PESTICIDE AND NITRATE CONTAMINATION
OF GROUND MATER NEAR ONTARIO, OREGON
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PESTICIDE AND NITRATE CONTAMINATION
OF GROUND MATER NEAR ONTARIO, OREGON
Glenn R. Bruck
U.S. Environmental Protection Agency
1200 Sixth Avenue
Seattle, Washington 98101
ABSTRACT
Since 1983 the agricultural area south and west of Ontario, Oregon,
has been investigated by the U.S. Environmental Protection Agency (EPA)
1n order to develop a better understanding of the high nitrate levels
found 1n ground water there. The study has Included samples from 50
wells, and nitrate levels as high as 49 mg/L n1trate-N have been found.
In May 1985, 34 wells were resampled for nitrates, and seven of those
samples were also analyzed for 13 pesticides and their common
metabolites. These 13 compounds are known ground water Teachers that are
commonly used 1n the area. All seven samples were found to contain the
pesticide DCPA In concentrations of between 0.11 and 290 ug/L. A second,
March 1986, round of 25 samples contained DCPA concentrations of between
0.27 and 275 ug/L, thus confirming the previous findings and Indicating
widespread contamination throughout the study area.
In this area DCPA is used extensively on onions, which are planted
1n the well-drained, deep silt-loam soils of the Owyhee-Greenleaf
series. The sampled wells penetrate an alluvial aquifer and are
perforated at depths of between 20 and 70 feet from the surface. Because
annual precipitation averages only about 12 inches per year, flood
irrigation provides an additional 5 acre-feet per acre of supplemental
water; thus, approximately 80 percent of the water available for ground
water recharge is from diverted surface waters.
All wells sampled in this survey are drinking water supplies. The
drinking water standard for nitrates is 10 mg/L as nitrate-N. In 1982
EPA established a drinking water draft health advisory for DCPA of
500 ug/L for longer-term exposure. At the present time, this has not
been exceeded, and public reaction has been subdued. EPA is cooperating
with agricultural, health, and environmental agencies on the state and
local level in a comprehensive ground water quality investigation
covering an area of approximately 10,000 acres. Preliminary results
indicate that there is contamination over much of this area.
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IntroductIon 2
Between 1979 and 1983, routIne monitoring of drinking water samples
from small water supply wells In the Ontario, Oregon, area (Figure ‘1)
Indicated a persistent problem of high nitrate—Nitrogen (nitrate-N)
levels. The drinking water standard for this contaminant is 10 mg/L
(parts per million), and many of the samples from wells had levels
ranging as high as 49 rng/L. U.S Environmental Protection Agency (EPA)
and state regulations, under the Safe Drinking Water Act, do not allow
the distribution of water that exceeds these standards for human
consumption. Several non-cotrinunity water systems in the area have had
great difficulty In achieving compliance with the standard, since the
removal of nitrates by water treatment is both difficult and expensive.
Due to their location, most of these systems could not readily connect
with the nearest community water supply.
In an effortto better understand this problem, EPA began a limited
ground water study aimed at determining if a specific nitrate source
could be identif led. Defining the problem would be the first step toward
the goal of ensuring that residents of the area were drinking the safest
water possible. Key to this study was the establishment of a ground
water sampling program Intended to both define the limits of the affected
area and provide nitrate concentration data that would be useful in
describing a subsurface plume of contamination.
Area Hydrogeology
From the onset of this project, it was realized that a basic
understanding of the hydrogeology In the area would be necessary.
Efforts were made to obtain all available driller’s well logs, along with
any ground water investigations and reports. A review of these disclosed
several limiting factors. The logs showed that the elevations of the
well heads are not precisely known; thus, there is no accurate way to
reference the water level measurements taken at any of the wells.
Without this, the true water table depth beneath the area cannot be
determined. Also, the geologic descriptions in the logs lacked accuracy,
making subsurface correlations difficult.
The logs do provide some valuable information regarding the well
construction. Each of the wells scheduled for sampling in the survey was
originally sealed to the surface, and the point of the shallowest
perforations in the well casings ranges from 20 to 70 feet from the
surface. The logs also reveal that the geology of the saturated zone
consists of interbedded gravel, sand, silt, and clay deposited as
alluvial sediments.
A single U.S. Geological Survey observation well, completed to a
depth of 135 feet. is located near the center of the study area. Water
levels in this well have been measured since 1950 and are presented as a
hydrograph in Figure 2. The data show a fairly constant ground water
level of between 9 and 11 feet from the surface, with seasonal
fluctuations as great as 7 feet. In a ground water report concerning the
region, it was observed that, overall, water table levels are stable,
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3
Figure 1
LOCATION MAP
Hydrograph of USGS Observation Well
‘‘ d t ’Y ‘YT’T HYT TY ’
1950
1 6u
I 9u’.
1570
6
S
S 10
0•
— IL 12
CC
— 14
• 16
a
Figure 2
(ColUns,1978)
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1ndIca lng that ground water recharge and discharge are generally In
balance (Collins, 1979).
The combination of limited published data and variable well log
infonnati on akes the Important task of estimating ground water flow
largely speculative. In the absence of more reliable data, it seems
reasonable to presi.aiie that ground water flow generally confonus to the
area’s surface topography. In the Ontario study area, this implies a
regional flow tending northward toward the Maiheur and Snake River flood
plains, where ft eventually parallels and then joins the respective flow
gradients of the rivers (Figure 3).
Land Elevations and
Estimated Ground Water Flow
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)bvious1y, there are many factors directly affecting ground water
flow that cannot be accounted for given the limited resources of this
Investigation. The area is crossed by unlined Irrigation canals and
ditches. Most parts of the study area have relatively low slopes of
2 percent or less, and there are many fields that receive intensive
irrigation. These factors could cause localized ground water mounding In
the water table and add greatly to the difficulty of estimating ground
water flow.
Nitrate Sampling and Results
In June 1983, EPA, in cooperation with Malheur County Sanltarjan’s
Office, collected the initial round of 45 samples for nitrate-N
analysis. The results showed concentrations ranging from 0 to 39.2 mg/L,
and the nitrate concentrations did not correlate with the depth of well
perforations. The concentration data are plotted In figure 4, which
shows contoured nitrate-N values in 10 mg/L intervals over the study
area. The results from these samples show that there Is not a
well—defined plume that would be characteristic of a single contaminant
source; instead, there appear to be broad areas with higher and lower
concentrations, Indicating the existence of several different sources of
nitrate—N. In addition, it is apparent that there are fairly large
quantities of the contaminant present throughout the area.
As part of the ongoing monitoring program, a second round of 35
samples was collected In August 1983. NItrate-N concentrations detected
In these samples ranged from 0 to 49 mg/I, and the results are plotted In
Figure 5. A contour pattern somewhat similar to that In the June 1983
data emerges. The areas of higher concentration have shifted to some
degree, but once again a distinctive flow pattern or plume does not
become readily apparent.
Pesticides
Resource limitations precluded sampling during 1984, but a third
round of sampling was scheduled for the spring of 1985.
With this third round of samples, It was decided to expand the scope
of the investigation in order to determine if any of the pesticides
comonly used In the area might also be affecting the ground water
quality.
The method used to establish which pesticides to look for in the
ground water samples consisted of comparing a list of pesticides known to
leach into ground water with a list of those conv only used in the area.
The overlap of these two became the final list of 13 target compounds
requested for laboratory analysis (Table 1). In the face of limited lab
resources, it was agreed that only seven samples could be afforded a
complete pesticide scan.
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N03-N ONTARiO, OREGON June, 1983
Figure 4
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N03-N ONTARIO, OREGON August, 1983
Figure 5
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TABLE 1 8
List of Pesticides for Analysis in
Ontario Ground Water Samples
May 1985
Comon Name
Alachior
Carbofuran
Cyanazi ne
OCPA
Di noseb
Di chi oropropane/propene
ED8
Fonofos
Hexazi none
Metribuzi n
Simazine
2,4-0
2,4-08
In May 1985, 34 ground water samples were collected for nitrate-N
analysis; seven of these were chosen for the more detailed pesticide
scans. The results of this third round of nitrate—N sampling are plotted
In Figure 6. A pattern somewhat similar to that of the previous two
rounds emerges. It Is significant to note that tn essence, there appears
to have been lIttle change in the nitrate situation over a two-year
period.
The results from these pesticide scans Indicated the presence of a
hydrolized form of the herbicide DCPA (Dirnethyl tetrachloroterephthalate)
In all seven of the samples, which were collected from widely—spaced
wells. The concentrations detected in the samples ranged from 0.1 to
290 ug/L (parts per billion).
Public Health Implications
In 1982 EPA established a drinking water draft Health Advisory of
500 ug/L DCPA (and metaboittes) for longer-term exposure. Since all of
the samples collected In this study were from drinking water wells and
the levels found were greater than one half of the advisory value, it was
Ininedlately realized that there was a potential for endangerment of
public health.
In light of these concerns, EPA, Oregon State Health Division, and
Malheur County Sanitarian’s Office jointly decided that the local or
county sanitarian’s office should publicly release the preliminary
pesticide results. A press release was drafted and issued to the local
newspapers.
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N03—N ONTARIO, OREGON May , 1985
Contour Interval lOmg/L
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Public reaction to the sample results was generally subdued, and the to
subsequent newspaper and television reports tended not to sensationalize
the problem.
Before deciding on the next course of action, EPA met with a diverse
group of state and local agencies representing agricultural, health, and
environmental concerns. During this meeting all of the agency
representatives agreed that the sampling effort should be greatly
expanded and that their respective agencies should participate in the
sampling and follow—up activities.
Two imediate objectives were identifIed: (1) to determine the size
of the affected area, and (2) to see if even higher levels were present
in unsampled areas. In order to meet these objectives, the following
recoimnendations were incorporated into a comprehensive sampling plan:
1. The agricultural agencies should proceed. to determine what
additional areas should be sampled, while the health and
environmental agencies should determine the most suitable well sites
and field conditions for sample collection.
2. The samples should be collected in the middle of March, before the
widespread application of agricultural chemicals begins for the
growing season.
3. Samples should be collected from those areas in the region where
onions had been grown because onion crops characteristically receive
6 lbs/acre active DCPA as a pre—emergent herbicide. Onion producers
constitute the largest users of OCPA in the region.
4. Aldicarb is comonly used in the area and has serious health
implications. Thus, EPA laboratory capabilities should be expanded
in order to analyze ten samples for that compound as well.
5. Between 90 and 100 samples should be collected over an agricultural
area totaling approximately 10,000 acres in the northern half of
Malheur County.
6. The samples should be divided between different laboratories in
order to maintain quality assurance. A three—way division between
EPA, Oregon State Univers ity, and a major producer of the herbicide
was decided upon.
Sample collection for this second follow—up effort was completed in
March 1986, when five sampling teams collected 96 samples.
As of this writing, results from all three laboratories are not yet
available. However, those samples from the Ontario portion of the study
area are available, since they were analyzed by the EPA laboratory. The
nitrate-N values in these samples range from 0.4 to 36 mg/i. and are
plotted in a contour pattern somewhat similar to that of the previous
nitrate-N data (Figure 7). OCPA was again detected In the samples in
concentrations ranging from 0 to 215 ugh. A contour plot of the DCPA
results is shown in Figure 8. Several of the samples collected from
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N03-N ONTARIO, OREGON March, 1986
Figure 7
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DCPA ONTARIO, OREGON March, 1986
Figure 8
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outside the Ontario area were found to contain DCPA as well, thus 13
indicating a more widespread problem. Results from the ten aldicarb
analyses were all negative.
Implications for the Regional Agricultural Economy
DCPA ‘is presently the only effective pre—emergent herbicide
registered for use on onions in Malheur County. Onions rank second,
behind cattle production, in terms of the county’s agricultural income.
Most farmers in the area would prefer to use the more effective herbicide
propachior either alone or with lower amounts of DCPA. The use of
propachlor on onions in Oregon was permitted from 1982 to 1984 through
section 18, emergency registrations, which were issued annually. In
1985, issuance was denied because the registrant had failed to make
adequate progress in pursuing a section 3 or ‘normal” registration to
allow for its use on onions.
Optimistic projections indicate that the data requirements for
propachlor registration (section 3) for existing crops could be met by
1988. Unless that data contains adequate information regarding use on
onions, the chemical will still remain unavailable to growers In Malheur
County. It Is these problems with propachior’s registration that have
resulted In the extensive use of the less effective DCPA.
Agriculture and Ground Water in the Ontario Area
In an EPA contract study, W. M. Mack provided a comprehensive
analysis of aerial photographs of the Ontario area (1983). The photos
cover the time period from 1946 to 1983; throughout that time the area
was predominantly crop land. No waste disposal sites or industrial
sources of water pollution could be identified. The report concluded by
stating that agricultural practices were the most likely source of
nitrate contamination of ground water.
Data from the present study further supports such a conclusion. The
role of agriculture appears to be significant for many reasons. Farming
is the major land use in the affected area. Irrigation records show that
the average cultivated field receives about 5 acre-feet per year of
diverted surface water, and the local soils consist of the
Owyhee-Greenleaf series, an alkaline, well—drained deep silt-loam
occurring on slopes of 0 to 2 percent. All of these factors combine to
create a high potential for ground water recharge from cultivated
fields. The dispersed patterns of nitrate—N and the widespread presence
of the herbicide DCPA in the ground water further highlight the
contribution of agriculture.
Nitrate-Pesticide Relationship
In recent years, it has been implied that the presence of nitrates
in ground water may connote pesticide contamination. As a test of this
hypothesis, an x-y plot of nitrate—N versus DCPA for the March 1986
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Figure 9
Plot of Nitrate—N vs DCPA
14
260
240
220
200
_. 180
160
140
120
U
U
80
60
40
20
0
0 10 20 30
NITRATE—N mg, 1(ppm)
40
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sample data Is presented fri Figure 9. The correlation coefficient for 15
this dátiWi proximately O.T. If such a correlation can be valfdated
through f tfi r studies, routine, low cost nitrate sample results could
be used ii FeTfm1nary Indicators of potential pesticide contamination.
Conci uslons
This study has been conducted under constraints that will not aFlow
absolute determinations of the direct causes of nitrate and pesticide
contamination. However, the data do indicate a significant contribution
from agricultural activities.
More information about the hydrogeology of the area is necessary,
since this will play a vital role in understanding the contamination
problems and any subsequent decision making.
Nitrate contamination of ground water is widespread throughout the
study area, with levels comonly exceeding the drinking water standard by
two or three times. Since the data indicate that agricultural practices
are a major contributor of nitrates here, the need for a careful
re-evaluation of nitrogen fertilizer use Is apparent.
DCPA contamination of ground water Is also widespread in the area,
with levels greater than one half the present health advisory. The
health implications of these high levels may at some point force the
agricultural, environmental, and health agencies to place limitations on
DCPA in the area. Before such decisions are made, the possible economic
impacts on the agricultural comunity that would result from such
restrictions must be carefully weighed.
At the time of writing, EPA drinking water and pesticide programs,
along with their state counterparts, are exploring options that may
mitigate these problems. Presently a continuing, long-term monitoring
program Is being implemented, and the propachior registration issues are
being carefully examined. Hopefully, reasonable and intelligent actions
can be taken before the ground water situation worsens.
REFERENCES CITED
Collins, C.A. 1979. Ground-Water Data in the Baker County-Northern
Maiheur County Area. U.S.G.S. Open File Report 79-695. 28 pp.
Mack, W.M. 1983. Aerial Photographic Analysis for Groundwater Contam-
ination, Ontario, Oregon. Environmental Monitoring Systems Laboratory,
U.S. EPA, Las Vegas, Nevada. 31 pp.
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