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18 MAYLAND ET AL. Knowledge of Se in the environment prior to the mid-1970s has been summarized in a number of excellent reviews: Allaway et al. (1967), Alla- way (1968), Anderson et al. (1961), Anderson and Scarf (1983), Brown and Shrift (1982), Gissel-Nielsen et al. (1984), Koller and Exon (1986), Martin (1973), Moxon and Rhian (1943), NAS-NRC (1974, 1976, 1983), Reuter (1975), Sharma and Singh (1983), and Shrift (1958, 1969, 1973). Selenium information is also given in the recent publications by Adriano (1986), Combs and Combs (1986), Ihnat (1989), and Levander (1986). Since the mid-1970s human activities have introduced new sources of Se into the ecosystem, and much attention is now being given to the anthropo- genic impact of Se cycling in seleniferous environments. This chapter will briefly describe Se in the various compartments that were characterized through the mid-1970s but will elaborate in more detail on the impact of human activities. However, those aspects associated with the central Califor- nia experience will be detailed in other chapters of this special publication. Low, medium or adequate, and high levels of bioavailable Se, as used in this chapter, will refer to those levels in the soil that result in concentra- tions of < 0.1, 0.1 to 1.0, and > 1.0 mg Se/kg, respectively, in the non-Se- accumulator plants growing on such soils. GEOCHEMISTRY Selenium occurs in nearly all materials of the earth's crust and is present in magmatic rocks in concentrations generally <0.05 mg/kg (Table 2-1). During volcanic activity, much of the Se escapes as high-temperature vola- tile gases (Reuter, 1975); thus, igneous rocks are usually low in Se. The ele- ment is rarely seen in elemental form (Se°) but may be found in association with sulfide ore bodies (Anderson et al., 1961). Selenium concentrations are higher in sedimentary rocks where it is as- sociated with the clay fraction, and thus, larger concentrations of Se are found in shales than in limestones or sandstone. Even in shales, the concentrations Table 2-1. Selenium concentration in magmatic and sedimentary rocks (compiled from Kabata-Pendias &Pendias, 1984-. Rock type Se conc., mg/kg Magmatic rocks Ultramafic rocks (Dunites, Pendotites, Pyroxeuites) 0.02-0.05 Mafic rocks (Basalts, Gabbros) 0.01-0.05 Intermediate rocks (Diorites, Syenites) 0.02-0.05 Acid rocks (Granites, Gueissas) 0.01-0.05 Acid rocks-volcanic (Rhyolites, Trachytes) 0.02-0.05 Sedimentary rocks Argillaceous sediments 0.4-0.6 Shales 0.6 Sandstones 0.05-0.08 Phosphorites 1-100 Limestones, dolomites 0.03-0.10