Numerous studies over the past 40+ years have established that pesticides & herbicides, typically applied at the land surface, can move downward through the soils unsaturated zone to the water table at detectable concentrations. This downward movement of pesticide degradation products, formed in situ, can also contribute to the contamination of ground water. Once reached ground water, depending upon the chemical structure of the compounds and the environmental conditions, pesticides and their degradation products can persist for years.
Scientists at the U.S. Geological Survey (USGS) investigated the occurrence of selected pesticides (plus degradation products) in ground water. The group of scientists examined several of the factors (e.g. oxidation-reduction (redox) conditions and ground-water residence times), that can influence the likelihood with which pesticides and their degradation products are detected in shallow ground water at four study sites across the United States. Click here for the paper which is free to non-subscribers of Journal of Environmental Quality for July 2008.