Groundwater is a precious natural resource that about 95% of rural Washington residents rely on as a source of drinking water. Both the public and agricultural communities are concerned about protecting and maintaining the quality of this water.

A pilot study monitoring vulnerable groundwater aquifers in Washington has detected eight pesticides in rural wells used for drinking water. Three of the chemicals are fumigants no longer in use. Researchers are assessing the significance of the other contaminants. Concentrations were below standards used to determine health risks in drinking water. Two-thirds of the wells sampled had no detectable pesticide levels.

Pesticides, used extensively in the production of agricultural crops, include insecticides, herbicides, and fungicides. Without pesticide applications, field crops would produce significantly lower yields due to insect damage, weed infestations, and plant diseases.

The toxic qualities that make pesticides effective for pest control create a potential for groundwater contamination. Growers, who also depend on water for drinking, can avoid contamination of groundwater by using proper procedures to apply and manage pesticides. To do this, growers must understand the factors that control the fate and transport of pesticides through soils.

Three major factors determine whether a pesticide is likely to reach groundwater. These are

1) pesticide properties,

2) soil properties, and

3) site conditions—including rainfall, irrigation, and depth to groundwater.

Pesticides do not move through all soils into groundwater at the same rate. For instance, aldicarb has been detected in groundwater sampled from New York state, but not in groundwater from Washington State. The fumigant EDB has been detected in Washington drinking water, while the herbicide dicamba has not been detected.

Learning about pesticide properties, soil properties and site conditions will help you understand why some pesticides have been found in groundwater while others have not, and why pesticides are found in some areas, but not in others.

Persistence is the ability of a pesticide to resist breakdown (degradation) into compounds that have different chemical structures and properties. Activity of microbes found in the soil, soil chemical reactions, and sunlight bring about this change. The longer a pesticide persists before it breaks down, the greater chance it has for contaminating groundwater.

In moist, warm soil, microorganisms can completely transform most pesticides into harmless inorganic molecules, such as carbon dioxide and water. This detoxification process occurs mainly in the biologically active zone where plant roots are abundant. It is important to keep pesticides from leaching out of the rooting zone because pesticides break down more slowly in the deeper soils and sediments.

The rate of pesticide breakdown (degradation) is estimated using a chemical property known as half-life. The chemical half-life is the time required for half of the original pesticide application to be transformed into its metabolites. In general, this is also the time required for half of the pesticide application to be deactivated.

Chemicals having short half-lives do not persist in the soil environment for a significant length of time. Chemicals with long half-lives are highly persistent and have a greater chance of leaching to groundwater.

To estimate potential persistence, scientists classify pesticides as:

1) nonpersistent chemicals having a half-life less than 30 days,

2) persistent chemicals having a half-life greater than 100 days, and

3) moderately persistent chemicals having an intermediate half-life (Table 1).

Adsorption is a process in which the pesticide forms chemical bonds to colloidal materials, such as soil organic matter and clay particles. Adsorption is an extremely important process affecting pesticide fate. Strongly adsorbed pesticides will be less mobile when leached through soil than are weakly adsorbed pesticides.

Factors controlling pesticide adsorption include the pH, temperature, and water content of the soil and amount and type of organic matter present. In general, pesticide adsorption relates inversely to pesticide solubility in water. Highly soluble pesticides are more weakly adsorbed in a given soil than are sparingly soluble pesticides. Thus, highly soluble pesticides pose a greater threat for contamination of groundwater.

An index known as the organic carbon partition coefficient measures the adsorption of pesticides to soil organic carbon. This index is the ratio of adsorbed to dissolved pesticide concentrations per 1% of soil organic carbon content. As the partition coefficient increases, the potential for the pesticide to contaminate groundwater decreases. Since the partition coefficient is normalized to 1% organic carbon, the total amount of pesticide adsorbed is obtained by multiplying the partition coefficient times the total soil organic carbon content. Thus, a given pesticide has greater potential for mobility when applied to a soil low in organic carbon than it has when applied to a soil high in organic carbon content.

To estimate potential for contaminating groundwater, classify pesticide mobilities according to organic carbon partition coefficient values. If the partition coefficient is less than 30, the pesticide is classified as mobile; pesticides having a coefficient greater than 300 are immobile. Pesticides having a partition coefficient between 30 and 300 are moderately mobile (Table 2).

Soil Permeability is a measure of how fast water can move downward through the soil. Texture and structure of the soil control soil permeability. Soils having coarse or sandy textures are generally more permeable than are loamy or clayey soils under wet conditions. Soils that have good structure generally have larger pores and greater permeability than soils that have poor structure. As soil permeability increases, the potential for pesticides reaching the groundwater by downward leaching increases.

Organic Matter. Many pesticides are adsorbed by soil organic matter, thereby reducing their rate of downward movement. Pesticide mobility and potential contamination of groundwater are greater in soils having low organic matter content than in soils having high organic matter content. To increase or maintain soil organic matter, add manure, reduce tillage operations, and incorporate crop residues at the soil surface.

Rainfall and Irrigation. Areas with high rates of rainfall or irrigation may have large amounts of water percolating (moving) through the soil, especially if there is no runoff. The potential for pesticides to leach to groundwater is high under such conditions, especially if the soils are highly permeable, if the soil is low in organic matter, and if the pesticide is persistent and only weakly adsorbed.

To minimize the potential for washing pesticides into groundwater (leaching), avoid applying pesticides just prior to a heavy irrigation or rainfall. Avoid overirrigation, especially early and late in the growing season when crops cannot take up excess water from the soil. Base irrigation frequency and amount on an assessment of the crop's water use characteristics and the soil water holding capacity.

Depth to Groundwater. The time required for pesticides to travel to groundwater decreases as the depth to groundwater decreases. Generally, the depth to groundwater is least in spring and greatest in late summer. If spring rains come shortly after application of pesticides and the water table is close to the surface, the potential for contaminating groundwater could be great.

The potential for a pesticide to contaminate groundwater depends:

1) on rate and method of pesticide application,

2) on pesticide persistence and mobility,

3) on soil permeability and organic matter content,

4) on frequency and timing of rainfall and irrigation, and

5) on depth to groundwater.

Many of these factors vary dramatically among the western, central, and eastern sections of Washington State. This diversity makes it impossible to estimate potential contamination without site-specific information on each factor. Ask your Cooperative Extension agent or Natural Resources Conservation Service personnel for this information.

Factors which lead to the greatest potential for contamination of groundwater are listed in Table 3. For sandy or gravelly soils which are low in organic matter content and underlain by shallow groundwater, avoid using chemicals that are persistent (Table 1) and mobile (Table 2). If irrigating, avoid excessive irrigation, and irrigation coinciding with or immediately following pesticide application.

Do not apply pesticides in excessive amounts or too frequently. You can greatly reduce the potential for contaminating groundwater if you apply pesticides at low rates and use them only when pest monitoring indicates a need for chemical control. Scientists prefer integrated pest management (IPM) over purely chemical pest control. Use of IPM encourages placement of beneficial organisms, such as predators, parasites, and pathogens as natural means of control. Examples of IPM include using pesticides that do not harm beneficial organisms; monitoring pest populations to time pesticide applications; and using cultural methods, such as crop rotation and delayed planting, in conjunction with reducing pesticide use.

Many pesticide labels carry instructions on proper application rates, safety procedures, and other restrictions designed to prevent groundwater contamination. Avoid the temptation to apply higher rates than the label directs. Using more is illegal and will not do a better job of controlling pests. Use of pesticides above legal limits will increase the cost of pest control, increase the resistance of pests to chemical controls, increase the risk of contaminating the groundwater, and invite legal penalties. Always measure pesticide amounts carefully, and use properly calibrated and maintained application equipment to avoid overapplication. Finally, avoid chemical spills, and use proper rinsing, disposal, and storage procedures for excess or unused chemicals.

For more information, see the following Extension publications, available through the

Bulletin Office
Washington State University
P.O. Box 645912
Pullman, WA 99164-5912

EB0753, Concepts of Integrated Pest Management in Washington, $1.00

EB1304, Simple Irrigation Schedule using Pan Evaporation, $1.00

EB1305, Sprinkler Irrigation: Application Rates and Depths, $1.00

PNW288, Irrigation Scheduling, 50¢

PNW320, Calibrating and Using a Backpack Sprayer, $1.50

MISC0091, Application of Herbicides through Irrigation Systems, $1.00

College of Agriculture and Home Economics

Use pesticides with care. Apply then only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

WSU Cooperative Extension bulletins contain material written and produced for public distribution. You may reprint written material, provided you do not use it to endorse a commercial product. Alternate formats of our educational materials are available upon request for persons with disabilities. Please contact the Information Department, College of Agriculture and Home Economics, Washington State University for more information.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Published October 1989. Revised August 1996. Subject code 376. A. EB1543