An Assessment System for Potential Groundwater Contamination from Agricultural
Pesticide Use in North Dakota
Extension Bulletin No. 63, March 1994
Bruce Seelig, Water Quality Specialist
Introduction
STEP 1. Aquifers versus groundwater
STEP 2. Pesticide use
STEP 3. Filtration potential
Appendix 1 - Trade and Common Names of Pesticides and
their Filtration Potential
A groundwater assessment system is proposed to help develop and
implement best management practices (BMP) to protect groundwater from pesticide
contamination. This system will help producers organize natural resource information into
groundwater sensitivity categories. BMP recommendations will be adapted for each
groundwater sensitivity category. To determine the groundwater sensitivity of a given
area, a guided path, or stepwise algorithm, is used (Figure 1).
Figure 1. Stepwise algorithm for determination
of ground water sensitivity to pesticides (first order of priority). (8KB
b&w diagram)
Key factors that determine vulnerability or sensitivity will be assessed to assign each
site to a specific category. The categories will emphasize similarities in factors, and
will represent a rating system only in the broadest sense. Instead, the focus will be on
placing sites in categories that allow logical development of an effective system of
management practices that protect groundwater.
STEP 1. Aquifers versus groundwater
The first step requires the user to determine if an aquifer with a useable supply of
water exists. Emphasis must be placed on protecting readily accessible groundwater or
shallow aquifers with useable water. In North Dakota, aquifers located in glacially
derived materials are of greatest value due to their generally good water quality, high
yields, and shallow depths. The water must be of such quality that it is useable for human
needs. The first order of priority is the determination of glacial or alluvial aquifers
under the area of interest. All glacial and alluvial aquifers will be considered as worthy
of protection, particularly those shallower than 50 feet.
Those aquifers that supply useable water to a significant number of people must also
take on a higher level of importance than those that don't. Glacial or alluvial aquifers
with useable water that are extensive enough to be used by significant numbers of people
are identified in the groundwater studies report (North Dakota Geological Survey and State
Water Commission) for each county in North Dakota.
In general, aquifers located in bedrock in North Dakota have poor quality water, are
deep, and have variable yields. As a whole, these aquifers are not worthy of the same
level of protection as glacial aquifers or alluvial aquifers. However, in some parts of
the state, particularly the unglaciated southwest, bedrock aquifers are the only source of
groundwater.
Within the county groundwater studies report, information about bedrock aquifers is
quite general and difficult to apply to a specific area. This makes the sensitivity
assessment more difficult, because important information about the aquifer is not as
easily extracted from the report. Only in areas of southwestern North Dakota, where
bedrock aquifers are the sole source of groundwater, should the assessment be extended to
include these aquifers as second order of priority.
STEP 2. Pesticide use
Distribution of land use has been recognized as an important factor in groundwater
protection from agricultural chemicals. Different types of land use will require different
levels of agricultural inputs. Land use is a general indicator of the amount and type of
pesticide applied above an aquifer. Pesticide use will be combined with land use in the
following land use -- pesticide categories: 1 ) cropland with pesticides; 2) hayland,
pastureland, forestland, and rangeland with pesticides; and 3) no pesticides.
STEP 3. Filtration potential
After the location of vulnerable aquifers and pesticide usage over them is assessed,
the site properties that affect pesticide movement must be determined. In simple terms,
the soil and geologic materials act as a filter to protect aquifers from contamination.
That filtering process is often referred to as pesticide "attenuation"
in scientific parlance. Attenuation can be defined as lessening the amount, force, or
value of something. In this case, the amount of pesticide is lessened as it is filtered
out on soil and geologic materials. An estimate of the potential for materials to
attenuate or filter out pesticides will be presented as the "filtration
potential" for this sensitivity assessment system.
Pesticide properties must also be accounted for when determining ground water
sensitivity. Pesticide half-life (T1/2) and organic carbon adsorption
coefficient (KOC) have been used to rate pesticide potential to leach. When the
KOC is divided by the T1/2 and then multiplied by 10, the resulting
number is called the Hornsby Index. A pesticide with a small Hornsby Index is more likely
to leach to groundwater than a pesticide with a large Hornsby Index.
Pesticide Filtration Potential Factors
The assessment of filtration potential of materials overlaying an aquifer will include
the following: 1 ) depth to the saturated aquifer combined with predominant waterflow
direction; 2) soil and geologic strata permeability; 3) soil organic matter content; 4)
pesticide KOC and T1/2.
Aquifer depth -- water flow direction
Depth to the saturated aquifer can be determined from the county groundwater studies
report. Depths less than 50 feet are considered to be shallow. Soils are an excellent
indicator of long term water flow direction. Water flow through a soil to the groundwater
can be categorized as recharge (downward through the soil to groundwater)
and discharge (upward through the soil from the groundwater). Flowthrough
is the term used to describe lateral movement of groundwater through the soil.
Depth to the saturated aquifer can be determined from a county groundwater
studies report by the ND Geological Survey and ND State Water Commission.
The presence and depth of calcium carbonate (lime) will be used to assess the long-term
water flow direction. In recharge areas the predominantly downward movement of water
removes calcium carbonate from the upper levels of the soil. For this assessment system,
soils of recharge areas lack calcium carbonate in the upper 30 inches of
the soil profile. Depth to calcium carbonate can be determined from a county soil survey
report (USDA, Soil Conservation Service). Presence of calcium carbonate in each soil
horizon is indicated by effervescence when dilute hydrochloric acid is applied to the
soil. This information is available in the soil series descriptions.
Irrigation increases the potential for groundwater recharge. Many factors such as
timing of water application, tile drainage, soil texture, and pumping of wells influence
groundwater recharge under irrigated fields. Despite these extenuating factors, the water
flow for irrigated soils will be considered recharge.
A groundwater recharge area overlaying a shallow aquifer constitutes low
potential for filtration of contaminants from percolating water. All other
combinations of ground water flow and aquifer depth have high filtration potential.
Soil and geologic material permeability
Soil permeability is closely related to soil texture. Soils in the sandy and
sandy-skeletal textural families that overlie sand and gravel geologic materials have low
potential for filtration. Soils in the fine textural family that overlie geologic
material finer than sand and gravel have high potential for filtration.
All other textures or combination of textures will have intermediate potential
for filtration. Family textural classification of soils can be determined from a county
soil survey. Texture of geologic material overlaying the aquifer can be determined from a
county groundwater studies report or sometimes from the county soil survey report.
Soil and geological material textures are important factors that control water
infiltration and flow. Illustration of infiltration and flow.
(57KB b&w illustration)
Organic matter content
Soil organic matter (o.m.) content has the largest influence on pesticide attenuation
compared to the other soil factors. Organic matter content of less than 2 percent in the A
horizon (very low to moderately low) will have low potential to filter
pesticides from percolating water. As o.m. content increases, filtration potential also
increases. Soils with more than 2 percent (moderate to very high) in the A horizon have a high
potential to filter pesticides from percolating water. Soil organic matter
classes are given in the map unit descriptions in most county soil survey reports (Table
1). If this information is not in the county soil survey report, the local SCS office
should be contacted.
Soils are an excellent indicator of long term water flow direction and contain organic
matter to which pesticides are absorbed.
Table 1. Soil organic matter content (percent) conversion
from soil mapping unit description.
------------------------------------
Organic Matter Organic Matter
Descriptor Content by Weight
------------------------------------
%
Very Low < 0.5
Low 0.5 - 1.0
Moderately Low 1.0 - 2.0
Moderate 2.0 - 4.0
High 4.0 - 8.0
Very High > 8.0
------------------------------------
Pesticide chemistry
The tendency for a pesticide to move with water through soils is also influenced by its
chemistry. This is referred to as leaching potential. It is just the opposite of
filtration potential or a pesticide's tendency to be removed from the water and trapped or
filtered by the soil. To remain consistent with the other pesticide filtration potential
factors, the pesticide chemistry factor will be expressed in terms of filtration
potential. The pesticide chemistry factors expressed as filtration potentials for
pesticides used in North Dakota are listed in Appendix 1.
The tendency for a pesticide to move with water through soils is strongly
influenced by its chemistry.
Groundwater Sensitivity Categories
The proposed groundwater sensitivity categories are designed to guide management not
dictate it. Groundwater sensitivity categories will reflect combinations of filtration
factors that require similar management practices to protect groundwater. Groundwater
sensitivity can be defined in four broad categories: 1 ) high; 2) high intermediate; and
3) low intermediate; and 4) low.
High sensitivity is defined as the worst case in which all four
pesticide filtration factors are low.
High intermediate sensitivity is defined as a combination of pesticide
filtration factors in which one or more of the factors are low. The high intermediate
category name should always be accompanied by modifiers for land use and factors that have
low potential to filter pesticides from percolating water.
Low intermediate sensitivity is defined as a combination of factors in
which none of the factors have low filtration potential, but not all have high filtration
potential.
Low sensitivity is defined as the best case in which all four
pesticide filtration factors are high.
Groundwater sensitivity categories will be the basis for the best management practices
(BMP) recommendations. Best management practices will be designed specifically to improve
those factors that cause low filtration potential. All situations for low filtration
potential occur within the high intermediate and high sensitivity categories.
Because BMPs will focus on specific factors, they will also be applicable to problems
that may develop in the low intermediate and low sensitivity categories. There is always
the possibility that a problem may develop even in the Low sensitivity category.
Therefore, the system of BMPs must also be applicable to those situations. Relating BMPs
to specific factors will make this possible, because the solution to problems associated
with a specific factor will be similar regardless of the groundwater sensitivity category.
Groundwater sensitivity category names will include a modifier for the land use --
pesticide category discussed in step 2, if pesticides are used. Land use modifiers are the
following: 1) cropland; 2) hayland; 3) pastureland, 4) rangeland; and 5)
forestland. The high intermediate category will also include a modifier for those
factors that have low potential to filter pesticides from percolating water. This modifier
will follow the land use -- pesticide modifier. High Intermediate modifiers are the
following: 1) high permeability; 2) shallow recharge; 3) low o.m.; and 4)
leachable pesticide.
An Example of Groundwater Assessment for Pesticide Contamination
Sec. 29, T. 155 N., R. 65 W. - Ramsey County
STEP 1.
The Ramsey County Groundwater Studies Report identifies three glacial aquifers in the
county. They are the Spiritwood, Mcville, and Starkweather aquifers (Figure 2). In Sec.
29, the Spiritwood aquifer underlies approximately the western two-thirds. One-third of
Sec. 29 can be eliminated from further consideration, because no glacial aquifer is
present.
Figure 2. Glacial aquifers of Ramsey County. (7KB
b&w map)
STEP 2.
For the sake of demonstration, it will be assumed that the S 1/2, Sec. 29 is cropland
with regular pesticide applications. The N 1/2, Sec. 29 is assumed to be pastureland with
no pesticide applications. Based on these assumptions, the majority of S 1/2, Sec. 29
overlies the Spiritwood Aquifer and will be placed in the cropland-pesticide used
category.
STEP 3.
Groundwater sensitivity analysis of Sec. 29, T. 155 N, R. 65 W. is an example of the
type of information that can be used to help make BMP recommendations for individual
producers. The log from a well dug in the SW /4, Sec. 20 just to the north of Sec. 29 is
assumed to be representative for the surrounding area including Sec. 29. In this area the
Spiritwood Aquifer is overlain by less than 50 feet of clayey or silty materials.
For this hypothetical example, the N 1/2, Sec. 29 is pastureland where pesticides are
not applied. The assessment of ground water sensitivity will be limited to cropland areas
and only those that occur directly over the aquifer. The area of concern is slightly
larger than the SW 1/4, Sec. 29. For this example, it is assumed that the area of cropland
over the aquifer is in a wheat-sunflower-summer fallow rotation. The pesticides 2,4-D,
trifluralin, and ethyl parathion are used for pest control on this farmland.
The county soil survey is the key to site-specific sensitivity analysis, because it can
be used to determine variability in critical filtration factors at a scale appropriate for
management decisions. There are 10 soil map units with 10 different soil series (Table 2)
in the portion of the field that overlies the aquifer (Figure 3). When the filtration
factors are assessed (Table 3), the categories of groundwater sensitivity can be located
in the field (Figure 4) according to soil map unit.
Table 2. Soil mapping units overlaying the Spiritwood Aquifer on Sec. 29,
T.155N., R. 65W., Ramsey County.
---------------------------------------------------------
Symbol Soil Mapping Unit
---------------------------------------------------------
1 Tonka silt loam
11 Svea-Barnes loams, 1 to 3 percent slopes
12B Barnes-Svea loams, 3 to 6 percent slopes
23 Hamerly-Cresbard loams, 1 to 3 percent slopes
24 Svea-Cresbard loams, 1 to 3 percent slopes
35 Overly silty clay loam, 0 to 3 percent slopes
39 Colvin silty clay loam
42 Fargo-Hegne silty clays
45 Hegne silty clay
46 Aberdeen-Fargo silty clay loams
---------------------------------------------------------
Table 3. Groundwater sensitivity categories for the Spiritwood Aquifer in Section 29, T.
155N., R. 65W., based on major soil series and pesticide filtration factor potentials.
----------------------------------------------------------------------
Aquifer Groundwater
Soil Depth- Material O.M. Pesticide Sensitivity
Series Water Flow Texture Content Chemistry Category
----------------------------------------------------------------------
Aberdeen high high high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Barnes high intermediate high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Colvin high intermediate high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Cresbard high high high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Fargo high high high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Hamerly high high high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Hegne high high high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Overly high intermediate high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Svea high intermediate high low (2,4-D) High
intermediate-
cropland-leachable
pesticide
----------------------------------------------------------------------
Tonka low high high low (2,4-D) High
(shallow- intermediate-
recharge) cropland-shallow
recharge, leachable
pesticide
----------------------------------------------------------------------
Figure 3. Detailed soil map for Sec. 29, T.155 N., R.65 W.
superimposed over the Spiritwood Aquifer. (14KB b&w map)
Figure 4. Groundwater sensitivity map for Sec. 29, T.1
55N., R.65 W. (20KB b&w map)
BMPs can be tailored according to the producer's needs and field distribution of the
following two categories: 1 ) High intermediate - cropland - shallow recharge, leachable
pesticide; and 2) High intermediate - cropland - leachable pesticide. The factors that
have low filtration potential will be the basis for BMP recommendations. They are
identified as modifiers to the High intermediate category, leachable
pesticide and shallow recharge.
A Note of Caution
Application of this assessment system depends on information extracted from a number of
different sources It's important that the limits of accuracy of that information are kept
in mind. In order to gather and compile data over large areas, it's necessary to group and
organize that information so it is manageable. County soil survey reports, water resources
reports, and agricultural statistics are examples of this process. Average or grouped data
lose some of their accuracy as the focus of study is narrowed to smaller and smaller
areas.
County soil survey information is critical to this assessment system and its
limitations should be understood. NDSU Extension Bulletin No. 60, "Soil Survey: The
Foundation for Productive Natural Resource Management," explains how to use this
information within the framework of its limitations.
Briefly, soil properties are organized into groups at several different levels. Each
group represents a range of soil properties. Also, soil mapping units, which are shown on
soil maps, are not pure units. They contain small percentages of completely different
soils that could not be separated out because the scale of the map did not permit it.
Therefore, an element of error exists in using this information.
Under most circumstances that error is probably quite acceptable for management of
large areas and farm fields. It is when the focus of study is narrowed to just a few acres
that the error becomes unacceptable and site specific information will be more
appropriate.
This system of groundwater assessment is primarily dedicated to categorizing pertinent
information, so that best management practices can be applied if necessary. It is not
intended to be used to impose management practices on land that may fall in one category
or another. Because an area is placed in the High sensitivity category does not mean that
the aquifer is contaminated, nor does it mean that contamination is imminent. It does
mean, however, the potential for contamination is greater than for other areas. On the
other hand, an area that is placed in the Low sensitivity category may contribute to
contamination, but it is not as likely.
Voluntary adjustment of management practices that help protect North Dakota's
groundwater resources is always desirable. However, in the case where management changes
for groundwater protection cause significant disruption of the producers operation, these
changes should only be recommended when the extent of ground water contamination has been
convincingly demonstrated by standard monitoring and analytical techniques. When this
occurs, BMP recommendations should be based on the groundwater sensitivity analysis of the
site.
It should also be noted that this system excludes many small aquifers, because they do
not receive extensive use. Groundwater sensitivity can be determined in these cases.
However, critical information about the aquifer will not be found in the county
groundwater studies report and must be determined from other sources.
Finally, this system does not assess the sensitivity of groundwater to contamination
from accidents or inappropriate handling and disposal of pesticides. The method of
determining groundwater sensitivity outlined above assumes that pesticides are applied as
recommended on the pesticide container label.
[ Appendix 1 -
Trade and Common Names of Pesticides and their Filtration Potential ]
Extension Bulletin No. 63, March 1994
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