estimates for nearby counties were based on the

National Ground Water report series and best professional

judgement. The Ground Water Atlas of the United States is a

comprehensive summary of the Nation’s ground-water resources, and is a basic reference for the location, geography,

geology, and hydrologic characteristics of the major aquifers in the Nation. The information was collected by the

U.S. Geological Survey and other agencies during the

course of many years of study. The USGS Ground Water Atlas was used in selected portions of the Nation (Miller,

1992; Planert and Williams, 1995; Robson and Banta, 1995;

Trapp and Horn, 1997) to determine which national principal aquifer(s) is (are) present in each county to indicate in a

general way, the type of aquifer from which public water

suppliers may be withdrawing water. Currently, there is insufficient information to link specific public water-supply

systems with specific aquifers or volumes withdrawn from

specific aquifers. Each principal aquifer is classified by one

of six types of permeable geologic material: (1) unconsolidated sand and gravel, including glacial deposits and major

alluvial aquifers along main watercourses, (2)

semiconsolidated sand, (3) sandstone, (4) carbonate rock,

(5) interbedded sandstone and carbonate rock, and (6) basalt

and other types of volcanic rock.

The principal aquifer was determined for each county

in the Nation through a geographic information system and

professional judgement in counties where no aquifer designation information existed. The median value of all arsenic

concentrations, which was based on three or more data

points in a given county, was then calculated for each

county and plotted on state maps that showed county out-
7

lines; this was useful in differentiating between nondetected

values and concentrations of arsenic equal to 1 μg/L. For

counties without sufficient data points to calculate median

values, the aquifers and arsenic concentrations in adjacent

counties, and the range of arsenic concentrations in adjacent

counties were used to estimate the arsenic concentration to

assign to the county. Under these conditions the estimate of

the median arsenic concentration in ground water in the

county was based on the following criteria:

• Geographic proximity (distance) of counties with available arsenic analyses. The estimated median value for a

county with no data was based on arsenic data from the

county or counties nearest to it. This is the simplest procedure and was used only when the counties were underlain by the same principal aquifer.

• The number of arsenic analyses in each county. When estimating the median arsenic concentration, the median

values from nearby counties having several or more

analyses weighed more heavily than a median value

based on only three data points.

• The range of arsenic values in a county. If the range of

arsenic concentrations was small, the median was used

for the county.

• The range of arsenic values in an aquifer. If there was a

relation between arsenic concentration and the principal

aquifer type underlying the county, the median arsenic

concentration estimated

for the county was consistent with the arsenic concentration in water from

that aquifer.

• The range of arsenic values at depth. If there was

a relation between arsenic

concentrations and depth

of well, the median arsenic concentration estimated for the county was

consistent with the depth

of nearby public watersupply wells.

• Number of aquifers. Where

more than one principal

aquifer underlies a given

county, the aquifer most

likely used for public supplies was chosen for estimating a median arsenic

concentration for the

county. In other cases, the

median value of arsenic in

water from all the associated aquifers was chosen for

the county.

NATIONAL ARSENIC OCCURRENCE ESTIMATES

Portrayals and estimates of the occurrence of arsenic

in ground water nationwide and associated public watersupply information were determined by using the data in selected counties from the arsenic point data base and the public water-supply data base that was constructed for the

project. The results of portrayals of arsenic concentrations

in the ground-water resource are based on the actual ranges

of concentrations within counties that have sufficient arsenic

data and selected nearby counties to which arsenic concentrations were estimated. The latter results are also used in

association with the public water-supply system data base to

estimate the percentage of public water-supply systems that

exceed targeted arsenic concentrations.

 Analyses for 18,850 potable water samples are in the

arsenic point data base, of which 2,262 were collected from

public water-supply systems and 16,602 were collected

from other potable ground-water sources (fig. 2). The ranges

in arsenic concentrations in ground water in counties across

the United States (fig. 2) exhibit some regional patterns,

generally with higher concentrations in the western states

than those in the eastern states. Welch and others (1999) describe the broad regional patterns in the occurrence of arsenic across the United States in more detail and relate the

patterns to groupings of the general geology.

Figure 2. Locations and concentration ranges of samples in the arsenic point

data base.

Arsenic in water from

18,850 wells and springs

Greater than 10 μg/L

5 to 9.9 μg/L

3 to 4.9 μg/L

Less than 2.9 μg/L

EXPLANATION

400 Miles0

400 Kilometers0

200 Miles0

200 Kilometers0

300 Miles0

300 Kilometers0
EXPLANATION

Less than five actual

Number of arsenic analyses

Five or more estimated

Five or more actual

Figure 3. National distribution of counties based on the

number of arsenic concentrations (actual and estimated)

selected for the arsenic occurrence analysis.

Occurrence of Arsenic in Ground-Water Resources

of Selected Counties in the United States

A total of 17,496 samples from 595 counties with five

or more arsenic data points were selected (fig. 3). The arsenic concentration data in the 595 counties are associated

with 47 percent of all large (more than 10,000 people

served) water-supply systems and 32 percent of all small

(between 1000 and 10,000 people served) systems in the

public water-supply data base. The definition of small and

large systems used by Frey and Edwards (1997) is adopted

here for consistency in later comparisons.

The number of counties associated with arsenic concentration data was increased beyond the 595 counties by

using a spatial estimating procedure. The spatial estimating

procedure, based on a 50-km search radius, increases overall

spatial coverage of the Nation because more of the small

counties in the eastern United States are included in the

analysis. This procedure increased the number of counties to

1,528 and accounted for 76 percent of all large systems and

61 percent of all small systems in the public water-supply

data base. In addition, the number of population-served system size classes in the public water-supply systems was increased to nine to further refine the display of data beyond

the two size classes used by Frey and Edwards (1997) for

the NAOS data and particularly to provide estimates of the

number of systems serving less than 1,000 people. The latter

application may be the most valuable use of the USGS arsenic data because many of the public water-supply samples

were collected from domestic wells, small community water

supplies, and other wells that may not tap the deeper aquifers typical of the larger water supplies represented by the

NAOS data. The percent of arsenic data points with concentrations that exceed 1, 2, 5, 10, 20, and 50 μg/L were estimated for each county and subsequently associated with the

numbers of public water-supply systems in nine populationserved size categories that occur in those counties (table 2).

The percentage of all public water-supply systems in the

Nation estimated to exceed targeted arsenic concentrations

are shown in table 3. Again, the USGS data represent the

ground-water resource in general, are not analyzed in relation to depth within aquifers, and are not restricted to wells

currently used for public drinking sources. In this way, the

increased spatial extent, large number of water samples, and

low detection limits used for the USGS data can be used to

determine where targeted concentrations of arsenic are

likely to occur in the ground-water resources within much

of the United States.

Median Arsenic Concentrations in Ground-Water

Resources of all Counties in the United States

A total of 18,850 samples from the 1,312 counties (out

of a total of 3,222 counties in the Nation) were available in

the arsenic point data base. The median concentration of all

18,850 samples is less than or equal to 1 μg/L; a summary

of median arsenic concentration, by state, is listed in table 2.

A summary of results of the estimation procedure for counties with no arsenic data is given in table 4.

LIMITATIONS OF THE U.S. GEOLOGICAL SURVEY

ARSENIC POINT DATA BASE AND ASSOCIATIONS

WITH PUBLIC WATER-SUPPLY INFORMATION USED

FOR DRINKING-WATER CHARACTERIZATIONS

Although a rigorous error analysis was not

possible in this study, most of the presumed major

contributors to uncertainty have been revealed by

various statistical, graphical, and qualitative techniques. As previously mentioned, sample design,

sampling point, minimum levels of detection, and

availability of data and interpretation of important

explanatory variables, such as depth within an aquifer, are important considerations for developing data bases

on the occurrence of a contaminant in ground water

(Raucher and others, 1994; Alley, 1993; Franke and others,

1997). Hamilton and others (1993) investigated the effects

of survey design and compared existing groundwater data from NWIS with data collected as part of a regional survey. The existing data for that study, although not

part of a random design, were useful for some simple assessments of regional ground-water quality, and the interpretations derived from the existing data were mostly descriptive rather than explanatory because of uncertainties

associated with the water-quality data and limitations in the

ancillary information. These and additional considerations

and uncertainties exist when using the arsenic point data

8
<100 100-500 501- 1,001- 3,301- 10,001- 50,001- 100,0001- >1,000,000

1,000 3,300 10,000 50,000 100,000 1,000,000

1 3,296 3,144 956 1,152 617 416 61 32 0.4

2 2,318 2,191 670 789 420 295 45 21 0.2

5 1,223 1,151 372 439 227 178 30 13 0.1

10 696 638 208 253 129 102 17 7 0.1

20 296 258 86 102 55 40 6 2 0

50 100 77 23 27 17 11 1 2 0

Table 2. The estimated number of public water-supply systems in selected counties that exceed targeted

arsenic concentrations in the associated ground-water resource for various public water-supply system

sizes categorized by population served

[μg/L, micrograms per liter]

Table 3. Percentage of all public water-supply

systems in the selected counties estimated to

exceed targeted arsenic concentrations in the

associated ground-water resource

[μg/L, micrograms per liter]

 1 35.9

 2 25.0

 5 13.6

10 7.6

20 3.1

50 1.0

Percentage of all public water-supply

systems estimated to exceed the

targeted arsenic concentration in the

associated ground-water resource

Arsenic

 (μg/L)

base. The principal question in considering the integrated

effects of these uncertainties is whether the USGS arsenic

concentration data, which have been collected for a variety

of purposes from a variety of wells, can be associated with

public water-supply system information in a meaningful

way and used as a source of information for various applications that require national and regional estimates of the occurrence of arsenic in ground-water resources used for

drinking-water supplies.

Arsenic Concentration Data

A variety of factors can affect the interpretation of the

arsenic point data base when used as an indicator of the suitability of the ground-water resource for drinking-water purposes, including

 —Source of sampled water,

 —Sample collection protocols,

 —Laboratory reporting level,

 —Temporal variability,

 —Density and location of sampling sites.

Source of sampled water

Sources of water represented in the arsenic point data

base include wells used for a variety of purposes, including

public water-supply, agricultural, industrial, municipal, observation, and private water-supply. Observation wells

sampled for research on arsenic geochemistry are also included in the data base. Additionally, some wells are located

near waste-disposal facilities. This areal distribution of data

may be more representative of the quality of water from

small and very small water supplies or unregulated private

wells than that of larger public water-supply systems. The

point of sample collection is typically at the wellhead for all

USGS data; thus, effects of water treatment and public water-supply distribution systems on arsenic concentrations are

not considered.

The local aquifer tapped for private domestic-water

supplies or other small public-water supplies (serving tens

to hundreds of people) compared to another aquifer(s) that

serves a larger municipal supply could also result in a

source of variability in arsenic concentrations in drinking

water. Private domestic well owners or very small public

suppliers generally are limited in the availability of the

aquifer used for water supply. This is in contrast to the large

public water suppliers who have more resources to develop

available aquifers and optimize location of wells. Thus,

large water suppliers may be able to avoid the use of an unArsenic

(μg/L)

Estimated number of public water-supply systems that exceed the targeted

arsenic concentration in the associated ground-water resource

Population served

9
10

STATE

Alabama 1 1 2 2

Alaska 4 9 22 29

Arizona 4 7.75 20.5 61

Arkansas 1 1 1 1

California 2 4 9 32.5

Colorado 1 2 3 4

Connecticut 1 1 1.5 2.5

Delaware 1 1 1 3

Florida 1 1 2 2

Georgia 1 1 1 1

Idaho 2 3 5.5 19

Illinois 1 1 1 1

Indiana 3.5 6.5 10 12

Iowa 1 1 1.5 1.5

Kansas 1 2 2 2

Kentucky 1 1 1 1

Louisiana 1 1 1 1

Maine 1 3.5 6.5 14.5

Maryland 1 1 1.5 2.5

Massachusetts 1 1 2 8

Michigan 1 3 6 6.5

Minnesota 1 1 1 1

Mississippi 1 1 1 1

Missouri 1 1 1 1

Montana 2 4 7 7

Nebraska 4 6 7.5 9.5

Nevada 8 15 29 70

New Hampshire 1 1 2 5

New Jersey 1 1 1 7

New Mexico 1.5 4 8 19.5

New York 1 1 1 1

North Carolina 1 1 1 1

North Dakota 2.5 3 4 7

Ohio 1 1 2 2

Oklahoma 1 2 2 3

Oregon 1.5 2.5 3 4

Pennsylvania 1 1 2 2.5

Puerto Rico 1 1 1 1

Rhode Island 1 1 1 1

South Carolina 1 1 1 1

South Dakota 1.5 3 4 7

Tennessee 1 1 1 1

Texas 1 1 1 2

Utah 2 4.5 9.5 12

Virginia 1 1 1.5 2.5

Vermont 1 1 1 1

Washington 1 2 3 5

West Virginia 1.05 2 3 3

Wisconsin 1 20 20 20

Wyoming 1 2 3 3

1 Includes actual and estimated values.

suitable aquifer that contains water with

high arsenic concentrations or other objectionable water quality (Franke and

others, 1997). Peters and others (1999)

showed that water from domestic wells in

New Hampshire contained substantially

more arsenic than water from municipal

sources that served larger numbers of

people. Welch and others (1999) have

linked high concentrations of arsenic in

ground-water resources with certain

types of geologic terranes. The difference

in the geologic setting of sited wells emphasizes the potential differences in the

quality of water that could occur in wells

used for large public water suppliers as

compared to those used by smaller water

suppliers. Other differences in small and

large public water-supply systems include the larger area of contribution required for wells that serve large watersupply systems. A comparison of the

estimates of the national occurrence of

arsenic made with data from the arsenic

point data base and with the NAOS data

base is presented in the following section

for small and large public water-supply

systems.

The procedure used for grouping

the different well types in the USGS arsenic point data base is the separation of

wells used for public water supply from

all other wells. The group of public water-supply wells in the USGS classification scheme includes wells withdrawing

water for bottling, commercial, medicinal, and institutional purposes, as well as

individual homeowner’s wells and public

water utility wells. The group of all other

wells includes wells used for research,

observation, and other wells not used for

drinking purposes. The two groupings

may not address concerns of local heterogeneity such as those noted in Peters and

others (1999) but provides insight in the

general approach of combining data from

both groups of wells. Uncertainties in using ambient ground-water data as indicators of public water supplies can severely

limit use of the data for determining the

suitability of water for drinking purposes.

The differences in arsenic concentration

data from wells that were used as public

water-supply sources were compared

with those that are not water-supply

Median

value1 of all

county medians

in state

Median value1 of

the 75th percentiles for each

county in state

Median value1

of maximums

for each county

in state

Median value1 of

the 95th percentiles for each

county in state

Table 4. Statistical summary of median arsenic concentrations estimated for all counties
11

Table 5. Summary of factors considered in making median arsenic concentration estimates for counties with no

arsenic data

[μg/L, micrograms per liter; n/a, not applicable; >, greater than]

sources. The nonparametric Wilcoxon rank-sum test is used

to determine whether the two groups come from the same

population (same median and other percentiles), or alternatively whether they differ only in location (central value or

median)(Helsel and Hirsch, 1992). In this way, if the only

interest in the data is to determine whether one group tends

to produce higher observations, the two groups do not even

need to have the same distribution (Helsel and Hirsch,

1992). The nonparametric Wilcoxon rank-sum test showed

that the two sources of data were statistically different overall (p=0.0001) with samples collected from the public water-supply wells having lower arsenic concentrations than

those in water from the other types of wells. This was not

unexpected because of the large number of data available;

even small differences between the two groups can be seen.

Differences at the high end of arsenic concentrations can be

explained in part by wells not used for drinking-water supplies. In some cases these wells likely include research

 Non-detect All are non-detects 81 4.24

1 Only non-detect and 1 μg/L 91 4.76

1 247 12.93

1 136 7.12

2 149 7.80

3 30 1.57

4 14 0.73

5 3 0.16

6 15 0.79

 Non-detected All are non-detects 70 3.66

1 667 34.92

2 250 13.09

3 111 5.81

4 32 1.67

5 4 0.21

6 4 0.21

8 2 0.10

8 4 0.21

Total n/a n/a 1,910 100

All county medians less than

or equal to 1 μg/L, and some

analyses > 1 μg/L

Most county medians less than

or equal to 1 μg/L, and

Some medians > 1 μg/L

Most county medians

> 1 μg/L

Geohydrologic

conditions in

county with no

arsenic data

Estimated median

arsenic concentration (μg/L)

Range of arsenic

concentrations in

adjacent counties

Number of

counties

with no

arsenic data

Percent of total

number of

counties with

no arsenic data

One aquifer or one

geologically similar

aquifer underlies an

adjacent county with

arsenic data

More than one aquifer

underlies an adjacent

county with arsenic

data

County has no principle aquifer defined
12

wells that purposely tapped ground water known to have

high concentrations of arsenic. Similarily, as previously

mentioned, samples collected from public water-supply

wells would tend to have lower arsenic concentrations than

other wells due to choices made upon well installation.

However, the medians for the two groups of wells are

equivalent because about half the arsenic data in each group

are below or at the reporting limit (1 μg/L) . The 75th percentile arsenic concentration is 3 μg/L for the public watersupply wells and 4 μg/L for the other types of wells (fig. 4).

For the 90th percentile (8 μg/L for public water-supply wells

and 13 μg/L for other types of wells) there is a slightly

larger difference in concentrations than for the 75th percentile of the data. The arsenic concentrations in the two groups

of wells actually differ by only 1 μg/L or less for the majority (about 75 percent) of the data.

Differences in arsenic concentrations between the public water-supply well group and the group of other types of

wells were statistically tested to determine the influence of

various physiographic regions (table 3; fig. 5) defined by

Fenneman (1931). The median concentrations of arsenic

were the same in both groups of wells for all the physiographic provinces analyzed (table 6). Only the Atlantic

Coastal Plain region had a statistically significant difference

in arsenic concentration in water between the two groups of

wells. The median value of arsenic is less than or equal to 1

μg/L (the minimum report level) for data collected from

both groups of wells in the Atlantic Coastal Plain; the mean

value of arsenic in ground water from the 646 public watersupply wells was 1 μg/L, and the mean value of the 2,047

nonpublic water-supply wells was 2 μg/L. The absolute

magnitude of the differences in the means

and medians of arsenic are small in the

Atlantic Coastal Plain

when considering the

large regional scales

over which comparisons were made; however, the differences

in the 95th and 99th

percentile concentrations are larger.

Differences in

the means and percentiles between the two

groups of wells in

other provinces may

be important even

though there was not

a statistically significant difference in the

two groups of wells.

For example, the difference in the mean arsenic concentrations between the two

groups of wells in the Rocky Mountain System (2 μg/L and

7 μg/L) could be significant to some applications where the

arithmetic mean is used. As previously mentioned, the difference in means and percentiles between the two groups of

wells in the Rocky Mountain physiographic province could

be controlled, in part, by research wells purposely located in

areas of high arsenic concentration. Overall, the combined

data sets (including public water-supply wells and other

types of wells) may be useful for some analyses of arsenic

in certain ranges of concentration; however, the most appropriate uses of the data may vary with physiographic province and size of the data set.

The regional classification used by Frey and Edwards

(1997) and adopted later in this study for estimating occurrence of arsenic and for susbsequent comparisons is based

on the boundaries of states with presumed similar geologic

sources of arsenic (table 7) and do not coincide with the

physiographic boundaries used in the previous comparisons.

For example, the Rocky Mountain System physiographic

province includes more than one region as defined in the

NAOS study (the Western, South Central, and North Central

regions). These regions, in turn, also include other physiographic provinces. Thus, data in any given physiographic

province may be used in one or more than one region defined by Frey and Edwards (1997).

Sample-collection protocols

Special field protocols were developed and evaluated

by the USGS in the early 1990’s after recognition that some

inorganic analytes were affected by contamination due to

sampling methods at the 1 μg/L level of quantitation

(Horowitz and others, 1994; Koterba and others, 1995).

Thus, data collected before institution of the “ultra-clean” or

“parts per billion” protocols (1992 or earlier) may be different than data collected afterwards. The USGS Office of Water Quality performed several tests to evaluate the effect of

the older, less stringent sampling protocols on the quality of

analytical results for the trace inorganic analytes. It was

shown that any potential random contamination of arsenic

due to improper handling in the field during sampling was

limited to concentrations at or below the detection limit of 1

μg/L. Therefore, arsenic data, unlike data for other analytes,

such as lead, zinc, and copper (Ivahnenko and others, 1996)

is not significantly affected by the change in field sampling

protocols. For the large-scale analysis of this study, arsenic

data was not censored on the basis of field sampling protocols.

Ground water can contain arsenic in particulate-bound

and dissolved phases. Unfiltered water samples provide the

best estimate of the combined mobile particulate-bound and

dissolved arsenic concentration. Comparison of analytical

results from unfiltered and filtered samples from 589 well

sites shows that concentrations of filtered samples are either

Figure 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.

90th percentile

15

10

5

0

0 0

Public watersupply wellsAll other

 wells

75th percentile

Arsenic concentration, in μg/L

median
13

the same or less than unfiltered samples with a few exceptions, particularly near the reporting limit (fig. 6). Samples

collected by USGS personnel are generally filtered to provide an estimate of dissolved concentrations and to exclude

artificially mobilized (by the sampling process, for example)

large particulates that may bear trace elements. The commonly used 0.45-μm filter, however, does not necessarily

exclude all particles from the sample, such as colloidal iron

oxides (Kennedy and others, 1974; Horowitz and others,

1994). Particulate-bound arsenic can pass through a 0.45 mm

pore-size filter (Chen and others, 1994; Edwards and others,

1998). Filtered arsenic concentration data can be the most

consistent way to characterize the quality of a ground-water

resource because the filtering process tends to homogenize

samples by removing variability caused by introduction of

solid material from the distribution system, well casings,

Table 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

[μg/L, micrograms per liter; ≥, greater than or equal to; <, less than]

(AP)

(IMP)

(IP) (RMS)

(AH)

(PMS)

Interior Plains

Appalachian

Highlands

Intermontane Atlantic Plain

Plateaus

Rocky Mountain

System

Pacific

Mountain

System

Figure 5. Major physiographic provinces of the conterminous United States.

Wilcoxon test

statistic*

(p>|z|)

Public watersupply wells

All other wells

Public watersupply wells

All other wells

Public watersupply wells

All other wells

Public watersupply wells

All other wells

Public watersupply wells

All other wells

Number of

samples

Mean

(μg/L)

Median

(μg/L)

95th

percentile

(μg/L)

99th

percentile

(μg/L)

1. Appalachian

Highlands

2. Atlantic Coast

Plain

3. Interior Highlands,

Interior Plains, and

Laurentian Upland

4. Intermontane

Plareaus

5. Pacific Mountain

System

6. Rocky Mountain

System

376

2,212

646

2,047

342

3,947

458

4,640

303

2,401

74

1,028

1

3

1

2

5

5

9

15

6

9

2

7

≤1

≤1

≤1

≤1

≤1

≤1

3

3

2

2

≤1

≤1

5

8

2

6

19

16

39

44

21

27

6

20

10

25

7

21

75

48

100

200

92

82

30

100

0.6552

0.0067

0.3289

0.1389

0.7159

0.6444

* A value < 0.05 indicates the two data sets are different

Physiographic

Province

States included in region

Western CA, NV, AZ, UT, ID, WA, OR, AK, HI

North Central MT, ND, SD, WY

South Central TX, NM, CO, NE, KS, OK, LA, MO, AR

Midwest Central MN, WI, MI, IA, IL, IN, OH

Southeast TN, MS, AL, GA, FL

Mid-Atlantic KY, WV, PA, MD, DE, VA, NC, SC

New England ME, VT, NH, MA, CT, RI, NY, NJ

Region used in

survey (Frey and

Edwards, 1997)

Table 7. Regions and associated states used in the National Arsenic Occurrence Survey
14

field equipment, pumping dynamics, aquifer materials, and

the like during sampling. Filtered samples are thus considered more representative of the water in the aquifer. Most of

the data in the USGS arsenic point data base were determined

from filtered samples. Therefore, the estimated arsenic occurrence information in this report is based on filtered (0.45

μm or smaller pore-size filter) samples.

Laboratory reporting

A change in USGS laboratory reporting procedure in

the mid-1980’s (Alexander and others, 1996) increased the

frequency with which arsenic analyses were reported as less

than 1 μg/L rather than as 1 μg/L. Arsenic concentration

data determined as part of USGS stream water-quality networks show an increase in the frequency of reported censored (less than) values. In 1985, about 36 percent of the arsenic analyses were censored, indicating that the change in

reporting occurred in late 1984 or 1985. Less than 15 percent of the results of arsenic analyses were censored from

1980–84 compared with more than 42 percent from 1986 to

1989. Therefore, arsenic concentrations that fall into these

two categories are treated as “less than or equal to” 1 μg/L

in this report (Helsel, 1990).

Temporal variability

Many wells in the arsenic point data base were

sampled more than once and one was sampled 72 times. All

analyses of arsenic concentrations in this report are based on

one data point per well with the most recent data point usually selected to represent a given well. Consequently, the

representativeness of the most recent data point (or any

single data point) from a given well may be limited if the

variability in concentration over time is significant to the intended use of the data. The variability in arsenic concentrations over time was analyzed for the 355 wells (regardless

of potability of the water collected from the wells) in the

USGS NWIS data base that contained 10 or more samples

collected over various time periods.

A general indication of the uncertainties contributed

by temporal trends in any given well were examined by regression of arsenic concentrations and time for each well.

The coefficient of determination (R2

) ranges from 0 to

greater than 0.8 for the range of mean arsenic concentrations and time periods in the data set (fig. 7). Most of the R2

values shown in figure 7 are low, indicating that there probably is no relation between arsenic concentration and time

for most of the wells. However, there are many R2 values

that indicate a significant relation. The data from several of

the wells with high R2 values were examined closely and it

was determined that while a few trends may exist, the high

values can often be explained by mechanisms that do not

limit the use of the data. For example, a temporal trend (and

associated high R2 value for arsenic and time) exists in

many wells because the samples were collected over a short

time period (days). This indicates that the data were collected for some short-term local objectives, perhaps during

the time when the well was being developed for its intended

use. The water chemistry data collected during those shorttime periods are more indicative of the changes associated

with well development than with the general geochemistry

of the aquifer, and the most recent sample is usually the

most appropriate to characterize the arsenic concentration at

that location. Wells used for irrigation purposes are also potential sources of temporal variability due to seasonal pumping schedules (Franke and others, 1997). It is beyond the

scope of this investigation to determine the causes of temporal trends, and although it is possible that some trends may

exist, most wells are not affected.

The coefficient of variation in the arsenic concentration over time data for 116 of the 355 wells were all zero,

reflecting the large number of concentrations consistently at

or below the reporting level. This indicates that the arsenic

concentration in water from those 116 wells is accurately

portrayed by any one of the measurements from each well.

The coefficient of variation is nearly constant as the arsenic

concentration becomes larger; this is depicted by the relation of mean arsenic concentration in each of the 355 wells

with the respective coefficient of variation for each well

(fig. 8A). This indicates that although the standard deviation

Figure 6. Relation between (A) arsenic concentrations

in filtered and unfiltered samples, and (B) the number of

samples with selected ratios of filtered to unfiltered concentrations.

500

400

Number of observations 300

200

100

0

0 to 0.1

Ratio of filtered to unfiltered arsenic concentrations

0.1 to 0.5 0.5 to 1.0 1.0 to 1.5 1.5 to 2.00 >2

B

10,000

1,000

100

10

1

1 10 100 1,000 10,000

Filtered arsenic, in μg/L

Unfiltered arsenic in μg/L

A
15

(fig. 8B) increases with increasing arsenic concentration, the

increase is constant with respect to the mean and therefore

the variability of arsenic concentrations over time is about

the same for any well regardless of arsenic concentration.

However, the magnitude of the standard deviation as compared to the mean arsenic concentrations (fig. 8B) indicates

that there is a range of concentrations and wells where uncertainty in using any one arsenic concentration selected at

random can be an important limitation in characterizing that

location. For example, the standard deviation in arsenic concentrations for those wells with a mean arsenic concentration of about 10 μg/L (fig. 8B) ranges from less than 1 to

greater than 10 μg/L. Many factors can contribute to this

variability, including sampling techniques, natural variability in geochemistry or source of arsenic contamination, and

changes in pumping of wells over time (particularly for public water-supply wells). A detailed assessment of the causes

of temporal variability was beyond the scope of this study;

however the effects of well depth, which tends to be a general indicator of aquifer and well type, was examined. No

relation between well depth and temporal variability of arsenic concentration was detected by the standard deviation

of arsenic concentration (fig. 9).

Density and location of sampling sites

Statistical and other portrayals of arsenic concentrations that combine areas with large amounts of arsenic data

with areas that have little or no data should be interpreted

with caution. For example, data are sparse for Minnesota,

South Carolina, and Maine (fig. 2). Large parts of the eastern United States, particularly where arsenic has historically

not been a problem in drinking-water sources, and (or)

where ground water is not used for public consumption,

have little or no data. Conversely, some parts of the western

United States have large amounts of data, particularly in areas where arsenic has historically been of concern in drinking-water sources.

To the extent that large gaps in arsenic

data are associated with areas with no historic

arsenic problems in drinking water, national

spatial generalizations are not affected; however. it is possible that high concentrations of

arsenic could exist in the unsampled ground

water in some of the areas. In some states the

areal distribution of data is small, particularly

those in certain types of land uses or geologic

terrains. For example, California has many data

points clustered in the central part of the state

and little or no data in other parts of the state.

Alaska is another example of this—sample

sites for Alaska are clustered in only a few

parts of the state and include an area near

Fairbanks that is affected by mining. Accordingly, the data for Alaska may be more representative of the effects of mining than of

Figure 7. Coefficients of determination of arsenic concentrations regressed with time as a function of mean

arsenic concentration for each of 355 wells with 10 or

more arsenic samples collected over time.

Mean concentration of arsenic, in μg/LMean concentration of arsenic, in g/L

Coefficient of determination (R–squared)

0

0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

1,00010010 10,000

0.01

0.1

1

10

100

1,000

1 10

Mean concentration of arsenic, in μg/L

Coefficient of variation

100 1,000 10,000

A

Figure 8. Coefficient of variation (A) and standard deviation (B) for

each of 355 wells with 10 or more arsenic samples collected over

time as a function of mean arsenic concentrations.

1

0.1

1

10

100

1,000

10,000

10 100 1,000 10,000

Mean concentration of arsenic, in μg/L

Standard deviation of 

arsenic concentration, in μg/L

B
16

ground water resources in the state as a whole. These, and

other, potential limitations in the data constrain some uses,

particularly as the spatial area of interest decreases. Regional

and national comparisons of the arsenic point data, such as

shown in the previous section “Source of sampled water”

and in a subsequent section “Association of the arsenic point

data base with public water-supply system information” provide additional insights into differences between regional

and national arsenic data interpretations.

Association of the Arsenic Point Data Base with

Public Water-Supply-System Information

As previously mentioned, the arsenic-concentration

data in the arsenic point data base were collected from a variety of well types for many different objectives and some of

the major limitations and uncertainties in using the data to

describe the occurrence of arsenic in the ground-water resource were discussed. Associating the arsenic-concentration data (in the ground-water resource) to the number and

size of public water-supply systems in order to perform subsequent applications addressing drinking-water issues can

introduce additional uncertainties and limitations. The additional uncertainties and limitations include those associated

with the public water-supply data base and the methods

used to relate arsenic-concentration data in the ambient

ground-water resource with the public water-supply data.

The public water-supply data base

The most current information on the population served

by each public water-supply system was subdivided by

source of water (table 8). One element of uncertainty in associating the arsenic concentration data with the public

water-supply system data is the assumption that all sources

Well depth, in meters

Standard deviation of 

arsenic concentration, in μg/L

1

0.1

1

10

100

1,000

10 100 1,000 10,000

Figure 9. Standard deviation for each of 355 wells with 10 or more

arsenic samples collected over time as a function of well depth.

of ground water are in the same county as

the public water supplier. This may not be

true if water is purchased from another public water supplier or if the source of ground

water is located in another county from the

listed public water supplier’s mailing address. In the cases where water is purchased

from other suppliers, this problem is probably not significant to national estimates of

arsenic occurrence because the number of

public water-supply systems that purchase

ground water from another supplier is only

about 4 percent of all such systems nationwide. The method of combining all public

water-supply systems that use any mix of

ground water and surface water with all systems that rely solely on ground water is another source of uncertainty when relating

this information with arsenic concentration

data. Most public

water-supply systems that use ground water

are not likely to use surface water as an additional source

because of financial and other constraints. The discrepancy

in accounting for the number of public water-supply systems that solely use ground water is in the larger system

sizes where abundant water sources are available and affordable.

The population numbers in table 8 were taken from the

USEPA Safe Drinking Water Information System (SDWIS)

and were compared with the Census Bureau population data

as an indicator of the integrity of the SDWIS data. The

SDWIS values of population served by public water-supply

system were compared on a county basis with population

data from the Census Bureau. Data collected by the Census

Bureau on housing units dependent on public water supply

per county during the 1990 Census were converted into a

percentage and multiplied by the 1996 population per

county. Large discrepancies in estimates of the number of

people served by a given public water-supply system in the

SDWIS data might be due to many factors, including (1) a

decimal error, (2) a combination of wholesale and retail

population served (as in Massachusetts), or (3) the retail sale

of water to an adjacent county. Discrepancies in low estimates of the number of people served by a given public water-supply system might be due to (1) a decimal error, or (2)

water purchased as retail from an adjacent county (as in

Washington, D.C.). Overall, the percent difference in the estimates of population served by public water-supply systems

between the two sources of data is 10 percent greater in the

SDWIS data than in the Census Bureau data. Differences in

estimates of population among states are typically within a

few percentage points, and the largest differences come from

heavily populated states as well as sparsely populated states.

Therefore, the SDWIS data were adequate for the purposes

of this project.
17

TOTAL

Alabama 4,637 2,262 647 1,728

Alaska 500 382 10 108

Arizona 3,994 3,919 36 39

Arkansas 2,145 900 92 1,153

California 32,821 27,520 311 4,990

Colorado 3,651 1,124 14 2,513

Connecticut 2,616 1,833 5 778

Delaware 578 437 141 —

Florida 14,727 13,655 629 443

Georgia 6,045 1,948 126 3,971

Hawaii 1,263 1,198 38 27

Idaho 796 760 1 35

Illinois 10,858 4,351 277 6,230

Indiana 4,091 2,096 13 1,982

Iowa 2,351 1,945 64 342

Kansas 2,386 1,679 115 590

Kentucky 3,858 419 139 3,300

Louisiana 4,602 2,680 41 1,881

Maine 605 198 3 404

Maryland 4,564 904 — 3,660

Massachusetts 7,969 3,540 65 4,364

Michigan 6,953 1,880 106 4,967

Minnesota 3,451 2,679 60 712

Mississippi 2,693 2,590 60 43

Missouri 4,876 2,321 360 2,195

Montana 610 323 6 281

Nebraska 1,236 1,211 14 11

Nevada 1,580 937 — 643

New Hampshire 730 451 1 278

New Jersey 7,642 5,535 1,996 111

New Mexico 1,454 1,346 12 96

New York 16,604 5,287 177 11,140

North Carolina 5,078 1,416 76 3,586

North Dakota 522 237 42 243

Ohio 9,811 3,646 421 5,744

Oklahoma 2,913 957 70 1,886

Oregon 2,495 1,607 180 708

Pennsylvania 10,571 4,404 290 5,877

Puerto Rico 5,038 4,089 — 949

Rhode Island 970 377 87 506

South Carolina 2,964 1,206 24 1,734

South Dakota 602 481 5 116

Tennessee 4,723 1,935 382 2,406

Texas 18,754 10,896 438 7,420

Utah 3,180 2,241 112 827

Vermont 471 227 3 241

Virginia 5,299 2,082 656 2,561

Washington 4,799 3,674 141 984

Washington DC 535 535 — —

West Virginia 1,462 382 58 1,022

Wisconsin 3,538 2,065 14 1,459

Wyoming 492 282 21 189

Description of the National Arsenic

Occurrence Survey

Uncertainties in estimates of the percentages of public water-supply systems with water

in the associated ground-water resource that exceeds targeted ranges of arsenic concentration

were examined by comparing the results with an

existing national survey for arsenic occurrence

in drinking water. The National Arsenic Occurrence Survey (NAOS) was based on a stratified

random sampling of public water-supply systems selected on the basis of water-source type,

system size, and geographic location. Two-hundred and seventy-five samples from raw groundwater sources (untreated ground water) and 161

from surface-water systems from 7 regions

(New England, Mid-Atlantic, Southeast, Midwest, Southcentral, Northcentral, and Western)

were included in the NAOS. Public water-supply system sizes were grouped into large (serving more than 10,000 people) and small (serving more than 1,000 and less than 10,000

people) categories. The USEPA’s SDWIS data

base indicates that of the total number of public

water-supply systems that use ground-water in

the Nation, about 80 percent serve populations

of less than 1,000. Thus, the NAOS data represents the larger systems and under-represents

the smaller systems. Frey and Edwards (1997)

discuss the procedure used to estimate the percent exceedances of arsenic in water for all public water-supply systems in the country. The

NAOS data were collected from raw-water

sources and only about one-third of the groundwater systems applied treatment. The projected

effects of water treatment were published for

those systems and are used here in the national

comparisons with the raw-water USGS data.

Frey and Edwards (1997) state that the arsenic

concentrations in finished water are substantially lower than those in raw-water sources

used for surface-water supplies but are comparable for ground-water supplies.

1. National Comparative Analysis of

Percentages of Public Water-Supply

Systems with Water that Exceeds

Targeted Arsenic Concentrations Estimated from the National Arsenic Occurrence Survey and the U.S. Geological

Survey Retrospective Analysis

Percentages of public water-supply systems that exceed targeted arsenic concentrations

in the USGS and NAOS data bases are shown in

Table 8. Safe Drinking Water Information System data for populations

served by ground water and surface water, by state, 1998

[—, no data]

Safe Drinking Water Information System

population served (in thousands)

State

Purchased some

ground water

Only surface

water

Total Some ground

water
18

1

Percent exceedance

0

10

20

30

40

Percent exceedance

0

10

20

30

40

2 5 10 20 50

125

Arsenic concentration, in μg/L

Arsenic concentration, in μg/L

10 20 50

* Note: Data for 20 and 50 μg/L were not available

* Note: Data for 20 and 50 μg/L were not available

U.S. Geological Survey

National Arsenic

Occurence Survey*

U.S. Geological Survey

National Arsenic

Occurence Survey*

figure 10. Generally, the USGS data show similar percentages (largest differences are within a few percentage points)

of exceedance in comparison to both the large and small

public water-supply system data in NAOS. Note that percent

exceedances are presented for 20 and 50 μg/L with USGS

data to demonstrate the completeness of the USGS data but

these concentration categories were not published for the

NAOS data.

2. Regional Comparative Analysis of Public WaterSupply Systems with Water that Exceeds Targeted

Arsenic Concentrations Estimated from the

National Arsenic Occurrence Survey and the U.S.

Geological Survey Retrospective Analysis

As previously mentioned, the NAOS data were stratified by 7 regions throughout the country. The percentage of

public water-supply systems with water that exceeded targeted arsenic concentrations in raw water of all samples

combined (including large and small public water-supply

systems) were published for each region (Frey and Edwards,

1997). These percentages of exceedance data were compared with the corresponding USGS data for a regional perspective. The regional arsenic concentrations that exceeded

5 μg/L (chosen as the example because the reporting limit

used in the National Inorganics and Radionuclide Survey

survey was 5 mg/L) are shown in figure 11. Generally, the

percentages of public water-supply systems with water that

exceeds targeted arsenic concentrations are similar on a region-by-region basis; however, there are some differences of

10 or more percent between the USGS and NAOS data for

the Mid-Atlantic and North Central regions. The percentexceedance differences for the targeted arsenic concentrations were compared within each region separately (fig. 12).

As an example, the percentages of exeedance for the targeted arsenic concentration data in the Western and Mid-Atlantic regions are compared in figures 12A and 12B. Comparisons between USGS and NAOS data were not possible

at this time for the arsenic concentrations of 2, 20, and 50

μg/L because the data were not available. The USGS data

are similar (within a few percent) to the NAOS data for the

Western region (fig. 12A) but larger differences occur in the

comparison of data from the Mid-Atlantic region (fig. 12B).

The large number of point arsenic concentrations in the

USGS data base for the Western region (7,699 points taken

in 163 counties) indicates that sample density is high in this

region. In contrast, the number of point arsenic concentrations in the Mid-Atlantic region were much lower (1,417

samples in 62 counties) than the Western and other regions

(fig. 12B).

3. Comparative Analysis of Arsenic Concentrations

in Raw and Finished Water

Frey and Edwards (1997) show that the projected arsenic concentrations in finished water in the NAOS groundwater data base are similar to the raw-water arsenic concentrations. Therefore, results of comparisons of arsenic

concentrations in the USGS raw-water data with those in the

NAOS finished-water data should be similar to comparisons

between USGS raw-water data and NAOS raw-water data.

This finding is supported by the similarities in the percentages

of public water-supply systems that exceed targeted concenA.

B.

Percent exceedance

West

Midwest

N Central

S Central

Southeast

Mid 

Atlantic

New England

0

10

20

30

40

5

15

25

35

45

National Regions

U.S. Geological Survey

National Arsenic

Occurence Survey

Figure 10. Percentage of public water-supply systems

with water estimated to exceed targeted arsenic concentrations for (A) large and (B) small public water-supply

systems. Percentage values calculated with USGS rawwater data and NAOS finished-water projections.

Figure 11. Percentage of public water-supply systems

that exceed arsenic concentrations of 5 μg/L in raw-water arsenic by region.
19

A.

B.

Figure 13. Percentage of public water-supply systems

that exceed targeted arsenic concentrations in the western region (NAOS finished-water projections).

Figure 12. Percentage of public water-supply systems

that exceed targeted arsenic concentrations in raw water

from the Western (A) and Mid-Atlantic (B) regions.

trations of arsenic in raw and finished water for all regions as

shown in the example for the Western region (fig. 13).

CONCLUSIONS

Two data bases were developed for this study—the

public water-supply data base and the arsenic point data

base. Each data base stands alone and can be used for various purposes. The public water-supply data base provides

information on public water-supply systems throughout the

Nation and was used to associate water-supply system population-served size classes with concentrations of arsenic in

potable ground-water resources of the United States. The arsenic point data base contains arsenic concentrations in

ground water and associated data from the USGS National

Water Information System and includes only potable (less

than 2,000 mg/L total dissolved solids and less than 50o

C)

water data. The large number of data points, broad spatial

scale, and consistent data-collection and analysis methodology, and the generally high-level quality of data in the arsenic point data base make it a unique source of information

on the occurrence of arsenic in the nation’s ground water.

The arsenic data in selected counties of the United

States includes only potable water data in those counties

that contain 5 or more sampling locations; the spatial distribution of sampling sites was augmented by including all

available data in the selected counties plus additional counties chosen through a spatial search criterion (sampling sites

selected by use of a 50-km search radius). This data set of

measured arsenic values was used with the public water-supply data base to produce estimates of percentages of public

water-supply systems in eight population-served size classes

that exceeded targeted arsenic concentrations in the associated ground-water resources. Targeted arsenic concentrations of 1, 2, 5, 10, 20, and 50 μg/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. 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.

 Statistical, graphical, and other qualitative evaluations

of the limitations of using the USGS arsenic point data base

for assessments of the arsenic concentrations in ground water used as a source of drinking water provide insight to the

presumed major contributors to uncertainty in the analysis.

The type and depth of well (public water-supply well and all

other types of wells) are limiting factors that affect interpretations of arsenic concentrations made with the arsenic point

data base. Large differences in some regional patterns of arsenic concentrations in water collected from public watersupply wells as compared to all other types of wells indicate

that the data are limited on some spatial scales; however, national-scale differences in patterns are much less pronounced

for the arsenic concentrations of interest to this study. The

much larger data set used in the national-scale analyses in

comparison to the regional-scale analyses is an important

factor in minimizing the limitations of using data collected

from the different types of wells. No temporal variability in

arsenic concentrations was detected in data from most of the

wells analyzed, particularly those with arsenic concentrations at or below reporting levels. However, the variability of

arsenic concentrations over time in water from some wells,

1

Percent exceedance

0

40

20

60

80

100

2 5 10 20 50

Arsenic concentration, in μg/L

* Note: Data for 2, 10 and 50 μg/L were not available

U.S. Geological Survey

National Arsenic

Occurence Survey

(raw-water samples)*

1

Percent exceedance

0

30

25

20

15

10

5

2 5 10 20 50

Arsenic concentration, in μg/L

* Note: Data for 2, 10 and 50 μg/L were not available

U.S. Geological Survey

National Arsenic

Occurence Survey

(raw-water samples)*

0

40

20

60

80

100

1

Percent exceedance

2 5 10 20 50

Arsenic concentration, in μg/L

* Note: Data for 2, 10 and 50 μg/L were not available

U.S. Geological Survey

National Arsenic

Occurence Survey

(finished-water 

 projections)*
20

although probably not reflecting trend, can limit interpretations of the spatial distribution of arsenic concentration in

ground water when only one arsenic concentration is used to

represent the well. Other factors such as the density of sampling locations severely limits use of the data in some parts

of the country. Limitations caused by associating the arsenic

concentration data with the public water-supply data base

were assessed by comparing estimates of percentage of public water-supply systems that exceed targeted arsenic concentrations with similar estimates made by use of data in the

National Arsenic Occurrence Survey (NAOS). The differences in percentages were small (less than about 2 percent)

for the national comparisons but were larger (as much as

about 10 percent in one region) for regional comparisons

that were made to highlight the effects of the sizes of the

datasets and hydrogeologic variability at those scales.

Existing estimates of the occurrence of arsenic in

ground water that is 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 may be the additional

insight gained by establishing relations between the 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. Limitations of spatial and temporal factors due to sample density, type and depth of well,

and variability in arsenic concentrations over time should be

considered in any application of the data. Descriptive rather

than explanatory applications of the data are most appropriate because of uncertainties associated with the water-quality data and limitations in the ancillary information. With

these qualifications, the USGS data represent the groundwater 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 can be used in

association with the public water-supply data to determine

where targeted concentrations of arsenic are likely to occur

in the ground-water resources used for drinking water

within much of the United States.

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