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SELENIUM IN SELENIFEROUS ENVIRONMENTS 21 <br />1.4 <br />1.2~ ~ - <br />\ Se03~ ~ H2 _ <br />I.0 HZO, <br />0.8 <br />HSe03 SeOq-- <br />0.6 Se ~~~\ - <br />o Se03 " SeOq - <br />_'. 0.2 - <br />t <br />W O.0 , ~ HZp Se <br />-0.2 Hp ~~. Seo3 - <br />Se <br /> <br />pH <br />Fig. 2-3. Variation of oxidation potentials for the predominating states of Se as a function <br />of pH. A line intersecting the point Eh = 0.4 and pH = 4 and the point Eh = 0.1 and pH <br />=9 divides the oxidized soils above and the reduced soils below (adapted from Lakin & David- <br />son, 1967 and NAS-NRC, 1976). <br />amined the bioavailability of soil Se by using ammonium bicarbonate-DTPA <br />(diethylenetriaminepenta acetic acid) as a soil extractant. Alfalfa (Medicago <br />sativa L.) contained in excess of 5 mg Se/kg when growing on mollisols con- <br />taining more than 45 mg extractable Se/kg soil. Alfalfa yields were reduced <br />when the extractable soil Se exceeded 500 mg/kg. The acutal amount of Se <br />for toxicosis will depend on species of plant, species and age of animal, and <br />other factors including concentrations of S04-S, P, and sesquioxides in the <br />soil. <br />Soils that were formed from either high or low Se parent materials, but <br />have poor aeration; acid pH; and high rainfall are likely to be soils that may <br />be deficient in either total or at least bioavailable Se. Plants growing on these <br />latter soils may not contain sufficient Seto meet animal requirements. More <br />detail on the uptake and accumulation of Se by agricultural crops is pre- <br />sented in Chapter 4 of this publication. <br />SELENIUM IN WATER <br />Selenium occurs as a minor constituent in drinking water in a concen- <br />tration range of <0.1 to 100 µg/L (NAS-NRC, 1983). Samples rarely ex- <br />ceed the 10 µg/L upper limit established by the 1977 Safe Drinking Water <br />Act of the USEPA. However, water derived from some of the Cretaceous <br />zones may have high concentrations of both Se and total soluble salts. For <br />example, water from wells drilled into any of the geologic formations of the <br />Cretaceous Colorado group in central Montana may contain as much as 1000 <br />µg Se/L (Donovan et al., 1981). The total dissolved solids in this water aver- <br />age 9260 mg/L. <br />