Introduction and Purpose
Domestic water wells supply water to 17.6% of the households in the
upper midwestern states (Table 1). In the spring of 1993, flood waters
covered some of the water wells in the Missouri and Mississippi river basins. River
flooding can affect groundwater quality by raising the water table, altering hydraulic
gradients, recharging from different areas, or flowing directly down the well casing. Many
residents who tested water from their domestic well after the flood waters receded
reported the presence of coliform bacteria or E. coli in these samples.
TABLE 1.
Sources of drinking water for households in nine Midwestern States
State |
Drinking Water Source* (percent) |
Number
of Households |
Public |
Private |
Other |
Iowa |
1,143,669 |
81.1 |
18.4 |
0.5 |
Illinois |
4,506,275 |
89.8 |
09.8 |
0.4 |
Kansas |
1,044,112 |
89.5 |
10.0 |
0.5 |
Minnesota |
1,848,445 |
83.7 |
15.3 |
1.0 |
Missouri |
2,199,129 |
73.0 |
26.2 |
0.8 |
Nebraska |
660,621 |
82.9 |
16.9 |
0.2 |
North Dakota |
276,340 |
79.0 |
19.2 |
1.8 |
South Dakota |
292,436 |
81.4 |
16.7 |
1.9 |
Wisconsin |
2,055,774 |
66.5 |
32.8 |
0.6 |
Total |
14,027,611 |
81.8 |
17.6 |
0.6 |
US |
102,263,678 |
84.2 |
14.8 |
1.0 |
*The US Census defines
a public water source as one that provides water for five or more houses, apartments,
or mobile homes and a private water source as one that provides water for four or fewer
houses, apartments, or mobile homes
Source: 1990 US Census
The coliform group of bacteria is recognized as a
microbial indicator of drinking water quality because these bacteria are commonly found in
the environment, are present in large numbers in feces, and are easily detected by simple
laboratory methods. E. coli, a member of the coliform group, is found only in
fecal material. The presence of coliform bacteria in a water system indicates
vulnerability to contamination and ineffective disinfection whereas the presence of E.
coli indicates fecal pollution. People drinking water with these bacteria are at
increased risk of contracting a waterborne disease.
In addition to measuring bacteria, samples were
collected for nitrate and atrazine analysis. The major sources of nitrate in groundwater
include fertilizers, animal manure, seepage from septic systems, and atmospheric fallout
from combustion of fossil fuel. Background levels of nitrate in ground water may reach 3
mg/L because of natural decomposition and soil bacteria. Higher nitrate levels are
associated with anthropogenic activity (Mueller et al., 1995). The Environmental
Protection Agency (EPA) established an MCL of 10 mg/L for nitrate-nitrogen in public water
systems (EPA, 1994) because infants are particularly susceptible to nitrate and may
develop methemoglobinemia (Coomley, 1945).
Triazines are organic herbicides introduced in the
1950s. These synthetic chemicals are among the most widely used and effective herbicides
in the world. In the Midwest, atrazine is used seasonally to control grassy and broadleaf
weeds in corn and wheat fields. The chemical is applied to the surface of the land and
degrades quickly when exposed to light. However, the half-life of atrazine in soil or
water is several months (EPA, 1984a). Atrazine is the most commonly found herbicide in
ground and surface water because of its high use, persistence in the environment, and
ability to dissolve in water. The chemical is mutagenic in bacteria and is considered a
possible human carcinogen (IARC, 1991).
The purpose of the survey was to measure levels of
coliform bacteria, E. coli, nitrate, and atrazine in water collected from
households that are supplied water from a domestic well water system in nine midwestern
states. This concern originated when many water samples from rural wells collected shortly
after the 1993 midwest floods tested positive for coliform bacteria or E. coli.
Public health officials from the affected states and from federal agencies met to discuss
the contamination. They concluded that the available data was insufficient to characterize
the nature and magnitude of the situation. They agreed to conduct a survey of the
geographic distribution of chemical and bacteriological contamination of water from
domestic well water systems in the affected states. The survey would collect information
on the construction, maintenance, and condition of the well. To correlate health effects
with contamination, participants in the survey would be asked whether they had a diarrheal
episode in the 2 weeks before the water sample was collected from their house.
Methods
Any household in the nine upper midwestern states that
used a domestic well to supply water for drinking, cooking, or bathing was eligible for
the survey. The EPA defines a public water system as having at least 15 service
connections or regularly serves an average of 25 people daily for 60 days out of the year
(EPA, 1995). In this survey, a domestic well had fewer than 15 service connections and
regularly served fewer than 25 people. Field personnel collected a water sample from the
household closest to and within 3 miles of each intersection of a 10 mile grid overlaid on
the 9 states. The grid was constructed by randomly choosing a starting point outside the
9-state region as the lower left corner (Gulf of Mexico). ArcInfo (Environmental Systems
Research Inc., 1993) was the primary geographic information system (GIS) used to construct
the sampling grid. This program also generated a list of the latitude and longitude of
each grid intersection, a unique identification number for each intersection, and printed
maps of each county showing the major rivers, roads, and railroads in the county, and the
location and the unique identification number of each sampling unit (the area within a
3-mile radius of the intersect) in the county ( Figure
1 ).
Figure 1.
A county map used to locate households to be sampled in the 1994 Midwest Well Water
Survey. Households nearest to the intersection and within the circle and county
sampled.
When a sampling unit included more than one county,
field personnel did not enter the adjacent county to collect that sample. Most field
personnel were familiar with the area in which they were assigned to collect samples.
Real-estate plats, U.S. Geological Survey quadrangles, and municipal maps were also used
to locate the households to be sampled. Field personnel were employed by the state agency
that was conducting the survey.
A systematic geographical sampling approach was used
because a list of domestic wells was not available and variables that affect water supply
and quality (e.g. geology, soil type, topography, land use, etc.) are not randomly
distributed. In addition, conducting a census of wells in each sampling unit would have
been difficult and time-consuming.
Collection of water samples
Water samples were collected from May to November 1994. Field personnel located the
household closest to the grid intersection and asked an adult resident for permission to
collect a water sample. An eligible household received water from a domestic well, had at
least one member who drank the water, and was within 3 miles of the intersection. In
addition, the well must not have been chlorinated in the previous 4 days because chlorine
that was used to disinfect the well may still be present. If the resident declined to
participate or the well did not meet enlistment criteria, the field personnel proceeded to
the next closest household. If no well was sampled in the designated sampling unit, field
personnel proceeded to the next sampling unit. When no households with wells could be
found in several sampling units within a county, the sampling unit within that county was
extended to a 5-mile radius from the grid intersection.
When a household member granted permission, field personnel marked the
approximate location of the sampled well on the survey map or recorded the latitude and
longitude of the sampled well if geographical positioning system instruments were
available. Water samples were collected from the faucet most commonly used to provide
drinking water. When possible, aerators, strainers, hoses, water treatment devices, or
other attachments were removed before the sample was collected. Taps were sanitized by
wiping the inside and outside of the tap with a paper towel or cotton-tipped swab
saturated with 100 mg/L sodium hypochlorite. The tap was opened fully for 3 to 5 minutes
prior to sampling, and then the water flow was reduced during sample collection. The
sample bottle cap was removed, and without rinsing, sufficient water was collected to fill
four-fifths of the container. Water was collected in polyethylene bottles for
bacteriologic analysis. Two milliliters of dilute sulfuric acid were added to the sample
bottle for nitrate and atrazine analysis. The caps were immediately replaced without
touching the interior of the cap or container. After collection, samples were placed on
ice until they were delivered to the state laboratory. Microbiology testing begun within
30 hours of collection.
Duplicate samples were chosen in advance. In each state, the survey
coordinator decided the rate at which duplicate samples were collected -- usually every
eighth, ninth or tenth household -- and maintained this frequency throughout the state.
Field surveyors collected the duplicate samples at the preselected rate. If no sample
could be collected at the designated site, the sample was collected at the next available
sample site.
Data Collection Form
In addition to collecting water samples, field
personnel interviewed survey participants to obtain information on the construction,
condition, and maintenance of the well; the potential sources of contamination; the number
of people drinking water from the well; and the occurrence of diarrhea in the household (Appendix I). For most wells, a sanitary survey was
performed to determine the condition of the well; the character of local geography; and
the nature, distance, and location of potential pollution sources in the area.
Laboratory Analysis
Coliform Bacteria and E. coli. A
10-tube assay (Colilert, IDEXX Laboratories Inc., 1994) measured the concentration of
coliform bacteria and E. coli in the water samples. In this procedure, an aliquot of the
sample is placed in each of ten tubes containing nutrient broth and indicator chemicals.
The broth turns yellow when coliform bacteria metabolize O-Nitrophenol-b-d-galactopyranoside and fluoresces under ultraviolet light when E. coli
breaks down 4-methylumbellifery-b-d-glucuronide. The medium contains chemicals tha
suppress the growth of noncoliform bacteria. The result, number of coliform bacteria or E.
coli per 100 mL, is a statistical estimate of the mean density of bacteria in a water
sample and is based on the number of samples testing positive. The assay had a
quantitative range from 1.1 (95% confidence interval 0.0., 5.9) to 23 (95% confidence
interval 8.1, 59.5) bacteria per 100 mL.
Nitrate. The colorimetric, automated,
cadmium reduction method (APHA, 1992) measured nitrate concentrations as milligrams
nitrate-nitrogen per liter (mg/L NO3-N). The preserved water sample was filtered and
passed through a column containing granulated copper-cadmium. This step converts nitrate
(NO3) tonitrite (NO2), which forms an azo dye when sulfanilamide couples with
N-(1-naphthyl)-ethylenediamine dihydrochloride. The azo dye is measured colorimetrically
and is proportional to the amount of nitrate in the sample. This assay had a limit of
detection of 0.01 mg/L.
Atrazine. An enzyme-linked
immunosorbent assay measured atrazine in the water samples (Ohmicron, 1995). This method
used atrazine-selective antibodies linked to a peroxidase enzyme detector system. In the
presence of atrazine, a colored product is formed that is inversely proportional to the
concentration of triazines in solution. As with most immunoassays, structurally related
chemicals may cross-react with the antibody. These include other triazines such as
cyanazine, simazine, and terbutryn and the atrazine metabolites 6-hydroxy atrazine and,
desisopropyl atrazine. This assay had a limit of detection of 0.05 ppb.
Quality Assurance
In an effort to produce data that is precise and
comparable, standard protocols for sample collection and analysis were established by the
laboratories conducting the water analysis. One quality control procedure involved
collecting duplicate samples for every eighth to tenth well. The difference between the
original and the duplicate samples for coliform bacteria, E. coli, or nitrate was not
statistically significant (p = 0.14, student=s t-test). Other quality control measures
used by the laboratories included standardized sample collection and transport procedures;
standard solutions, reagents, and preservatives; and use of analytical reagents with the
same lot number for the Colilert and the atrazine assays. Laboratories also performed
routine internal quality control procedures.
Data Analysis
Data entry. State survey coordinators mailed
completed data collection forms, county maps, lists of well identification numbers, and
the latitude and longitude of each well, when available, to CDC. Forms were examined for
completeness and logged into a program that monitored the progress of each form in the
data-entry process. The latitude and longitude of each well were entered into an ArcInfo
data base. The data were double-entered. Each state's well survey manager reviewed a data
base of the information of the wells sampled in their state.
Contamination levels. The EPA
established limits on the level of contaminants in drinking water to ensure that public
water systems deliver water that is safe for human consumption. These limits are known as
the maximum contaminant levels (MCLs) -- the highest allowable amount of a contaminant
that a public water supply can deliver to a consumer. A violation occurs when an MCL is
exceeded. The MCL is 10 mg/L for nitrate and 3 ppb for atrazine (EPA, 1994). For
bacteriological monitoring, the EPA established the total coliform rule, which states that
any water sample that tests positive for coliform bacteria must be analyzed for fecal
coliform or E. coli. A positive test result is when coliform bacteria or E. coli
concentration is at least one per 100 mL of sample. A repeat test is conducted for each
positive sample and samples are collected within 24 hours of a positive test result. A
violation occurs when coliform bacteria or E. coli are present in both the initial and
repeat sample. While these standards pertain to public water systems, they served as
guidelines for assessing the quality of water collected in this survey. Thus a water
sample collected from a household served by a domestic well was considered to be
contaminated if coliform bacteria or E. coli concentrations were detected, if nitrate
concentrations exceeded 10 mg/L, or if atrazine levels were above 3 ppb.
Statistical analysis. Odds ratios were
calculated to order to determine the strength of the association between a well feature
(e.g., depth, presence of cracks in casing, pesticide use near the wellhead) and the
presence of contaminants in the water samples (coliform bacteria, E. coli, nitrate, or
atrazine). Results for atrazine are not reported because only 0.2% of the samples had
levels that exceeded the MCL. An odds ratio less than one indicates that the well feature
was associated with a lower contamination rate than the wells without that feature, an
odds ratio greater than one implies that the well feature was associated with a higher
contamination rate, and an odds ratio of one shows that the well feature had no
association with the contamination rate. To examine the association between well
construction and contamination, we chose drilled wells as the reference because they
constituted the largest group and had samples with one of the lowest rates of
contamination.
Epi Info version 6.0 was used for the descriptive
analysis and calculation of odds ratios (Dean et al., 1994). SAS version 6.10 (SAS
Institute Inc., 1991) was used to run the logistic regression to examine for associations
between the analytes and well depth, age, and casing diameter. ArcInfo (Environmental
Systems Research Inc., 1993) and MapInfo (MapInfo Inc., 1994) were used in the descriptive
analysis of the spatial distribution of the analytes. ArcInfo was also used to examine for
associations between the analytes and well location, political boundaries, bodies of
water, soil type, household income, and the presence of multiple analytes in water
samples.
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