PESTICIDE MANAGEMENT FOR WATER QUALITY
Principles and Practices
Harold M. van Es
Assistant Professor of Soil and Water Management
Department of Soil, Crop and Atmospheric Sciences
Editorial assistance by
Nancy M. Trautmann
Research Support Specialist
New York State Water Resources Institute
John L. Hutson, Department of Soil, Crop and Atmospheric Sciences, Cornell
University Tammo S. Steenhuis, Department of Agricultural and Biological
Engineering, Cornell University Frederick B. Gaffney, Soil Conservation Service,
New York Don W. Goss, Soil Conservation Service, Ft. Worth, Texas Frederick L.
Gilbert, Soil Conservation Service, New York
Illustrations were developed by Lisa M. Richter and W.J. Davis Kathy F. Narde
provided support for word processing Cover illustration by W.J. Davis
This material is based upon work supported by the U.S. Department of Agriculture
- Extension Service under Special Project Number 89-EWQI-1-9112.
Over 50 percent of the nation's population relies on groundwater for drinking
water, and in rural areas this increases to 90 percent. In New York State, over
six million people are served by groundwater. Within the past decade, concern
has developed about pesticide contamination of groundwater resources. In a 1988
survey by the US Environmental Protection Agency, 46 pesticides from normal
agricultural applications were detected in groundwater of 26 states. In New York
State, eleven pesticides used for agricultural purposes were detected in
groundwater. Studies on Long Island have shown widespread groundwater
contamination by pesticides, including aldicarb, carbofuran, and oxamyl. In
experiments on fine-textured soil, atrazine, alachlor, and carbofuran were
detected in tile outflow shortly after application.
Streams, lakes, estuaries, and other surface water resources also are
susceptible to pesticide contamination. Although pesticides become more diluted
in surface waters, they can upset their ecological balance or cause
contamination of drinking water supplies.
Pesticide contamination of water resources poses a potentially serious threat to
human and animal health. Agriculture, however, relies on pesticides for
production of the high-quality crops. The potential for pesticide contamination
of water resources can be reduced considerably through use of careful designed
pesticide management practices, guided by the following goals: (I) the
prevention of pest infestations, (2) the use of pesticides only when necessary,
(3) the inclusion of environmental considerations in selection and application
of pesticides, and (4) the use of management practices which reduce pesticide
This publication intends to explain the fates of pesticides after they have been
introduced into the environment. In addition, processes which affect pesticide
movement and management practices which reduce their losses are discussed. We
hope that this information will foster the use of sound management practices
which protect the quality of New York State's water resources.
FATE OF PESTICIDES
After a pesticide is applied to a field, it may meet a variety of fates (Figure
1). Some may be lost to the atmosphere through volatilization, carried away to
surface waters by runoff and erosion, or broken down in the sunlight by
photolysis. Pesticides which have entered into soil may be taken up by plants
(and subsequently removed), degraded into other chemical forms, or leached
downward, possibly to groundwater. The remainder is retained in the soil and
continues to be available for plant uptake, degradation, or leaching. How much
meets each of these fates depends on many factors, including:
- the properties of the pesticide
- the properties of the soil
- the conditions of the site
- management practices
If a pesticide is not readily degraded and moves freely with water percolating
downward through the soil, it may reach groundwater. If, however, the pesticide
is either insoluble or tightly bound to soil particles, then it is more likely
to be retained in the upper soil layers and small amounts may be lost to surface
waters through runoff or erosion.
Studying the pathways for contamination of water resources by pesticides is
complicated by the multitude of chemicals and environmental conditions.
Approximately 40,000 different pesticide products are registered in this
country, composed of over 1,000 active ingredients. Many pesticide products are
different formulations or mixtures of the same active ingredients. New
pesticides are continuously added, while others are removed from the market.
Pesticide fate is also affected by a multitude of weather, soil, and geologic
conditions, in addition to characteristics of the pesticide itself. In order to
effectively manage pesticides, the properties of the chemicals, the environment,
and their interaction need to be studied. These factors will be discussed in the
Pesticides are lost to water resources through (i) surface loss (runoff and
erosion) to streams, lakes, and estuaries, and (ii) leaching through the soil to
groundwater. The potential for pesticide contamination of surface and
groundwater depends on three factors: (1) the presence of the pesticide, (2) the
mobility of the pesticide, and (3) the quantity of water moving across the soil
surface and/or through the soil profile. Water contamination by pesticides can
also occur through volatilization and subsequent precipitation into surface
water bodies or through wind erosion. For most conditions, however, these
pathways are insignificant.
Rainfall and Soil Water
Water flow is the most important transport mechanism for pesticides. The
assessment of the potential for pesticide loss to surface or groundwater should
therefore include an evaluation of the site-specific water balance. Water is
added to the soil through precipitation or irrigation, which either infiltrates
into the soil or runs off the soil surface. The fraction of water that
infiltrates compared to the fraction that runs off depends largely on the
intensity of precipitation and the infiltration capacity of the soil. For
example, if rainfall rates are high and the soil is a compacted clay loam,
little water will enter the soil and most will be lost through runoff. This is
especially the case when the soil is near saturation and therefore has a low
capacity for absorbing additional water from precipitation. When runoff carries
dissolved pesticides or those adsorbed to eroding soil particles, contamination
of surface water resources can result.
Water that infiltrates into the soil either is stored within the soil profile or
percolates downward toward groundwater, depending on the soil water conditions.
When soil conditions are dry, the added water will increase soil water storage.
This soil water is then available for evapotranspiration, a combination of
transpiration by plants and evaporation from the soil surface. If the
moisture-holding capacity of the soil is exceeded, the excess water percolates
downward through the soil profile to groundwater. The amount of runoff and water
percolation at any location varies over the course of the year, as shown in
Figure 2. The arrow sizes in this figure indicate the relative magnitude of the
component of the water budget. During the winter, soils in the northern part of
the U.S. are likely to be frozen and impermeable to water. Snowmelt, rain, and
low evapotranspiration rates in the spring generate wet soil conditions and
downward movement of water to groundwater. The potential for runoff is high
during this period because the near-saturated or partially frozen soils have low
water infiltration capacities. In addition, runoff and erosion often are
aggravated by the lack of crop canopy which protects the soil from direct
raindrop impact. During the summer, high rates of evaporation and plant water
uptake may reduce soil water storage, leaving none to percolate downward. Summer
rains only partially recharge the soil profile, and the soil's moisture holding
capacity is typically not exceeded. Except for during high-intensity
thunderstorms, runoff and erosion potentials are generally low during the summer
because of the protective crop canopy and the higher water absorption capacity
of the soil. In the late fall, evapotranspiration rates strongly decrease, and
groundwater recharge occurs when the moisture-holding capacity of the soil is
exceeded. Runoff and erosion potentials also increase during this period.
Because late fall and especially early spring tend to be the seasons of greatest
runoff, erosion, and groundwater recharge, pesticide management should aim at
keeping soil pesticide concentrations low at these times of the year.
Surface Loss of Pesticides
If pesticides are applied to the soil surface without incorporation, they are
susceptible to loss through runoff and erosion during high- intensity rainfall
events. Surface losses will likely result in contamination of streams, lakes,
and estuaries. The potential for surface loss depends on pesticide properties,
soil type, and the length of time after application. Pesticides that attach
easily to soil particles or are very insoluble tend to remain close to the soil
surface. At times when soils are not protected by crop canopies, high intensity
rainfall may cause erosion and removal of sediment particles to streams. This is
of particular concern with persistent pesticides on highly erodible soils. The
highest risk period is immediately after application, before the pesticides have
moved into the soil or have degraded.
Pesticide Leaching and Groundwater
The most productive aquifers in the Northeast consist of sand and gravel
deposits, which generally are found in river valleys. These lands also tend to
be intensively used for agriculture and urban development and therefore are at
high risk for groundwater contamination. Once groundwater becomes contaminated
at concentrations which are higher than the drinking water standard, the wells
tapping into this water have to be closed. Reclamation of contaminated
groundwater is typically prohibitively expensive.
Groundwater originates as recharge, the water that percolates downward through
soil to the depth at which all soil pores are saturated. Recharge areas may be
very small or may extend over many square miles. Depending on local geology and
groundwater flow characteristics, water in any given well may be recharged from
the land directly adjacent to the well or from areas miles away. Shallow wells
typically are recharged by water originating from adjacent land.
Recharge takes place intermittently during and immediately following periods of
rainfall or snowmelt. Under wet soil conditions, recharge carries with it any
pesticides that are dissolved in the soil solution. Pesticides applied in the
spring may move downward rapidly, especially through coarse-textured (sandy,
gravelly) soils. Once the chemical has moved beyond the root zone, it cannot be
absorbed by plants and its rate of degradation is reduced, so its potential for
groundwater contamination is greatly increased.
During the summer little leaching of pesticides occurs. Precipitation
replenishes dry soil and is subsequently taken up by plants. Timely rains are
then beneficial, because they promote vigorous plant growth and decrease
susceptibility to pests. Pesticides applied in early summer have on the average
a lower potential for leaching than those applied earlier in the growing season,
because drier soil conditions and deeper roots typically prevent water from
percolating to depths below the root zone.
The toxic effect of a pesticide on humans and animals is directly related to its
concentration. Therefore, dilution plays an important role in maintaining
pesticide concentrations below health standards. Dilution may occur both over
space and time. In regions where agriculture coexists with other land uses (e.g.
forestry), recharge and runoff are diluted with waters from adjacent lands.
Dilution over time occurs because pesticide losses from agricultural land may be
high during only short periods in the growing season (e.g. immediately after
application). Shallow wells, however, are very sensitive to such fluctuations
and pesticide concentrations may be far above health standards during those
periods of heavy leaching.
In order to evaluate a pesticide's potential for loss to surface or groundwater,
one needs to determine its characteristics with respect to degradation,
adsorption, solubility and volatility.
Most pesticides are organic compounds which degrade under typical environmental
conditions. Pesticide degradation includes several types of processes:
- Microbial degradation. Soils and plants hold populations of microorganisms which
derive energy from the degradation of organic compounds such as pesticides. Two
important processes are distinguished: (1) mineralization, in which the compound
is completely degraded to carbon dioxide (CO2), and (2) cometabolization, in
which the chemical is transformed into other chemical compounds.
- Photochemical reactions, or decomposition through exposure to sunlight. These
reactions are called photolysis.
- Chemical reactions. Pesticides may react with air, water, and other chemicals in
soil and plants through oxidation, reduction, and hydrolysis. Figure 3 shows an
example of the oxidation and hydrolysis reactions for aldicarb. Through a series
of reactions, aldicarb degrades into several other compounds.
Degradation reactions typically produce harmless products such as carbon
dioxide, but can also yield compounds which themselves are toxic, such as the
aldicarb sulfoxide and aldicarb sulfone produced as intermediary products of
aldicarb degradation (Figure 3). Figure 4 illustrates the degradation of a
typical herbicide in soil and the formation and subsequent degradation of an
intermediary breakdown product.
The rate of degradation for a pesticide typically is expressed as half-life, the
period of time required for half of the original quantity to be degraded. The
half-life for each compound depends on many factors, including:
- Chemical structure. Some types of chemical compounds are more easily degraded
through chemical or microbial reactions than others,
- The amount of sunlight, if photochemical reactions play a role.
- Soil type. Soil properties affect pesticide degradation in many ways. In
general, the higher the organic matter content and moisture-holding capacity of
the soil, the higher the rate of pesticide degradation in that soil.
- Temperature. The rates of microbial and chemical reactions increase with
temperature, so pesticide degradation occurs faster as the soil and air become
- Soil water content. Microbial and chemical reactions are favored by moist soil
conditions, so degradation occurs fastest when soils are not too dry. When soils
are water-saturated, however, oxygen supplies gradually decrease and an
anaerobic environment develops. This typically slows pesticide degradation.
- Position in the soil. The upper layers of the soil profile are chemically and
biologically most reactive. Once a pesticide has moved below the rooting depth,
the degradation rate becomes very low because of decreased microbial activity.
A low degradation rate (long half-life) indicates that the pesticide tends to
persist in the environment and therefore to remain available for potential
contamination of surface or groundwater. The fate of a pesticide, however, also
depends on its mobility and on weather, soil type, and local geology.
Soil organic matter and, to a lesser extent, clay particles can bind pesticides.
Pesticides which are strongly adsorbed to soil are not carried downward through
the soil profile with percolating water. However, strongly adsorbed pesticides
can be carried with eroded soil particles by surface runoff. The pesticides may
subsequently desorb from soil particles and become surface water contaminants.
A pesticide's tendency to be adsorbed by soil is expressed by its adsorption
High K(oc) values indicate a tendency for the chemical to be adsorbed by soil
particles rather than remain in the soil solution. Since pesticides bond mainly
to soil organic carbon, the division by the percentage organic carbon in soil
makes the adsorption coefficient a pesticide-specific property, independent of
soil type. Adsorption coefficients less than 500 indicate a considerable
potential for losses through leaching.
The tendency of a chemical to dissolve in water is expressed by its solubility.
Pesticides with solubilities below the threshold value of 30 mg/l are considered
to have relatively low potentials for leaching. Pesticides with solubility
values higher than 30 mg/l may have a high leaching potential if the degradation
rate and the soil adsorption coefficient are low. If poorly soluble pesticides
are applied to soil but not incorporated, they have a high potential for loss
through runoff or erosion.
The potential for a pesticide to volatilize, or become a gas, is expressed by
its Henry's Law Constant:
A high value for this constant indicates a tendency for the pesticide to
volatilize and be lost to the atmosphere. Gaseous losses can be reduced through
soil incorporation. Although exchange of soil air with the atmosphere does take
place, the rate is so slow that volatilization losses of incorporated pesticides
are very low. Pesticides which have volatilized can be redeposited through rain
and thereby reach off-target areas. For most pesticides, loss through
volatilization is insignificant compared with leaching or surface losses. The
main pathway for atmospheric loss of a pesticide is through drift of spray mist
under windy conditions, which is relatively independent of a pesticide's
Pesticide Fate in Soil
For most pesticides, the potential for surface loss or leaching to groundwater
depends mainly on the half-life, solubility, and adsorption coefficient. In
general, pesticides with a long half-life have a higher potential of reaching
surface or groundwater because they are exposed to the hydrologic forces for a
longer period of time. Adsorption coefficient and solubility are the main
determinants for pesticide mobility. Pesticides which are insoluble or have high
adsorption coefficients tend to remain near the soil surface and are more
susceptible to surface loss. Soluble pesticides with low adsorption coefficients
have high leaching potentials.
Pesticide Data Base
The USDA Agricultural Research Service and Soil Conservation Service have
developed a pesticide data base which allows for rapid determination of leaching
and surface loss potentials. The data base was developed with the GLEAMS
computer model, which was used to simulate leaching and surface losses for a
large number of pesticides in various soils. Statistical methods were
subsequently used to evaluate the interactions between pesticide properties
(solubility, adsorption coefficient and half-life) and soil properties (surface
horizon thickness, organic matter content of the surface horizon, surface
texture, subsurface texture, and hydrologic soil group). The variables that
provided the best estimate of surface loss and leaching were selected, and these
predictors were then used to classify all pesticides into groups (large, medium,
and small) according to their potential for leaching or surface loss. Supplement
A lists these potentials for pesticides used in the Northeastern US and provides
further explanation on the interpretation of these classifications. The data
base can be used in conjunction with soils information (see next sections) to
make rapid evaluations of potentials for water contamination.
In addition to the potential for transport to surface or groundwater, a
pesticide needs to be evaluated for its toxicity once it has reached reach
groundwater need to be evaluated for their risk to human health. The U.S.
Environmental Protection Agency has issued guidelines for lifetime health
advisory levels (HAL's) for commonly used pesticides, which includes a margin of
safety to protect humans. Water containing pesticides in concentrations at or
below this level is believed to be acceptable for drinking every day over the
course of a lifetime. HAL's (Table I ) are typically expressed in concentrations
of micrograms per liter, which is equivalent to parts per billion (ppb). For
example, alachlor has a HAL of 0.4 ppb and carbofuran has one of 40 ppb.
Table 1. USEPA Pesticide Health Advisory Levels
Name HAL(ug/l) Name HAL(ug/1)
Acifluorfen 1 Disulfoton 0.3
Alachlor 0.4 Diuron 10
Aldicarb 10 Endrin 0.3
Aldicarb Sulfoxide 10 Ethylene Dibromide 0.004
Aldicarb Sulfone 40 Ethylene Thiourea 0.2
Ametryn 60 Fenamiphos 2
Atrazine 3 Fluometuron 90
Baygon 3 Heptachlor (Epoxide) 0.004
Bentazon 20 Hexachlorobenzene 0.02
Bromacil 90 Hexazinone 200
Butylate 700 Methomyl 200
Carbaryl 700 Methoxychlor 400
Carbofuran 40 Metolachlor 100
Carboxin 700 Metribuzin 200
Chloramben 100 Oxamyl 200
Chlordane 0.03 Pentachlorophenol (PCP) 200
Chlorothalonil 2 Picloram 500
Cyanazine 10 Prometon 100
Dalapon 200 Pronamide 50
2,4-D 70 Propachlor 90
DBCP 0.03 Propazine 10
Diazinon 0.6 Propham 100
Dicamba 200 Simazine 4
1 ,2-Dichloropropane 0.6 2,4,5 T 70
1 ,3-Dichloropropane 0.2 2,4,5 TP (Silvex) 50
Dieldrin 0.002 Tebuthiuron 500
Dinoseb 7 Terbacil 90
Diphenamid 200 Terbufos 1
Pesticides that are easily lost to streams and lakes need to be evaluated for
their toxicity to aquatic life. In watersheds where protection of certain
aquatic species is important, such pesticides need to be checked for potential
harmful effects. For most pesticides, this information is noted on the label and
in chemical reference manuals.
PESTICIDES AND SOILS
Soil type is a major factor determining how much water percolates through the
soil profile and how much runs off the surface. The soil's permeability
determines the potential for water transport through the soil profile. This can
typically be inferred from soil texture.
Coarse-textured sands and gravels have high infiltration capacities, and water
tends to percolate through the soil rather than to run off over the soil
surface. Therefore, coarse-textured soils generally have high potentials for
leaching of pesticides to groundwater but low potentials for surface loss to
streams and lakes. Fine-textured soils such as clays and clay loams generally
have low infiltration capacities, so surface runoff is relatively high compared
to percolation. The potential for pesticide surface loss therefore is high and
the potential for leaching is low. However, such soils may exhibit large pores
formed as shrinkage cracks (when dry), worm holes or root channels. These may
extend into the soil for several feet and can act as conduits for of chemicals .
The amount of organic matter in a soil determines its potential for pesticide
adsorption. Soils high in organic matter have a low leaching potential. In
addition, high organic matter also may reduce the potential for surface loss by
providing good soil aggregation in the plow layer, which increases the
infiltration rate and therefore reduces runoff and erosion. Some pesticides are
adsorbed by soil minerals. For example, paraquat is highly adsorbed by clay
particles. Soils with virtually no clay, such as sands, therefore may allow for
leaching of such chemicals.
Local geology and the depth to groundwater also affect the potential for
leaching of pesticides to groundwater. Deep groundwater resources are typically
more protected from contamination, although in time they can be affected if
improper management practices continue to be used. In regions where agricultural
or urban lands are interspersed with forest land, the recharge from forested
lands may dilute the contaminants from agricultural and urban sources.
Groundwater closer to the soil surface is more likely to become contaminated.
Great care should be taken if shallow wells are located near sites where
pesticides are applied.
The type of geological formation underlying the lands on which pesticides are
applied can also affect the contamination of groundwater. Rock formations that
are impermeable to water (aquitards), prevent recharge and may protect an
underlying aquifer. On the other hand, some formations contain large fractures,
which may cause channeling of contaminated water to deeper aquifers. Of
particular concern are karst areas where soils are underlain by limestone
materials which contain large channels permitting rapid flow and little
filtration of the water recharging groundwater.
Soils Data Base
County soil surveys yield information that can be used for site-specific
evaluation of soil potentials for leaching and surface loss of pesticides. The
USDA Soil Conservation Service has developed a data base which rates soils for
their potentials for surface loss and leaching of pesticides. Using pesticide
movement simulations with the GLEAMS computer model, as described earlier,
predictors were developed to rate soils for their pollution potentials. Three
ratings (high, intermediate, nominal) were established for both surface loss and
leaching potentials. Supplement B lists these rankings for the soils of New York
As discussed above, the movement of pesticides in soil depends on pesticide
characteristics, including half-life, solubility, and adsorption coefficient,
and on soil characteristics, including organic matter content, texture, and
permeability. The interaction of pesticide and soil properties defines the
potential for a pesticide to reach surface or groundwater.
Computer models can be applied to simulate movement of chemicals over the soil
surface and through the soil profile. These models have the potential to
integrate physical processes and to depict the interaction of soil and pesticide
properties. They are therefore useful tools for estimating pollution potentials.
As an example, the model LEACHM (Wagenet and Hutson, 1989) was used to simulate
leaching of four pesticides applied to two soil types. Table 2 shows the
chemical properties for the commonly used herbicides (pesticides used for weed
control) atrazine, cyanazine (Bladex), and alachlor (Lasso), and the insecticide
methyl parathion (Pencap- M). Atrazine has the lowest degradation rate, as
indicated by a half-life (T l/2) of 71 days compared to 30 or less for the
others. Methyl parathion is most highly adsorbed by soils, as indicated by its
high Ko value. The Henry's Law constants (H) vary considerably between the
pesticides but are all too low for volatilization to be a major pathway for
pesticide loss. The solubility of all four pesticides is high and does not
generate differences in movement. The pesticide properties with the greatest
influence on leaching are the degradation rate (half-life) and the soil
Table II. Properties of Simulated Pesticides
T 1/2 K oc H Solubility
(days) (m3/kg) (mg/1)
Atrazine 71 160 2.5 E-7 33
Cyanazine 15 183 4.4 E-6 171
Alachlor 30 200 1.3 E-6 242
Methyl Parathion 15 5100 9.3 E-8 60
Atrazine, due to its persistence, has the highest potential for leaching to
groundwater. Cyanazine, which can be used as a substitute for atrazine, has a
much shorter half-life and therefore a lower potential for leaching. Methyl
parathion has a low leaching potential due to its high adsorption coefficient
and low persistence.
The two soil types used for simulation were a Plymouth loamy sand and a
Rhinebeck silt loam. The Plymouth soil is a highly-permeable, well-drained soil
found mainly on Long Island, while the Rhinebeck is a slowly-permeable,
poorly-drained soil developed from lake-laid sediments.
In the LEACHM simulation, the three herbicides were applied preplant on April 26
at a rate of 2 quarts per acre; the insecticide methyl parathion was applied on
July I at a rate of 2 quarts per acre. The 1968 weather data for Ithaca, NY were
used because they represent a year with average rainfall conditions.
Figure 5 shows the distribution of the pesticides in the soil profile on April
29, July 1, and October 26 for the Plymouth Soil and on October 26 for the
Rhinebeck soil. On April 29th, three days after application, which included a
0.36" rainfall event, the pesticides moved downward to a depth of 6 inches. Only
3.7% of the atrazine degraded in the first three days, compared with 15.2% of
the cyanazine and 10.5% of the alachlor. This can also be deduced from the
half-lifes in Table 2.
By July 28, atrazine, cyanazine, and alachlor had moved down to a depth of about
14 inches. Little difference in depth of movement was observed between these
three herbicides because their adsorption coefficients are similar (Table 2).
The insecticide methyl parathion was present in the largest quantities because
it had been applied on July 1 rather than April 26. Due to its high adsorption
coefficient, however, methyl parathion remained in the surface layer (0-4
inches). The amounts of pesticide left in the profile varied according to their
half-lifes: only 4.0% of the original quantity of cyanazine was left, compared
to 40.1% for atrazine and 11.2% for alachlor. Of the late-applied methyl
parathion, 39.7% remained in the soil. Most of the herbicide movement during the
period April 29 to July 28 took place in the first month, in which
evapotranspiration was low.
At the end of the growing season (October 26), the quantities of pesticide
remaining in the soil were substantially reduced. Only 0.2% of the cyanazine,
1.4% of the alachlor, and 0.6% of the methyl parathion were left in the soil.
However, 16.9% of the atrazine remained in the soil profile, indicating the
higher persistence of this chemical. Because of this persistence, high
application rates may cause carry over effects during the next growing season.
Between July 28 and October 28 little further downward movement took place
because dry soil conditions and plant uptake prevented significant downward
water movement. From October 28 onward, the removal crop at the end of the
growing season and the wetting of the soil profile caused increased downward
water flow. The increased percolation rate after the growing season resulted in
a leaching loss of 0.4% of the originally applied quantity of atrazine by the
end of December. Leaching losses at the bottom of the soil profile were 0.025%
for alachlor and considerably less for cyanazine and methyl parathion. The EPA
Health Advisory level for atrazine is 3 parts per billion (ppb); for alachlor
this level is 0.4 ppb. Therefore, the application of alachlor poses a similar
human health threat as atrazine, despite the fact that lower quantities are
leached. Cyanazine has a Health Advisory Level of 10 ppb and is therefore safer
than atrazine in terms of human toxicity as well as quantities reaching
The quantities of pesticide remaining in the soil at the end of the growing
season for the Rhinebeck soil also are shown in Figure 5. The total quantities
were very similar to those for the Plymouth soil. The downward movement,
however, was less due to the lower permeability and the higher organic matter
content of the Rhinebeck soil. Although atrazine was still abundant, it remained
in the top 10 inches of the profile. Leaching losses at the end of the year were
less than 0.01% for all four pesticides. Because of the higher runoff and
erosion potential for the Rhinebeck soil, surface loss of highly adsorbed
pesticides like methyl parathion may be of concern.
The simulations revealed that most of the downward movement of pesticides took
place during the early part of the growing season when soils were wet,
evapotranspiration was low, and pesticides were present in the largest
quantities. If rainfall during the spring and summer had been excessive, the
downward movement would have been higher. Delaying herbicide applications to
post-emergence (based on need) would therefore have reduced early pesticide
leaching. The simulations also revealed that persistent pesticides, such as
atrazine, may become subject to leaching after the growing season. When the soil
profile wets up, the downward movement of water and leftover pesticide increases
and causes further leaching loss. Due to its high leaching potential and
widespread usage, atrazine has been detected in many groundwater surveys.
The simulations presented here assumed that pesticide transport occurred through
so called "matrix flow", which does not account for movement through warm holes,
cracks, and other macropores. Pesticides can move rapidly through such large
continuous pores, thereby by passing the soil matrix and reaching groundwater
much faster than if only matrix flow occurs. Differences between "bypass flow"
and "matrix flow' are especially apparent in fine-textured soils where matrix
flow is low, but cracks and wormholes are very common. In a field experiment on
a Rhinebeck soil in New York, atrazine, alachlor, and carbofuran were detected
in tile outflow within days after application. Fast downward movement occurred
during the first major rainfall events after application. In a wet year, the
more persistent atrazine remained in tile outflow through the entire growing
season. Other studies have confirmed that heavy rainfall after pesticide
application may cause rapid downward movement./ Non incorporated,
surface-applied pesticides are especially subject to this process because ponded
water easily dissolves the chemical; and enters directly into macropores that
are open at the soil surface. The simulations discussed above are therefore very
conservative because they do not account for bypass flow processes.
Experiments on sandy soils have also shown uneven flow patterns referred to as
"fingering", which may cause downward movement of pesticides through zones of
preferred water flow.
The pesticide and soils data bases discussed earlier provide means for
evaluation of soil-pesticide interactions (see Supplement C). If a pesticide has
a large leaching potential and the soil is permeable (high soil leaching
potential), leaching loss is likely to occur. The use of atrazine on a Plymouth
soil illustrates such a situation. If the soil is slowly permeable (e.g. on
Rhinebeck soil), leaching is less likely to occur unless many macropores are
present. Supplement C shows how the leaching and surface loss potentials for
pesticides and soils can be combined into a matrix which allows for evaluation
of the soil-pesticide interaction. This information must then be supplemental
with Health Advisory Levels for drinking water, toxicity to aquatic life and the
relative importance of the affected water bodies as a source of drinking water
or as a natural habitant. Subsequently, management practices can be identified
which reduce environmental impacts of pest control.
PEST MANAGEMENT FOR WATER QUALITY
The key to reducing the potential for pesticide contamination of water resources
is the use of planned pest management. This may include avoiding unnecessary
pesticide applications, use of targeted and economical applications, and use of
cultural or biological practices that substitute for or complement pesticide
use. In addition, pesticide selection and crop management should be carried out
according to the site-specific needs for reduction of water contamination. The
management plan requires evaluation of the nature of the water quality problem
through consideration of the relative priorities for protection of various
surface and groundwater resources, and the vulnerability of these water
resources to contamination by pesticides.
Soil and Crop Management
Pest infestations can be minimized by using soil and crop management practices
which provide for vigorous plant growth. These practices include:
- appropriate seedbed preparation and planting,
- optimization of timing of crop planting and harvesting,
- maintenance of optimum soil nutrient and pH levels,
- use of appropriate crop rotations,
- use of good water management practices (drainage and irrigation), as
- avoidance or alleviation of soil compaction, and
- use of soil and water conservation practices that reduce surface loss or
Cornell Cooperative Extension recommendations for these practices are published
in grower's guides (Cornell Recommends), handbooks and fact sheets. Rotation of
crops provides plants with better resistance against diseases, insects, and weed
competition. It also improves soil aggregation, increases infiltration rates and
soil-water storage capacity, and decreases surface loss potentials, especially
if the rotation includes sod.
Soil compaction from machinery traffic, especially under wet soil conditions,
may have a negative effect on water quality. Infiltration capacity and drainage
rates are lowered, increasing the potential for surface loss. In addition, root
restriction causes reduced plant growth and may necessitate increased pesticide
use to compensate for the plants' increased susceptibility to diseases, insects,
and weed pressure. Compaction can be alleviated in various ways including deep
tillage and rotation to sod.
Soil management affects the potential for pesticide contamination of water
resources by changing the soil characteristics at and beneath the soil surface.
Some of the most economical and effective practices that reduce surface loss of
pesticides at the edge of fields are sod-based rotations, sod waterways,
contouring, reduced tillage, and vegetative buffer strips.
Use of conservation tillage and no-till affects the potential for pesticide
transport in various ways. These practices typically reduce the runoff and
erosion volume, but also tend to use higher concentrations of chemicals near the
soil surface. This is especially the case for no-till, which relies more heavily
on the use of pesticides that are applied to the soil surface and not
incorporated. The reduction in runoff and erosion volume usually outweighs the
increased concentration of pesticides in water and sediment. Depending on soil
conditions, weather, and management practices, however, surface losses of
pesticides may be higher under these conservation tillage practices.
Tillage also affects the leaching potential of pesticides. Surface soil
loosening increases water flow in the plow layer. Tillage also destroys the
continuity of macropores, especially worm borrows, and thereby cuts off the
bypass flow mechanism. Figure 7 illustrates the movement of a dye tracer through
a Rhinebeck clay loam soil. In the plow layer, which was loosened several weeks
before dye application, flow occurred through approximately half the soil
volume. The location of furrow slices are recognizable in the flow pattern. In
the subsoil, dye movement was concentrated in macropores, resulting in patches
of dye accumulation in the lower part of the soil profile. With time after
tillage, the plow layer recompacts and new worm holes and cracks will develop.
Due to the lack of soil disturbance, soil under no-tillage contains many
continuous pores which may extend deep into the soil profile. Field experiments
have confirmed that the bypass flow process is very significant under no-tillage
and causes more rapid downward movement of pesticides at higher concentrations,
even in fine-textured soils. Therefore, if pesticide leaching is of concern,
periodic soil disturbance may be appropriate.
Low-input crop management focuses on cultural practices in order to decrease
pesticide use. For example, banded herbicide applications combined with
mechanical cultivation and the use of interseedings can greatly reduce herbicide
use in row crops. By reducing chemical inputs and maintaining similar or
slightly lower yields, low-input agriculture increases net returns and reduces
the environmental risks of crop production.
Some crops, especially fruits and vegetables are grown without the use of
pesticides. They are mostly produced to be sold as fresh produce to special
markets. Crops produced without the use of pesticides generally require
increased labor input, which is offset by higher market prices. Pest management
for certified organic crops is typically limited to cultural and biological
Irrigation of crops and water of lawns may increase the potential for movement
of pesticides to groundwater or surface water. If the rate of water application
is higher than the infiltration capacity of the soil, runoff results. Pesticides
located at the soil surface may be carried to streams and lakes with this runoff
water. The rate of water application should therefore always be lower than the
infiltration rate of the soil.
Excessive irrigation rates also may increase the potential for leaching of
pesticides to groundwater. If more water is supplied than is required to
recharge the water storage capacity in the root zone, excess water and dissolved
chemicals will percolate to greater depths. Irrigation scheduling should
therefore also incorporate information on soil water storage capacity and
rooting depth of the crop. For example, shallow-rooted vegetable crops growing
on sandy soil require more frequent water applications at lower quantities than
do deeply rooted field crops on a loamy soil. Similarly, homeowners on sandy
soils may water their lawns more frequently, but use lower quantities of water
compared to those on loamy or clayey soils. Alternatively, they may conserve
water and reduce leaching by selecting drought-resistant varieties.
Subsurface drainage may increase pesticide contamination of streams and lakes by
diverting water flow from groundwater to surface water and providing a shortcut
for drainage water. However, it also provides for a superior plant growth
environment and therefore insures more vigorous plant growth and higher
resistance to pests. The interaction of drainage with pesticide management has
not been researched extensively.
The effect of subrogation, in which water is supplied through drainage tile
lines, on the movement of pesticides is unknown. Since this method does not
induce net downward movement of water, it is expected not to cause additional
leaching of pesticides, as may be the case with conventional irrigation
Integration Pest Management
Integrated Pest Management (IPM) seeks to reduce pesticide use to the minimum
level necessary to produce high quality food and agricultural products, at the
same time protecting human health and environmental quality.
The New York State IPM program operates under five objectives:
- to minimize crop losses caused by insects, weeds, and plant diseases,
- to optimize the use of cultural management techniques, biological pest controls,
and resistant varieties,
- to maximize the effectiveness of pesticide use,
- to reduce pest management costs, and
- to minimize the development of pesticide resistance.
IPM encourages beneficial organisms such as predators, parasites, and pathogens
as a natural means of control. Field monitoring, or "scouting," is used to
detect pest infestations so that pesticide applications can be targeted to times
of need and unnecessary applications are avoided. Such field monitoring can
significantly reduce pesticide use while protecting crop yields. In New York
State, for example, onion growers who followed IPM thresholds based on field
scouts' weekly monitoring reports were able to reduce insecticide use by 54% and
to save $24 per acre in insecticide costs. Thrips populations were 42% lower
than on farms which were not participating in the field scouting program, and
quality of the harvested onions was unaffected. Various estimates suggest that
the adoption of the currently available IPM practices would permit a 40-50%
reduction in the use of insecticides within a 5-year period and 70-80% in the
next ten years, without sacrifice of crop yield or grower profits.
Most groundwater contamination problems are associated with application of
pesticides to control soil-dwelling pests such as nematodes, weeds, pathogens,
and insects. IPM techniques of greatest importance in reducing groundwater
contamination therefore are those which minimize the use of soil-applied
pesticides. Such methods include crop rotation, fallowing, resistant cultivars,
and use of less persistent pesticides.
Integrated Pest Management programs are conducted by Cornell Cooperative
Extension and local crop consulting organizations.
Pesticide Management Practices
The evaluation of soil-pesticide interactions can be used to reduce the
pollution potential of pesticides. First, the most effective pest control method
should be selected based on Cornell recommendations. In cases in which various
chemicals can economically be applied to remedy an infestation, the pesticide
with the least environmental impact should be selected. This includes the
evaluation of the site-specific leaching and surface loss potentials
(Supplements A, B, and C). In addition, the chemical's toxicity to human and
aquatic life and the importance of the affected water bodies as drinking water
supplies or natural habitats need to be considered.
Pesticides should be applied when they are most effective, which is influenced
by temperature and moisture conditions. Pests under dormant or stressed
conditions may be less susceptible to pesticide treatment. Pesticide efficacy
can also be reduced by continuous use of pesticides of similar chemistry, which
can cause pesticide resistance. Pesticide applications should be avoided under
adverse weather conditions. This includes spraying under windy conditions,
surface application before high-intensity rainfall and application on
water-saturated soil. For volatile pesticides, application under high-
temperature conditions should be avoided.
Finally, pollution from pesticides can be reduced by proper operation, safety,
and maintenance practices, as listed in the New York Pesticide Applicator
Training Manuals. Application equipment should be maintained in proper working
condition. It should be calibrated at the beginning of the growing season and
then re-calibrated regularly during the growing season. The required quantity of
pesticide should be carefully measured to insure proper application rates and to
prevent leftover tank mixes. If pesticides require dilution with water,
prevention of backsiphoning to water supplies must be insured. Chemigation
systems, which provide for pesticide application through irrigation, must be
fitted with an appropriate anti-backsiphoning device.
Loading and mixing areas located near wells, high runoff areas or surface water
bodies are a common source for contamination of surface or groundwater by
pesticides. Sites for loading and mixing of pesticides should be located as far
away from points of entry to surface or groundwater as possible. Wells which re
used as a source for drinking water need especially be protected from pesticide
Comprehensive pest management plans allow for safe pesticide use and reduce the
potential for contamination of water resources. Additional information on the
judicious use of pesticides and how to protect and monitor the quality of water
resources can be obtained from Cornell Cooperative Extension, the USDA Soil
Conservation Service, the Department of Environmental Conservation, and the
local Soil and Water Conservation Districts.
Alexander, M. 1099. Biodegradation and Our Microbial Allies in Soils and Waters.
pp. 18-19 in New York's Food and Life Sciences Quarterly, Vol. 18, nos. 1 and 2.
USEPA. 1989. Drinking Water Health Advisory: Pesticides. Lewis Publishers,
Williams, W. M., P. W. Holden, D. W. Parsons, and M. W. Lorber. 1988. Pesticides
in Ground Water Data Base, 1988 Interim Report. U.S. Environmental Protection
Agency, Office of Pesticide Programs.
Wagenet, R. J., and J. L. Hutson. 1989. LEACHM Leaching Estimation and Chemistry
Model. Continuum: Vol. 2, Version 2. NYS Water Resources Institute, Center for
Environmental Research, 468 Hollister Hall, Cornell University, Ithaca, NY
Walker, M. J., and K. S. Porter. 1989. Assessment of Pesticides in Upstate New
York Groundwater. New York State Water Resources Institute, Cornell University.