Leaching and Run-off Excessive rain or irrigation can cause pesticide leaching. Water that moves down through soil or through cracks and worm tunnels transports water soluble substances. The herbicides 2,4-D and Tordon are leached easily in soil. Chemicals, such as paraquat, that are strongly adsorbed onto clay and humus show limited downward movement. The leaching of pesticides can contaminate ground water.
Pesticides also move by run-off and erosion.
Such movement most likely occurs on sloping land during periods of intense rain or excessive irrigation. This can lead to contamination of lakes and other surface water. In Colorado, atrazine was reported in subsurface water. For the most part, pesticide concentrations in ground water have been lower than the health advisory limits set by the U.S. Environmental Protection Agency. However, the concentrations of diazinon, chlordane and a few herbicides exceeded these limits at certain locations.