Management practices, or the methods used to apply pesticides, are another factor determining leaching potential. Injection or incorporation into the soil, as in the case of nematicides, makes the pesticide most readily available for leaching. Most of the pesticides that have been detected in groundwater have been incorporated into the soil rather than sprayed onto growing crops.
The rate and timing of a pesticide's application also are critical in determining whether it will leach to groundwater. The larger the amount used and the closer the time of application to a heavy rainfall or irrigation, the more likely some pesticide will leach to groundwater. Particular care should be taken with chemigation because of the risks of back-siphoning and leaching.
As indicated, the great complexity of soils, environmental conditions, chemical pesticides, and pesticide-soil-water interactions make it impossible for scientists to determine exactly what will happen to a pesticide once it has entered the environment. However, based on research, experience and scientific deduction, reliable conclusions can be drawn that will help managers make wise decisions about what pesticides to use in certain situations and the possibilities of risks to the groundwater environment.
It is important to remember that pesticide and groundwater relationships are site-specific, and even minor changes in the soil-crop-environment-pesticide relationship can change the potential for groundwater contamination. However, some general principles of pesticide management can help to control the situation and should be considered when developing any farm-chemical management plan. These principles are outlined on the following pages.