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in Fig. 3, selenium reduction can occur under moderately reducing conditions (+100 to +400 mv). Under these same conditions, organic acid oxidation occurs (Masscheleyn and Patrick 1993). Thus, depolymerization or mineralization to inorganic selenium is a source of selenium for aquatic organisms and plant-root uptake (Masscheleyn and Patrick 1993). Conversion in soils has been reported as relatively slow because of inhibition by stable ferric selenites (Geering et al. 1968). However, another study, which also referred to selenium oxidation as slow, reports that the conversion is biotic and yields both selenate and selenite (Losi and Frankenberger 1998). In contrast to the slow oxidation, reduction of oxidized forms can occur in a matter of hours (Losi and Frankenberger 1998). Guo et al. (1999) reports that selenate reduction occurs within about 30 hours and that the reduced forms are stable even under extreme leaching conditions. Another study reports that selenate reduction is fast, within a day typically, and is microbially mediated. These investigators also report that the reduction potential is correlated to the amount of selenium present and not the amount of organic matter (Zhang and Moore 1997). Adding organic matter in column studies does cause selenate reduction on the column. The latter experiment also shows that more sorption seems to occur with intermittent leaching (Neal and Sposito 1991). 2.2.1 Cycling in Wetlands In wetlands where organic matter accumulates, some selenium can be expected to be present in an inorganic form, chiefly selenite, sorbed to inorganic minerals, primarily Fe-oxyhydroxicdes, or covalently bonded to organic matter. When wetlands are reduced further, the Fe may dissolve releasing the selenium. A removal mechanism in wetlands is the microbial production of dimethyl diselenide. Dimethyl diselenide is very insoluble in water and has a very high vapor pressure, so it is not persistent (Masscheleyn and Patrick 1993). Studies of pond sediments from Kesterson indicate that the selenium in the ponds and drains was mostly selenium-11, but that selenium would be available if the sediments dried out (Martens and Suarez 1997). An experiment was conducted in which it was shown that selenium was immobile in untreated sediments, but air drying and freeze drying followed by water leaching mobilized it. The data from this experiment indicate that the highest concentrations were found almost immediately (Alemi et al. 1988b). Apparently, approximately 99% of the selenium in the ponds at Kesterson is the red, monoclinic elemental form. Rapid reoxidation of selenium was observed in some of the previously ponded sediments. Sixty percent was reoxidized to a mixture of selenium-IV and selenium-VI in unamended sediments within two days of sampling, but if organic matter was added, the reoxidation did not occur, at least over the 2-day period. Reoxidation occurred merely with exposure to air (Tokunaga et al. 1996). The latter results seem to contrast with those of Losi and Frankenberger (1998) who report that reoxidation is biotic and slow. Finally, another study reports that the reduction occurred only in the top 1 cm of sediment (Tokunaga et al. 1997). 2.3 Discussion Selenium cycling in soils and lower trophic levels has been studied extensively. The inorganic behavior of selenium appears to be well defined, but the combined geochemical/biochemical cycling is less well established. Uncertainties remain with respect to the structures of selenium metabolites in biological material and with respect to the kinetics regarding its remobilization from sediments. The latter issue is of great concern. An improved ability to predict selenium 13