Xenobiotics are chemical substances that are foreign to the biological system. They include naturally occurring compounds, drugs, and environmental agents (Mondofacto online medical dictionary at www.mondofacto.com).The classes of xenobiotics include: pesticides, polyaromatic hydrocarbons (PAHs), polychlorinated aromatics, solvents, hydrocarbons, and others (surfactants, silicones, and plastics).
Xenobiotics levels in soils are generally low (less than 100 ppm) unless they are concentrated by application as in the case of pesticides, by spills or by waste disposal. They can occur in soils in solid, dissolved and gaseous phases and all undergo microbial and abiotic (chemical) transformations (Logan, 2000).
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Pesticides are the most important xenobiotic pollutants because of their widespread use in agriculture. In many developing countries, unregulated use of pesticides by poor farmers contributes not only to environmental pollution but to health problems as well.
In the soil, pesticides can be temporarily fixed through adsorption by soil particles. The persistence or decomposition of pesticides in the soil is influenced by soil moisture, organic matter content, redox potential, soil acidity, soil temperature, texture, adsorption potential and clay minerals (Schactschabel et al., 1998; Sonon and Schwab, 2004).
References
Logan, T.J. 2000. Soils and environmental quality. In: Handbook of Soil Science (M.E. Sumner, ed.). CRC Press, Boca Raton, pp: G155-G169.
Schactschabel P., H.P. Blume, G. Brümmer, K.H. Hartge and U. Schwertmann. 1998. Lehrbuch der Bodenkunde (14th ed.). Ferdinand Enke Verlag, Stuttgart.
Sonon, L.S. and P.A. Scwab. 2004. Transport and persistence of nitrate, atrazine and alachlor in large intact soil columns under two levels of moisture contents. Soil Science 169: 541-553.