United States
Environmental Protection
Agency
Office of Water
Office of Pesticides and
Toxic Substances
Fall 1990
vvEPA National Pesticide Survey
Survey Design
Survey
Design
The U.S. Environmental Protection Agency (EPA) has completed its five-year
National Survey of Pesticides in Drinking Water Wells (NPS). A joint project of EPA's
Office of Drinking Water (ODW) and Office of Pesticide Programs (OPP), the Survey
was designed to assess the extent and severity of the presence of pesticides and
nitrate in drinking water wells nationwide, and the relationship of pesticide use and
ground-water vulnerability to the presence of pesticides and nitrate. To achieve these
goals, EPA tested water samples from more than 1,300 community water system
(CWS) wells and rural domestic wells for the presence of 101 pesticides, 25 pesticide
degradates, and nitrate (a total of 127 Survey analytes). EPA also collected detailed
information about the wells' characteristics and condition, as well as surrounding
circumstances such as nearby pesticide use and agricultural activities.
The focus of the Survey was on the quality of water in drinking water wells
before treatment rather than on the quality of drinking water at the tap. The Survey
does not provide a representative assessment of the presence of pesticides or nitrate
for specific local areas, counties, or States, nor does it assess the presence of
pesticides or nitrate in surface or ground water.
This fact sheet describes the key elements of the Survey design, including
design tasks, scheduling, questionnaires, and design review. These elements ensure
that the design was statistically sound and capable of producing information of a
known and useful quality.
The primary task in designing the Survey was to select a sample of wells that
would be representative of the nation's drinking water wells. Accomplishing this task
involved a complex process of scientific and statistical calculations, as well as budget,
policy, and program considerations. The basic elements of the Survey design and the
process of choosing wells are described below.
Defining the Population. EPA's first step in survey design was to define
carefully the two major categories of wells that it wanted to study. They were
community water system wells (managed by public water suppliers) and rural
domestic wells.
• Community water systems are defined as systems of piped drinking
water with at least 15 connections and/or 25 or more permanent
residents of the service area that have at least one working well
used to obtain drinking water.
• Rural domestic wells are defined as drinking water wells supplying
occupied housing units located in rural areas of the United States,
except for wells located on government reservations. Rural
households are defined as households located outside of
incorporated or unincorporated places with populations of 2,500 or
A/PS Survey Design
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more, or located outside of areas designated as urban fringe by the
Census Bureau.
With these overall categories of wells defined, EPA next set up a process for
selecting wells from those two categories for the Survey.
Determining the Size and Accuracy of the Survey. In order to determine how
many wells to visit for data collection, EPA first needed to identify approximately how
many drinking water wells exist in the United States. This process was easier for
community water systems than for rural domestic wells because a list of all public
water systems, with their addresses, is contained in the Federal Reporting Data
System (FRDS), which is maintained by EPA. From FRDS, EPA estimated that there
were approximately 51,000 CWSs with wells in the United States. EPA did not have a
comprehensive list of rural domestic wells to serve as the foundation for well selection,
as it did for CWSs. Using data from the Census Bureau for 1980, EPA estimated that
there were approximately 13 million rural domestic wells in the country, but the specific
owners and addresses of these rural domestic wells were not known.
EPA chose a survey design technique called "stratification" to ensure that survey
data would meet its objectives. This technique was used to improve the precision of
the estimates by selecting extra wells from areas with substantial agricultural activity
and high susceptibility to ground-water pollution (vulnerability). EPA developed criteria
for separating the population of CWS wells and rural domestic wells into four
categories of pesticide use and three relative ground-water vulnerability measures.
This design ensures that the range of variability that exists nationally with respect to
the agricultural use of pesticides and ground-water vulnerability is reflected in the
sample of wells.
EPA identified five subgroups of wells for which it was interested in obtaining
information. These subgroups were community water system wells in counties with
relatively high average ground-water vulnerability; rural domestic wells in counties with
relatively high average ground-water vulnerability; rural domestic wells in counties with
high pesticide use; rural domestic wells in counties with both high pesticide use and
relatively high average ground-water vulnerability; and rural domestic wells in "cropped
and vulnerable" parts of counties (high pesticide use and relatively high ground-water
vulnerability).
Two of the most difficult design questions were determining how many wells to
include in the Survey and determining the level of precision that would be sought for
the NPS national estimates. These two questions were connected, because greater
precision is usually obtained by collecting more data. Resolving these questions
would have been simpler if the Survey designers had known in advance what
proportion of wells in the nation contained pesticides, but answering that question was
one of the purposes of the Survey. Although many State studies have been
conducted for specific pesticides, no reliable national estimates of well water
contamination existed. EPA evaluated alternative precision requirements and costs for
collecting data from different numbers of wells to determine the Survey size that would
meet EPA's requirements and budget.
The Survey designers ultimately selected wells for data collection so that the
Survey provided a 90 percent probability of detecting the presence of pesticides in the
CWS wells sampled, assuming 0.5 percent of all community water system wells in the
country contained pesticides. The rural domestic well Survey design was structured
with different probabilities of detection for the several subgroups of interest, with the
greatest emphasis placed on the cropped and vulnerable subcounty areas, where
EPA was interested in obtaining very precise estimates of pesticide occurrence. EPA
assumed that 1 percent of rural domestic wells in these areas would contain
NPS Survey Design
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pesticides and designed the Survey to have about a 97 percent probability of
detection in "cropped and vulnerable" areas if the assumption proved accurate. EPA
concluded that sampling approximately 1,300 wells (564 public wells and 734 private
wells) would meet the Survey's accuracy specifications and provide a representative
national assessment of the number of wells containing pesticides.
Selecting Wells for the Survey. Because the exact number and location of
rural domestic wells was unknown, EPA chose a survey design composed of several
steps (stages) for those wells. The design began with a sampling of counties, and
then characterized pesticide use and ground-water vulnerability for subcounty areas.
This eventually allowed small enough geographic areas to be delineated to enable the
sampling of individual rural domestic wells. This procedure was not needed for
community water system wells, because their number and location were known.
The first step in well selection was common to both CWS wells and rural
domestic wells. Each of the 3,137 counties or county equivalents in the U.S. was
characterized according to pesticide use and ground-water vulnerability to ensure that
the variability in agricultural pesticide use and ground-water vulnerability was reflected
in the Survey. EPA used data on agricultural pesticide use obtained from a marketing
research source and information on the proportion of the county area that was in
agricultural production to rank agricultural pesticide use for each county as high,
medium, low, or uncommon. Ground-water vulnerability of each county was estimated
using a numerical classification system called Agricultural DRASTIC, which assesses
seven factors: (depth of water, recharge, aquifer media, soil media, topography,
impact of unsaturated zone, conductivity of the aquifer). The model was modified for
the Survey to evaluate the vulnerability of aquifers to pesticide and nitrate
contamination, and one of the subsidiary purposes of the Survey was to assess the
effectiveness of the DRASTIC classification. Each area was evaluated and received a
score of high, moderate, or low, based on information obtained from U.S. Geological
Survey maps, U.S. Department of Agriculture soil survey maps and other resources
from State agencies, associations, and universities. The following two maps show the
national characterization of pesticide use and ground-water vulnerability by county.
Exhibit 1 shows the resulting table of 12 strata for pesticide use and relative
ground-water vulnerability and the number of U.S. counties ranked within each
stratum.
Exhibit 1: Strata for NPS Survey Design
Strata
1
2
3
4
5
6
7
8
9
10
11
12
Ground-Water Vulnerability Number of
Pesticide Use (as estimated by DRASTIC) Counties
High
High
High
Moderate
Moderate
Moderate
Low
Low
Low
Uncommon
Uncommon
Uncommon
High
Moderate
Low
High
Moderate
Low
High
Moderate
Low
High
Moderate
Low
106
234
129
110
204
267
193
375
404
186
513
416
3,137
A/PS Survey Design
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First-Stage Characterization of Pesticide Use by County
for the United States
£ 2
NPS Survey Design
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First-Stage Characterization of Relative Ground-Water Vulnerability
by County for the United States
WPS Survey Design
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Scheduling
Question-
naires
The selection of the final sample of CWSs and domestic wells and the later
stages of stratification in the selection process are described below.
Community Water Systems. After the first stage of stratification, EPA randomly
selected approximately 7,000 CWSs from the total population of eligible systems
contained in FRDS. The selection process, though random, was designed to sample a
slightly larger proportion of those CWSs in the high ground-water vulnerability stratum
because EPA wanted to ensure that estimates for areas sensitive to contamination
achieved greater precision in the Survey. For the second stage of stratification, EPA
contacted the systems by telephone to determine their operating status, confirm the
number of wells, and obtain cooperation for sampling. Based on the results of this
screening process, EPA selected eligible systems in about 390 counties distributed
throughout all 50 States. EPA gathered water samples from 566 community water
system wells. At those community water systems chosen for sampling that had more
than one eligible well, the actual well to be sampled was chosen randomly. CWSs
with large numbers of eligible wells could be chosen for sampling more than once, but
a different well was sampled each time the CWS was included. Wells were sampled
from all twelve strata.
Domestic Wells. EPA did not have a comprehensive list of domestic wells to
serve as the basis for well selection, as it did for CWS wells. EPA used additional data
to select domestic wells for the Survey. In the 90 counties in 38 States that EPA
selected as areas for domestic well sampling in the first stage of stratification, a more
detailed assessment of ground-water vulnerability was conducted that resulted in
development of subcounty DRASTIC maps for each of these counties. Additional
information on cropping intensity (a surrogate measure for agricultural pesticide use)
was also collected and combined with the subcounty DRASTIC maps to delineate
areas with varying degrees of ground-water vulnerability .to pesticide contamination
(second-stage stratification). These areas were used to select a slightly larger
proportion of wells in the cropped and vulnerable parts of counties during the final
stage of well selection. During this last stage, EPA conducted telephone interviews
with well owners within each of the 90 counties to determine the eligibility of potentially
participating households and obtain their cooperation. Based on the results of the
subcounty scores and telephone interviews, EPA gathered water samples from 783
rural domestic drinking water wells for the Survey. Wells were sampled from all twelve
strata.
EPA prepared a sampling schedule that provided for well water samples to be
taken from both community water system wells and rural domestic wells during all
seasons and pesticide application cycles. EPA's scheduling approach minimized the
effect on NPS findings of weather conditions and pesticide applications in any
particular season. To accomplish this, sampling at the selected community water
systems and rural domestic wells in each stratum was randomly assigned to two week
time periods across the data collection period (April 1988 to February 1990).
During the well selection and actual sampling, EPA used questionnaires
designed for the Survey to collect detailed information on individual wells, water
systems, agricultural and non-agricultural pesticide use, and on geologic
characteristics in the surrounding areas. The questionnaires proved to be very
valuable in gathering important information from CWS owners and operators, domestic
well owners, county agricultural extension agents, and farmers. The questionnaire
data, together with the results of the water quality analyses, provide the basis for
exploring the relationships between pesticide use, ground-water vulnerability, and the
probability of contamination of drinking water wells. The analysis of these
relationships will be provided in the Survey Phase II report, expected Spring 1991.
WPS Survey Design
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Design
Review
Survey
Design
Implemented
Where to Go
for More
Information
A subpanel of EPA's Federal Insecticide, Fungicide, and Rodenticide Act
(FIFRA) Scientific Advisory Panel (SAP) reviewed the design of the Survey in 1985 and
again in 1987. The SAP subpanel made recommendations on several aspects of the
design, including the type and complexity of data needed to carry out stratification, the
proposed method for selecting CWSs, and the potential problem of seasonal
variations influencing the presence of contaminants in the wells. EPA made
appropriate changes to the Survey design to incorporate the subpanel's
recommendations.
EPA conducted sampling of CWS wells and rural domestic wells nationally from
April 1988 to February 1990. Laboratory analysis was completed in May 1990. EPA's
Phase I Report on the Survey findings was released in Fall 1990. A Phase II Report,
including the relational analysis of results, is scheduled for release in Spring 1991.
This fact sheet is part of a series of NPS outreach materials, fact sheets and
reports. The following additional fact sheets are available through EPA's Public
Information Center (401 M Street SW, Washington DC 20460, 202-382-2080):
Project Summary
Summary Results
Fact Sheet for each
detected analyte
Analytical Methods
How EPA Will Use
The NPS Results
Survey Analytes
Glossary
Quality Assurance/
Quality Control
Additional information on the Survey and on pesticides in general can be
obtained from the following sources:
U.S. EPA Safe Drinking Water Hotline
1 -800-426-4791 (In Washington, DC - 382-5533)
Monday-Friday, 8:30 am to 4:30 pm Eastern Time
National Pesticide Telecommunications Network
1-800-858-7378
24 hours a day
U.S. EPA Office of Pesticide Programs (OPP) Docket
401 M Street, SW Room NEG004
Washington, DC 20460
(202) 382-3587
National Technical Information Service (NTIS)
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
Information on regulation of
pesticides in drinking
water
Information on health
effects and safe
handling of pesticides
Background documents
for Survey (available
for review)
Copies of the
NPS Phase I Report
(available 1991) and
NPS Phase II Report
(when available)
If you are concerned about the presence of pesticides and nitrate in your private water
well, contact you local or State health department. Other experts in your State
environmental agency or agriculture and health department may also be helpful to
you. If you receive your drinking water from a community water system and have
questions about your water quality, contact your local community water system
owner/operator or the State water supply agency.
WPS Survey Design
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