Em4885 irrigation management practices to protect ground water

4 CHAPTER
SECTION 1 - OVERALL GOOD PRACTICES
IP 4.01.01 - Assess the Risk of Contamination of Ground and Surface
Waters due to Chemical Leaching and Runoff
Objective
Use chemical formulations that are minimally susceptible to leaching and runoff
considering the soil they are used on.
Description
Some combinations of soil and chemical are more susceptible to leaching and runoff than
others. The Washington State Water Quality Guide contains ratings of leaching and runoff
potential for chemicals. It also provides a method for determining potential leaching and
runoff for specific soils. If a soil/chemical combination is found to be susceptible to
leaching or erosion, either another combination should be sought or special care taken
in the use of the material.
Particular factors that affect potential contamination of ground water are listed in WSU
Cooperative Extension bulletin EB1543, Pesticide Movement in Soils-Ground Water
Protection, (published October 1989) as:
1. Rate and method of application. Excessive and improper applications must
be avoided.
2. Pesticide persistence and mobility. Some chemicals will persist longer than others.
Some are more mobile then others, either because of their solubility or their
adsorptive ability.
3. Soil permeability and organic matter content. More permeable soils are susceptible
to leaching of nutrients and chemicals. Organics tend to tie up chemicals and
prevent them from leaching.
4. Frequency and timing of rainfall and irrigations. Unexpected rainfall or excessive
irrigations can cause excessive deep percolation, surface runoff, or both. These are
prime transport mechanisms for moving chemicals to surface and ground
water bodies.
5. Depth to ground water. The deeper the aquifer, the more chance that the pesticide will
either convert to a less harmful form or be tied up by organics or clay soils layers.
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