Prior to the late 1970's ground water contamination from field applied pesticides was virtually unexpected. It was assumed that pesticides in the natural environment would either break down or that the soil, sand, gravel, and rock formations would be adequate to cleanse water of its contaminants before it reached ground water. Now it is clear that human activities can lead to contamination of ground water. The toxicity of pesticides makes them effective in controlling pests, but this toxicity means that pesticides must be properly applied and managed so that their potential to affect health and to contaminate the ground water is minimized. To reduce the risk of contaminating the ground water, it is essential to understand the factors that affect the behavior of pesticides in the natural environment.
Ground water is found in spaces between soil particles and rocks, and within cracks of bedrock.
The top of the saturated zone is the water table. Major reservoirs of ground water are called aquifers. Aquifers are recharged by rain, snowmelt, or interchange with surface waters. Ground water is not stationary, but moves vertically or horizontally following the slope of the water table. Ground water may be contaminated by a variety of elements including domestic waste, nitrate nitrogen, bacteria, viruses, synthetic organic chemicals, heavy metals, petroleum residues and combustion products from roadways.