Bioavailability of xenobiotics in the soil environment.

When synthetic, xenobiotic compounds such as agrochemicals and industrial chemicals are utilized, they eventually reach the soil environment where they are subject to degradation, leaching, volatilization, sorption, and uptake by organisms. The simplest assumption is that such chemicals in soil are totally available to microorganisms, plant roots, and soil fauna via direct, contact exposure; subsequently these organisms are consumed as part of food web processes and bioaccumulation may occur, increasing exposures to higher organisms up the food chain. However, studies in the last two decades have revealed that chemical residues in the environment are not completely bioavailable, so that their uptake by biota is less than the total amount present in soil (Alexander 1995; Gevao et al. 2003; Paine et al. 1996). Therefore, the toxicity, biodegradability, and efficacy of xenobiotics are dependent on their soil bioavailability, rendering this concept profoundly important to chemical risk assessment and pesticide registration.