bulk physicochemical attributes of soil are a direct function of the local geology (e.g., the amount of clay and type of clay mineral present, the carbonate content of the soil, or the availability of mineral iron oxides).

Viral particles behave in a fashion resembling passive charged colloids, where the charges of the viral and mineral surfaces are the important characteristic (Ashbolt, 2004). Viral colloids interact with other charged surfaces, particularly clays, and this interaction can be modeled using well-established double-layer models for solute sorption and exchange on mineral surfaces. Soil mineralogy and mineral surface geochemical properties provide a clear geological component to viral transport.

A recently advanced theory on microbial transport in porous media involves the nutrient requirements of the microbial population and the nutrient content of the soil or aquifer minerals (Rogers and Bennett, 2004). In a sand and gravel aquifer contaminated with crude oil, field and laboratory experiments showed that different minerals support dramatically different microbial populations independent of the mineral surface charge. The primary control of surface attachment and adhesion is the nutrient content of the mineral, with the critical nutrient varying with the dominant metabolic guild. Although physical filtration is the principal factor controlling microbial transport in fine-grained soils, in coarse-grained soils both the mineral surface charge and the mineral chemical composition also influence transport.

Microbial metabolism also represents an indirect threat to public health, through biogeochemical cycling of elements, alteration of soil gas composition, weathering of minerals, and altering element speciation (Chapelle, 2001; Ehrlich, 1996). For many slow geochemical processes, microbial catalysis is the primary mechanism for rapid and significant change in metal speciation and mobility (Huang et al., 2004; Islam et al., 2004). Microorganisms are now recognized as an important factor in agriculturally important metal chemistry, particularly for iron (Burd et al., 2000), as well as in the chemistry of toxic metal contaminants. Basic geological attributes such as mineralogy and permeability directly influence soil pH, moisture content, and redox potential and, as a result, influence the dominant microbial community.

HEALTH EFFECTS OF TRACE ELEMENTS AND METALS IN EARTH MATERIALS

Both toxic and beneficial trace elements are naturally present in soils as a consequence of soil parent minerals and as a result of atmospheric deposition of natural materials (e.g., volcanic ash). They are also present as a result of anthropogenic inputs, including application of treated sew-



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