Chlorpyrifos may enter the soil environment by direct application to control soil-dwelling insect pests or indirectly through spray drift or rainfall wash-off following spray application to plant foliage. Chlorpyrifos residues within the soil profile are broken down by the action of soil microbes and also by chemical processes. On the soil surface, chlorpyrifos residues are also broken down by the action of sunlight and may also evaporate into the air where breakdown by sunlight also occurs. On the soil surface, the product’s half-life can be as short as a few days or a couple of weeks, whereas within the soil profile typical half-lives would be from four to eight weeks. Variations in soil properties (acidity/alkalinity, organic matter content, microbe populations, etc.) and environmental conditions (temperature, moisture, etc.) can yield somewhat shorter or longer half-lives. For example, in some alkaline soils with high pH, breakdown can occur significantly more rapidly due to chemical hydrolysis.
Chlorpyrifos binds strongly to soil minerals and organic matter, and this binding reduces the likelihood of surface runoff effects and leaching into surface water and groundwater. Large-scale field runoff studies have confirmed that even under relatively severe conditions (heavy rainstorms closely following application), generally less than one percent of the applied chlorpyrifos can move off the edge of treated fields through runoff water and eroding soil particles. Farm management practices that reduce the degree of sediment erosion, such as planting of grass strips along field borders, also reduce the potential movement of chlorpyrifos and many other farm chemicals.
Racke, K.D. “Environmental Fate of Chlorpyrifos.” Reviews of Environmental Contamination and Toxicology, 1993, Volume 131:1-154.
Poletika, N.N. et al. “Chlorpyrifos and atrazine removal from runoff by vegetated filter strips: experiments and predictive modeling.” Journal of Environmental Quality, 2009, Volume 38(3):1042-1052.