Selenium - The Risk Assessment Information System

2.0 SUMMARY OF ECO-SSLs FOR SELENIUM
Selenium is a naturally occurring element found in all environmental media: air, soil, sediment,
and water. In nature it is found in the sulfide ores of heavy metals and predominates in
approximately 40 minerals including clausthalite, naumannite, tiemannite, and berzelianite
(Budavari, 1996; Fishbein, 1991). Selenium is also found in volcanic rock, sandstone, shale,
carbonates, bedrock, coal oil, and mineral oil (Kent and Spycher, 1994; Langer, 1993).
Selenium may be released to the environment from natural sources such as volcanic eruptions,
leaching and weathering of rocks, and volatilization as a result of biomethylation by plants and
bacteria (ATSDR, 1996). Anthropogenic releases of selenium may result from use in the
manufacture and production of glass, pigments, rubber, metal alloys, textiles, petroleum, medical
therapeutic agents, anti-dandruff shampoos, veterinary medicines, fungicides, gaseous insulators,
and photographic emulsions (USDOI, 1985; ATSDR, 1996). The burning of coal, oil and solid
waste may also contribute to selenium in the environment (NRC, 1976; ATSDR, 1996).
Background concentrations
reported for many metals in U.S.
soils are described in Attachment
1-4 of the Eco-SSL guidance
(U.S. EPA, 2003). Typical
background concentrations of
selenium in U.S. soils are plotted
in Figure 2-1 for both eastern and
western U.S. soils.
In soils, the chemical forms of
selenium are largely dependent
on pH and oxidation-reduction
potentials (McNeal and
Balistrieri, 1989). Selenium can
exist in the 2-, 0, 4+, and 6+
oxidation states (McNeal and
Balistrieri, 1989). Speciation of
selenium in soils is also
Conc (mg/kg dw)
influenced by the chemical and mineralogical composition of the soil,
microbial intervention, and the nature of the adsorbing surfaces (Neal,
1990). Selenium has a sorptive affinity for hydrous metal oxides, clays,
and organic materials. In well-aerated alkaline soils, inorganic selenium
exists primarily as the oxyanions selenite (Se(4+)) and selenate (Se(6+)).
4
3.5
3
2.5
2
1.5
1
0.5
0
East West
Figure 2-1 Typical Background
Concentrations of Selenium
in U.S. Soils
Maximum
Eco-SSL for Selenium 2
July 2007
95th
75th
50th
25th
5th Percentile
Selenite is soluble, but can strongly adsorb to soil minerals and organic material (Tokunaga et al.,
1997), while selenate is the most mobile of selenium compounds because of its high water
solubility and inability to adsorb to soil particles (ATSDR 1996). Alkaline soils formed from
parent materials high in selenium are also high in biologically available selenium (Mayland et al.,
1989). In poorly aerated soils, inorganic Se predominates as the relatively soluble selenide and