Quality Water for Idaho Current Information Series No. 865

Pesticides are one of the major technological developments of the twentieth century. During the past 30 years, however, concern has arisen about the threats they can pose to wildlife and mankind.

Pesticides have extended human longevity and improved the quality of life. Insect control programs have saved millions of lives by combatting the vectors of diseases such as malaria, yellow fever and typhus. The use of pesticides is also important in modern agriculture, for without chemicals to control various insect, weed, plant disease, nematode and rodent pests, our food supply would be inadequate, poor in quality and more expensive. Growers of high-value crops in Idaho depend on pesticides for economical crop management.

Pesticides are poisons, though, and may be dangerous when misused. Fish kills, reproductive failure in birds and acute illnesses in people have all been attributed to exposure to or ingestion of pesticides, usually as a reslut of use at an improper stage of crop growth, illegal use, misapplication, spills or careless disposal of unused pesticides or pesticide containers.

Both farmers and the general public desire careful management of pesticides in order to avoid contamination of our surface and ground waters. 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 have been found in the groundwater of some geographic areas but not of others.

Pesticide Movement
Once applied to cropland, a pesticide may be taken up by plants, adsorbed to plant surfaces, broken down by sunlight (photdegradation), or ingested by animals, insects, worms or microorganisms in the soil (Fig. 1). It may move downward in the soil and either adhere to soil particles or dissolve in soil water. The pesticide may vaporize and enter the atmosphere (volatilization) or break down via microbial and chemical pathways into less toxic compounds. Pesticides may be leached out of the root zone by rain or irrigation water or wash off the surface of the land. Pesticides applied to the soil and immediately incorporated are protected from photodegradation, volatilization and dew, which can cause hydrolysis (decomposition by reaction with water).

Fig. 1. Pesticide movement and degradation pathways in a soil-bedrock system.

Properly applied pesticides can reach surface and underground waters in two ways: in runoff and by leaching. Runoff is the physical transport of pollutants (chemicals or soil) over the ground surface by rainwater, snowmelt or irrigation water that does not penetrate the soil. In the leaching process, pollutants are carried through the soil by rain or irrigation water as it moves downward. In many parts of Idaho, leaching is likely to be a more serious problem than runoff because of the permeability of our soils.

Pesticides that are susceptible to leaching do not move through all soils and into groundwater at the same rate. Six major factors determine whether a pesticide is likely to reach groundwater:

Pesticide Properties
The physical and chemical properties that make pesticides effective for pest control also create a potential for groundwater contamination. The fate of a pesticide applied to soil depends largely on two of its properties: persistence and adsorption (adsorption is inversely related to solubility).

Persistence -- Persistence is the "lasting power" of a pesticide. Most pesticides in the soil break down or "degrade" over time as a result of several chemical and microbiological reactions. Generally, chemical reactions result in only partial deactivation of pesticides whereas soil microoorganisms can completely break down many pesticides to carbon dioxide, water and other inorganic constituents. Some pesticides produce intermediate substances called metabolites as they degrade. The biological activity of these substances may or may not have environmental significance. Microbes decrease rapidly below the root zone, so pesticides leached below this depth are less likely to be microbially degraded. However, some pesticides will continue to degrade by chemical reactions after they have left the root zone.

Degradation time is measured in half-life. Half-life refers to the amount of time it takes for a pesticide in soil to reach half the activity level it had at the time of application (i.e., for a pesticide with a half-life of 30 days, 50 percent of the pesticide will have degraded after 30 days).

Pesticides having short half-lives often do not persist in the soil long enough to leach into groundwater. Chemicals with long half-lives are highly persistent and have a greater chance of leaching into groundwater.

To describe potential persistence, scientists classify pesticides as follows:

1. Non-persistent chemicals
   Half-life less than 30 days
2. Moderately persistent chemicals
   Half-life of 30 to 100 days
3. Persistent chemicals
   Half-life greater than 100 days

Adsorption -- The adsorption process binds pesticides to soil particles, like iron filings or paper clips stick to a magnet. Adsorption occurs because of the attraction between chemicals and soil particles. Pesticide molecules that are positively charged, for example, are attracted to and can bind to negatively charged clay particles. Strongly adsorbed pesticides are less subject to leaching through soil than weakly adsorbed pesticides. On the other hand, strongly adsorbed pesticides are more subject to loss via surface runoff (erosion) (Table 2).

Factors controlling pesticide adsorption include pesticide charge; soil pH, temperature and water content; the presence of previously adsorbed chemicals that have a stronger bond to soil particles; and the amount and type of organic matter present. In general, pesticide adsorption relates inversely to pesticide solubility in water. Highly soluble pesticides are weakly adsorbed and pose a greater threat of groundwater contamination (Table 2).

Soil Properties
Soil Permeability -- Soil permeability is a measure of how fast water can move downward through the soil. Soil texture and structure control soil permeability. Soils having coarse or sandy textures are generally more permeable than loamy or clayey soils. Soils with good structure generally have larger pores and greater permeability than soils with poor structure. As soil permeability increases, the potential for pesticides to reach 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 a low organic matter content than in soils having a high organic matter content. To increase or maintain soil organic matter, add manure, reduce tillage operations and incorporate crop residues at the soil surface.

Site Conditions
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. Under such conditions, the potential for pesticides to leach to groundwater is high, expecially 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 leaching, avoid applying pesticides just before a heavy irrigation or rainfall. Avoid over-irrigation, 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 crops's water use characteristics and the soil's water-holding capacity.

Depth to Groundwater -- The time it takes 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 pesticide application and the water table is close to the surface, a greater potential for groundwater contamination exists.

Determining the Potential for Pesticide Contamination of Groundwater
The potential for a pesticide to contaminate groundwater depends on a combination of the following:

1. Rate of pesticide application
2. Method of pesticide application
3. Pesticide persistence and mobility
4. Soil permeability and organic matter content
5. Frequency and timing of rainfall and irrigation
6. Depth to groundwater

Factors that lead to the greatest potential for contamination of groundwater are listed in Table 3. In sandy or gravelly soils that 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, especially when the irrigation coincides with or immediately follows a pesticide application.

Pesticide Selection and Use
Pesticides should be part of an overall agricultural pest management strategy that includes biological controls, resistant crop varieties, certified seed, cultural methods, pest monitoring and other applicable practices referred to altogether as Integrated Pest Management or IPM. When field scouting and thresholds indicate a pesticide is needed, selection should be based on effectiveness, toxicity to nontarget species, cost, adsorption (solubility), persistence and site characteristics such as soil permeability.

Some of the pesticides listed in Table 2 have severely restricted use due to acute toxicity or long half-life. An important purpose of the pesticide container's label is to instruct users to apply the pesticide safely and with minimum threat to nontarget specis both on and off the application site. Pesticide users assume the responsibility to follow label instructions. To do otherwise is unsafe and unlawful.

Need More Information?
Pesticide recommendations for various crops and pests may be obtained from the University of Idaho Cooperative Extension System. Contact your county Extension office for information.

Acknowledgement -- The authors thank the USDA Soil Conservation Service for the use of its pesticide database.

The Authors -- Robert L. Mahler is soil scientist and Extension water quality coordinator, Hugh W. Homan is Extension entomologist and Gene P. Carpenter is entomologist and Extension pesticide coordinator, all in the University of Idaho Department of Plant, Soil and Entomological Sciences, Moscow.

This publication is one of a series on water quality issues produced by the University of Idaho Cooperative Extension System for the people of Idaho. The material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 90-EWQUI-1-9216.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

5M 5-90, 5M 1-91 (reprint) Printed with special grant funds from USDA

Comments to author: karenl@uidaho.edu

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Revised: January 3, 2002

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