BOX 5.1

Anthropogenic Contaminants and Natural Attenuation

Soil microbial populations form one component of the “natural attenuation” approach to the remediation of contaminated soil (NRC, 1993, 2000a), where the subsurface microbial community degrades contaminants. This interaction often (but not inevitably) results in a decrease in the concentration of toxic components and eventual remediation of the contaminated soil, providing protection for the food supply.

Growing evidence exists for a link between the geochemical and mineralogical properties of a subsurface system and the efficiency of biotransformation of organic contaminants (Rogers et al., 1998a, 1998b; Rogers, 2000; Rogers and Bennett, 2004; Bennett et al., 2000, 2001). In a typical hydrocarbon-contaminated soil or aquifer, the biogeochemical system is carbon substrate rich but nutrient poor, and the overall attenuation efficiency is limited by available ferric iron, nitrogen, and/or phosphorus (Chapelle, 2001). Laboratory and field experiments have demonstrated that the inorganic nutrient content of the constituent minerals directly influences the rate of hydrocarbon degradation by anaerobic microorganisms. In methanogenic regions, phosphate is the critical nutrient, whereas in iron-reducing zones the availability of nitrogen and iron constitute the limiting nutrients. Soil mineralogy is therefore a key control on the microbial detoxification of soil and a fundamental part of the microbial habitat description.

Another important example of indirect benefits of soil microbes, and the influence of the earth sciences on what we eat, is in the efficient breakdown of the various organic pesticides used to enhance agricultural yields (Corona-Cruz et al., 1999; Ragnarsdottir, 2000). After application, a valuable attribute of an effective pesticide is efficient action on the target plant or insect pest, followed by rapid degradation to limit runoff or indirect damage to valuable organisms (Maier et al., 2000). Both abiotic and biotic mechanisms are critical in degrading organic pesticides, and geological factors are important for both. In particular, soil pH, clay content, and moisture content are important factors for microbial degradation of organic pesticides.

infect agricultural products and decrease yields or damage the products (Tate, 2000). Although the geological controls on direct infection by plant or human pathogenic organisms have not been extensively investigated, the geochemical environment is a fundamental attribute of the soil habitat, and the basic physical attributes of soil such as pH, temperature, moisture content, porosity, permeability, and organic matter content are all important for the viability of pathogenic organisms and the structure and species composition of the soil microbial community (Kodama, 1999). The



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