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Water Quality


Principles and Practices

written by

Harold M. van Es
Assistant Professor of Soil and Water Management
Department of Soil, Crop and Atmospheric Sciences
Cornell University

Editorial assistance by

Nancy M. Trautmann
Research Support Specialist
New York State Water Resources Institute

Contributions from:

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.

October 1990


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 loss.

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.


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

Env fateIf 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 following sections.

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.

Soil/water cond

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.


Pesticide Degradation

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:

Aldicarb degrad

  • 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:

Typ herb degrad

  • 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 warmer .
  • 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 coefficient:


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 chemical characteristics.

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.

Pesticide Toxicity

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.


Soil Type

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 State.

Soil-Pesticide Interaction

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 adsorption coefficient.

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.

Sim pest distrib
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 groundwater.

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.


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 appropriate,
  • avoidance or alleviation of soil compaction, and
  • use of soil and water conservation practices that reduce surface loss or leaching.

Pub pix

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.

Pref flow patt

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 control methods.


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 practices.

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 contamination.

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, Chelsea, MI.

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 14853.

Walker, M. J., and K. S. Porter. 1989. Assessment of Pesticides in Upstate New York Groundwater. New York State Water Resources Institute, Cornell University.