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10 McNEAL & BALISTRIERI strongly influenced by pH (Merrill et al., 1986; Balistrieri & Chao, 1987, 1989). The other process is the formation of metal selenides or Se-sulfides. Sulfide minerals such as galena and pyrite tend to act as carriers for Se. Some sulfide minerals have been reported to contain up to 20QIo Se (Nazarenko & Ermakov, 1972). Thus, adsorption and mineral formation limit the mo- bility and bioavailability of Se in the environment. CONCLUSIONS Several generalizations can be made based on the brief reviews of the chemistry of Se and its cycling through the environment. 1. Selenium is widely distributed throughout the environment by phys- ical, chemical, and biological processes. Its distribution and the processes controlling its behavior in the environment are strongly in- fluenced by the chemical and biological reactivities of its various ox- idation states and the pH and redox conditions of the environment. 2. Selenate (SeO4-) is thermodynamically stable in alkaline and well- oxidized environments. Selenate salts are very soluble and SeO4 - is not as strongly adsorbed as SeO3 - . These characteristics allow for easy leaching of SeO4- from soils and transport to groundwaters. Selenate is also the form most readily taken up by plants. 3. Selenite (SeO3-) tends to occur in mildly oxidizing, neutral pH en- vironments. Its salts are not as soluble as Selenate salts. Selenite is strongly adsorbed by particles and both chemical and biological processes can reduce it to elemental Se (Se°). These latter two processes limit the mobility of Se in the environment and its availa- bility to plants and animals. However, oxidation of SeO3- to Se04- in alkaline and well-oxidized environments, although slow, can in- crease the mobility of Se and its assimilation by plants. 4. selenides (SeZ-) and Se-enriched sulfides exist in reducing, acidic, and organic-rich environments. Metal SeZ- or Se-sulfide salts are very insoluble and, generally, resistant to oxidation. This makes these forms of Se unavailable to plants and animals. REFERENCES Adriano, D.C. 1986. Trace elements in the terrestrial environment. Springer-Verlag, New York. Allaway, W.H. 1968. Control of the environmental levels of selenium. Trace Subst. Environ. Health 2:181. Balistrieri, L.S., and T.T. Chao. 1987. Selenium adsorption by goethite. Soil Sci. Soc. Am. J. 51:1145-1151. Balistrieri, L.S., and T.T. Chao. 1989. Adsorption of Se by amorphous iron oxyhydroxide and manganese dioxide. Geochim. Cosmochim. Acta (in press). Barnes, I. 1986. Sources of selenium. p. 41-51. In Selenium and agricultural drainage: Implica- tions for San Francisco Bay and the California environment. Proc. Second Selenium Symp., Berkeley, CA. 23 Mar. 1985. The Bay Institute of San Francisco, Tiburon, CA.