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GEOCHEMISTRY & OCCURRENCE OF SELENIUM Plains of the USA or the prairie regions of Canada is the Se-enriched Pierre Shale formed during the Cretaceous period. In these areas, certain plants tend to be enriched in Se (Mayland, 1985). However, the uptake of Se by plants from soils does not necessarily cor- respond to the total Se content of the soils. In general, the most bioavailable form of Se in soils is considered to be the water-soluble fraction (Kabata- Pendias &Pendias, 1984). Selenium can also be enriched in certain phos- phate rocks (Table 1-1). This has implications for agricultural environments where phosphate fertilizers are used (Robbins & Carter, 1970; Mayland, 1985). Most natural waters tend to have low concentrations of Se (<0.01 mg/L). Notable exceptions can occur if waters are alkaline or if they leach and drain seleniferous rocks and soils. One example of the pH effect is in the data of Scott and Voegeli (1961). When the pH of various Colorado rivers falls in the range of 6.1 to 6.9, the river water Se concentration is generally <0.01 mg/L. However, when the pH of the water is between 7.8 and 8.2, the Se concentrations range from 0.01 to 0.4 mg/L. One recent example of waters leaching and draining seleniferous rocks and soils is in the San Joaquin Valley. Studies have reported up to 4.2 mg Se/L in water draining irrigated land in the valley (Barnes, 1986; Sylvester, 1986). The form of Se in the waters is Se04-, the most mobile and bioavail- able form of Se. This Se-enriched water is carried to the Kesterson Reservoir where it caused toxicity problems for the wildlife (Kilness & Simmons, 1986). Biological activity plays a strong role in the distribution of Se in the environment. The accumulation of Se in the plants and animals is highly vari- able (Table 1-1) and, as noted before, can favorably or adversely affect their growth, survival, and reproduction (Eisler, 1985). Selenium can be adsorbed by plants and incorporated into their tissues. Both Se-containing amino acids and proteins have been identified (Shrift, 1973). The extent of Se accumula- tion depends on the type of plant; the speciation of Se; and the pH, salinity, and calcium carbonate (CaC03) content of the soil (Kabata-Pendias & Pen- dias, 1984). In turn, Se-enriched plants can be potentially hazardous to livestock and humans. The speciation and form of Se can change as a result of biological and physical processes. Selenate and Se03 - can be reduced to red amorphous Se by numerous bacteria and yeasts (NAS-NRC, 1976). This process signifi- cantly limits the availability of Se to plants. Inorganic Se compounds can be converted to volatile organic compounds such as dimethyl selenide or dimethyl diselenide by plants, microorganisms, and animals (Shrift, 1973; Chau et al., 1976). The formation of methylated Se compounds by animals appears to be one mechanism for Se detoxification as the toxicity of dimethyl selenide is about 1/500 to 1/1000 of the toxicity of See- (Vokal-Borek, 1979). Selenite can also be oxidized to Se04-, presumably by weathering processes (Vokal-Borek, 1979), but the reaction kinetics are thought to be slow (Cary et al., 1967). Several inorganic chemical processes control the distribution of Se. One is the adsorption of Se by particles. Selenite has a stronger affinity for parti- cles, particularly Fe oxyhydroxides, than Se04-, and that adsorption is