6.2.1. Air

Combustion of coal and other fossil fuels is the primary source of airborne selenium compounds. In air, elemental selenium burns to form selenium dioxide; however, during the combustion of fossil fuels, essentially all of the selenium dioxide produced should be reduced to elemental selenium by the sulfur dioxide that results from the combustion of these materials (NAS 1976a). Estimates of the quantity of selenium released to the air from fossil fuel combustion vary. Estimated annual selenium air emissions from stationary sources in the United States for 1969–1971, 1978, and 1983 were 900, 1,240, and 1,560 tons selenium/year, respectively (EPA 1974; Lee and Duffield 1979). Dulka and Risby (1976) estimated yearly releases of selenium to the air from fossil fuel combustion to be 1,000 tons. Harr (1978) estimated that 1,500 tons were released annually, with additional air releases from industrial and municipal wastes totaling 2,700 tons and 360 tons, respectively. Selenium releases to the air are likely to increase as more coal is burned in the future. The estimated selenium emissions from Canadian non-ferrous smelters (stack plus fugitive) were 3.02 tons in 1993 (Skeaff and Dubreuil 1997).

Table 6-1

Releases to the Environment from Facilities that Produce, Process, or Use Selenium.

Table 6-2

Releases to the Environment from Facilities that Produce, Process, or Use Selenium Compounds.

Incineration of rubber tires, paper, and municipal waste is an additional source of atmospheric selenium. Hashimoto et al. (1970) reported selenium concentrations in rubber tires to be 1.3 mg/kg. Seventy different kinds of paper have been found to contain selenium (West 1967). Combustion of municipal solid waste results in stack emissions ranging from 0.00098 to 0.00216 pounds (0.44–0.98 g) of selenium per ton of refuse (Johnson 1970).

The amount of selenium contributed to the air by other sources is not known. Microbial action within the soil may also contribute selenium to the air (Fishbein 1983). Selenium biomethylation volatilizes about 3,000 tons of selenium per year into the atmosphere, which eventually returns to earth in rainfall (NAS 1976a). Volcanic gas is suspected to be the major natural source of atmospheric selenium. Certain plants metabolize inorganic selenium compounds to volatile selenium in the forms of dimethyl selenide (Lewis et al. 1971) and dimethyl diselenide (Evans et al. 1968). Animals are also capable of volatilizing selenium and releasing dimethyl selenide in expired air (Schultz and Lewis 1940).

Fly ash settling ponds (which contain high concentrations of selenium) and hazardous waste sites where selenium compounds were disposed of in the past are potential sources of atmospheric selenium through fugitive dust emissions. Selenium emissions from these potential sources have not been quantified.

According to TRI, an estimated total of at least 65,481 pounds of elemental selenium and 576,929 pounds of selenium compounds were discharged to the air from manufacturing and processing facilities in the United States in 2000 (TRI00 2002) (see Tables 6-1 and 6-2). The data listed in the TRI tables should be used with caution since only certain types of facilities are required to report. This is not an exhaustive list.

Selenium has been identified in air at 13 of the 508 NPL hazardous waste sites where it was detected in some environmental media (HazDat 2003).

6.2.2. Water

Surface waters can receive selenium from the atmosphere by dry and wet deposition, from adjoining waters that may contain selenium, from surface runoff, and from subsurface drainage. Sewage treatment plants are another source of selenium releases to water. Effluents from sewage treatment plants and oil refineries appear to be the major sources of selenium in the San Francisco estuarine system (Cutter 1989). In a study of direct discharges from oil refineries in San Francisco Bay, the average selenium concentration in the effluent was 0.067 mg/L with a range of 0.0066–0.156 mg/L (Barceloux 1999; Cutter 1989). Approximately 50–76% of the total selenium in the effluents was selenite. This proportion of selenite is higher than that found in natural estuary sources in the San Francisco Bay (Cutter 1989). About 150,000–460,000 tons of selenium per year are deposited in coal fly ash (Andren and Klein 1975; Doran 1982). Selenium from fly ash settling ponds and hazardous waste sites could reach surface water via runoff or could reach groundwater via leaching. Concentrations of 0.10–0.25 mg/L in a settling basin effluent from coal fly ash in North Carolina were reported by Lemly (1985). Overflow from the ash basin of a coal fired electric generating facility to Belews Lake resulted in surface water selenium concentrations of 0.005–0.020 mg/L in the lake basin. These levels have been reduced considerably since 1986 when the discharge of selenium laden waste water to the lake was discontinued. The peak selenium concentration in 1996 was >0.001 mg/L (Lemly 1997). Selenium concentrations as high as 0.28 mg/L have been reported for raw sewage, 0.045 mg/L for primary effluent, and 0.050 mg/L for secondary effluent (Baird et al. 1972). Irrigation drainage from seleniferous soils can increase selenium concentrations in surface water and has resulted in levels that are toxic to wildfowl at Kesterson National Wildlife Refuge in California (Ohlendorf et al. 1986a, 1988). Selenium was found to be released during coal mining because of the oxidation of selenium-bearing pyrite (Dreher and Finkelman 1992).

According to the TRI, an estimated total of 1,019 pounds of elemental selenium and 56,048 pounds of selenium compounds were discharged to surface water from manufacturing and processing facilities in the United States in 2000 (see Tables 6-1 and 6-2). The data listed in the TRI tables should be used with caution since only certain types of facilities are required to report. This is not an exhaustive list.

Selenium has been identified in groundwater at 271 sites and surface water at 106 sites of the 508 NPL hazardous waste sites where it was detected in some environmental media (HazDat 2003).

6.2.3. Soil

The primary factor that controls selenium concentrations in soil is the selenium content of the parent bedrock materials that release selenium via weathering processes and leaching (NAS 1976a). Natural weathering processes are thought to release about 100,000–200,000 metric tons of selenium per year (Andren and Klein 1975). Atmospheric deposition of selenium also contributes to selenium in the soil. In the past, selenium was used in pesticide products, but because of its stability in soils and subsequent contamination of food crops, its use in pesticide products is now restricted. The release of selenium to soil from fly ash settling ponds and hazardous waste sites has not been quantified.

According to the TRI, an estimated total of 157,521 pounds of elemental selenium and 7,209,933 pounds of selenium compounds were discharged to land from manufacturing and processing facilities in the United States in 2000 (TRI00 2002). In addition, 40,246 pounds of selenium and 27,699 pounds of selenium compounds were injected underground (see Table 6-2). The data listed in the TRI tables should be used with caution since only certain types of facilities are required to report. This is not an exhaustive list.

Selenium has been identified in soil at 188 sites and sediment at 113 of the 508 NPL hazardous waste sites where it was detected in some environmental media (HazDat 2003).

6.3.1. Transport and Partitioning

The volatile selenium compounds that partition into the atmosphere include the inorganic compounds, selenium dioxide and hydrogen selenide, and the organic compounds, dimethyl selenide and dimethyl diselenide. Hydrogen selenide is highly reactive in air and is rapidly oxidized to elemental selenium and water (NAS 1976a), but the other compounds can persist in air.

Selenium compounds released to the atmosphere can be removed by dry or wet deposition to soils or to surface water. The annual wet deposition rate of selenium at two rural/agricultural sites in Queenstown, Maryland and St. Mary’s, Maryland were 287 and 140 µg/m²-year, respectively (Scudlark et al. 1994). Selenium concentrations ranging from 0.04 to 1.4 µg/L have been detected in rain and snow (Hashimoto and Winchester 1967). Kubota and coworkers (1975) reported selenium concentrations of 0.02–0.37 µg/L in rainwater at several locations in the United States and Denmark. Selenium was detected at average concentrations of 5.60–7.86 µg/L during four rainfall events in Riyadh, Saudi Arabia (Alabdula’aly and Khan 2000).

The forms of selenium expected to be found in surface water and the water contained in soils are the salts of selenic and selenious acids. Selenic acid (H₂SeO₄) is a strong acid. The soluble selenate salts of this acid are expected to occur in alkaline waters. Sodium selenate is one of the most mobile selenium compounds in the environment because of its high solubility and inability to adsorb onto soil particles (NAS 1976a). Selenious acid (H₂SeO₃) is a weak acid, and the diselenite ion predominates in waters between pH 3.5 and 9. Most selenites are less soluble in water than the corresponding selenates (NAS 1980b).

Selenium in an aquatic environment is bioaccumulated by aquatic organisms (Chau and Riley 1965; Ohlendorf et al. 1986a; Rudd and Turner 1983a; Saiki and Lowe 1987). Lemly (1985) has reported bioconcentration factors (BCFs) of 150–1,850 and bioaccumulation factors (BAFs) of 1,746–3,975 for selenium in freshwater. In the Kesterson National Wildlife Refuge in the San Joaquin Valley of California, elevated levels of selenium have been measured (dry weight) in algae (average 35 mg/kg), midge larvae (139 mg/kg), dragonfly and damselfly nymphs (average 122 and 175 mg/kg, respectively), and mosquito fish (170 mg/kg) (Ohlendorf et al. 1986b). For comparison, the mean concentrations of selenium found in fish throughout the United States in the 1976–1977, 1978–1979, and 1980–1981 National Pesticide Monitoring Program were 0.56, 0.46, and 0.47 mg/kg wet weight, respectively (Lowe et al. 1985; May and McKinney 1981; Ohlendorf et al. 1986b). Similarly, Lemly (1985) found elevated selenium concentrations in aquatic organisms living in a power plant cooling reservoir in North Carolina. The degree of bioaccumulation of selenium exhibited a stable pattern over several years, with selenium concentrations (wet weight) as follows: fish (6–35 mg/kg) > benthic insects (12–15 mg/kg) > annelids (10–12 mg/kg) > molluscs and crustaceans (5–9 mg/kg) > periphyton (4–6 mg/kg) (Ohlendorf et al. 1986a). In fish, selenium was concentrated in visceral tissue (25–35 mg/kg wet weight) more than in skeletal muscle (6–11 mg/kg wet weight). Adams (1976) reported BCFs of 62.1, 14.3, 6.3, 3.2, and 10.5 for selenium in the viscera, gill, head and tail, muscle, and whole trout, respectively. The BCFs and BAFs for selenium in visceral tissue (i.e., heart, hepatopancreas, spleen, and gonads) of fish have been estimated to range from 35 to 1,850 and from 1,058 to 3,980, respectively (Lemly 1982, 1985). Lemly (1985) also estimated BAFs for selenium in skeletal muscle of fish to range from 485 to 1,746, depending on the species. Maier et al. (1988) estimated selenium BAFs for algae to range from 100 to 2,600, and Besser et al. (1993) estimated BCFs of 16,000 for algae, 200,000 for daphnids, and 5,000 for bluegills from exposures to 1 µg/L selenomethionine. Selenite was more concentrated than selenate for algae and daphnids, whereas bluegills concentrated both inorganic species about equally (Besser et al. 1993). Selenium accumulation from selenomethionine occurred more readily than from selenite or selenate (Besser et al. 1989).

Some evidence indicates that selenium might biomagnify in aquatic organisms under natural conditions (Lemly 1985; Maier et al. 1988; NCDNR 1986; Sandholm et al. 1973). Biomagnification is evidenced by progressively higher concentrations of an element or substance in organisms at successively higher trophic levels. More than 50% of the selenium contained in sediments in the ponds and the reservoir in the Kesterson National Wildlife Refuge in California occurs in organic forms (Maier et al. 1988), resulting from the synthesis and bioaccumulation of organic selenium before the plants die and decay on the bottom.

In soils, pH and Eh are determining factors in the transport and partitioning of selenium. Elemental selenium is essentially insoluble and may represent a major inert “sink” for selenium introduced into the environment under anaerobic conditions (NAS 1976b). Heavy metal selenides and selenium sulfides, which are also insoluble, predominate in acidic (low pH) soils and in soils with high amounts of organic matter. Selenium in this form is immobile and will remain in the soil. The selenides of other metals such as copper and cadmium are of low solubility (NAS 1976b). Sodium and potassium selenites dominate in neutral, well-drained mineral soils, where some soluble metal selenites may be found as well. In alkaline (pH>7.5), well-oxidized soil environments, selenates are the major selenium species. Because of their high solubility and low tendency to adsorb onto soil particles, the selenates are very mobile (Kabatas-Pendias and Pendias 1984) and are readily taken up by biological systems (Klaassen et al. 1986) or leached through the soil. Gerritse et al. (1982) found selenium to be very mobile in sewage sludge leachate. They reported K_d values (distribution coefficient = [concentration of selenium sorbed on soil or rock]/[concentration of selenium in solution]) of 14.9 mL/g for sandy loam and 5.91 mL/g for sludge-treated sandy soils. Selenite forms stable ferric oxide-selenite adsorption complexes in acid or neutral soils (Geering et al. 1968).

When environments favor the soluble forms of selenium (alkaline and oxidizing conditions), these forms can be accumulated by plants. In addition, although both selenite (Se⁴⁺) and selenate (Se⁶⁺) are soluble forms of selenium, selenate was found to be the preferred form of selenium taken up by plants (Baňuelos and Meek 1990). Preferential uptake of selenate may be caused by its tendency to be less strongly adsorbed to soil particles and organic matter than selenite (Baňuelos and Meek 1990). Selenium uptake by plants is influenced by many factors including soil type, pH, colloidal content, concentration of organic material, oxidation-reduction potentials in the root-soil environment, and total level of selenium in the soil (Fishbein 1983; Robberecht et al. 1982). In acidic soils (pH 4.5–6.5) and under high moisture conditions, selenium is in the form of selenite and is bound to colloids as iron hydroxide selenium complexes. These complexes are insoluble and generally not bioavailable to plants (Galgan and Frank 1995). In basic soils (pH 7.5–8.5), selenium is present as soluble selenate. Soluble selenates (principally sodium selenate) appear to be responsible for most of the naturally occurring accumulation of high levels of selenium by plants, although much of the total selenium in soil may be present in other forms (NAS 1976a). The use of lime and plant ash as fertilizers, which would raise the pH of the soil and favor the formation of selenate, has been implicated as a contributing factor in the accumulation of selenium in crops grown in high selenium soil found in certain regions of China (Yang et al. 1988).

6.3.2. Transformation and Degradation

6.4.1. Air

Background ambient air concentrations of selenium are generally in the ng/m³ range (Harrison et al. 1971; John et al. 1973; Peirson et al. 1973). Dams et al. (1970) found concentrations of selenium in suspended air particulate matter of 2.5 ng/m³ in Niles, Michigan, and 3.8 ng/m³ in East Chicago, Indiana. During 1968–1969, 18 air samples collected around Buffalo, New York, showed a range of 3.7–9.7 ng/m³ (Pillay et al. 1971). Based on these results, the National Academy of Sciences (NAS 1976a) has estimated that the average selenium concentration in the air is well below 10 ng/m³. A monitoring study to determine the seasonal variation of pollutants in the air of Alaska was conducted from 1984 to 1987 (Sturges and Shaw 1993). The average concentrations of selenium in Poker Flats, Alaska were 0.035 ng/m³ (June 1 through January 31, 1984–1987) and 0.067 ng/m³ (February 1 through May 31, 1984–1987). The nearly 2-fold increase in concentration during the spring months were attributed to local marine biogenic volatilization of selenium, and not a coal burning origin (Sturges and Shaw 1993). Selenium was detected in the ambient atmosphere at seven sites in the United Kingdom at concentrations ranging from 0.1 to 42.3 ng/m³ (Lee et al. 1994). The lowest levels were observed in the rural areas of Chilton and Windermere with mean concentrations of 1.3 and 0.9 ng/m³, respectively. The highest mean concentration of 16.7 ng/m³ was observed in the industrial area of Walsall.

6.4.2. Water

Selenium has been detected in surface waters and groundwaters in the United States at generally low concentrations. Selenium has been detected in oceans at an average value of 9x10⁻⁵ mg/L (0.09 µg selenium/L) (Schutz and Turekiam 1965). In a study of selenium concentrations in major watersheds of the United States, selenium was detected in only 2 of 535 samples (>0.5%) at a concentration greater than the lowest detection limit of 0.010 mg/L (Lakin and Davidson 1967). Examination of the EPA STORET database for the state of North Carolina revealed that only 3.3% of 657 samples of surface water contained more than 0.001 mg/L, and the highest value was 0.012 mg/L (NCDNR 1986). Watersheds that receive selenium-contaminated waste water have high levels of selenium in surface water samples. The selenium concentration in Lake Belews, North Carolina has dropped from a maximum value of about 0.020 mg/L (pre -1986) to >0.001 mg/L in 1996, due to the discontinued release of selenium laden waste water from a local coal fired power plant (Lemly 1997). The selenium concentration in portions of Pigeon River and Pigeon Lake, Michigan which receive waste water input from a coal fly ash disposal facility, were >0.001–0.0075 mg/L (Besser et al. 1996).

High selenium levels are more likely to be found in irrigation return waters, seeps, springs, and shallow wells where seleniferous soils may contribute to the selenium content of the water. Glover et al. (1979) found that under unusual geological conditions, selenium concentrations in groundwater may reach 0.60 mg/L. In another study conducted in a seleniferous area of South Dakota, 34 of 44 wells did not show any measurable selenium; however, the remaining 10 had concentrations ranging from 0.050 to 0.339 mg/L (Smith and Westfall 1937). Selenium concentrations determined in 107 irrigation and 44 livestock well waters in the San Joaquin area of California exceeded 0.010 mg/L in 26 wells, but exceeded 0.020 mg/L in only 11 wells (Oster et al. 1988a). The maximum concentration was 0.272 mg/L (Oster et al. 1988a).

Selenium accumulation in agricultural drainage waters and basins has been documented in the western United States, particularly in California. The problem was first discovered in the Kesterson Wildlife Refuge in the San Joaquin Valley of California. In 1975, the U.S. Bureau of Reclamation finished construction of an 85-mile subsurface agricultural water drain that terminated in a series of evaporation ponds called Kesterson Reservoir. By 1983, however, it was confirmed that the drain waters contained high concentrations of selenium (>1.35 mg/L in some areas) leached from the soil by application of irrigation water (Maier et al. 1988). Because the high selenium levels produced death and deformities in fish and waterfowl, delivery of subsurface water to Kesterson was terminated in 1986 (Lewis 1988). Measurements of trace elements in the 27 other evaporation basins in the San Joaquin Valley have revealed only 3 basins with total selenium exceeding 0.10 mg/L and only 50 acres of evaporation basin cells with selenium concentrations in excess of 1.0 mg/L (CRWQCB 1988).

6.4.3. Sediment and Soil

Selenium is estimated to be the 69th most abundant element in the earth’s crust, with an average concentration of 0.05–0.09 mg/kg (Glover et al. 1979). Chemically, selenium closely resembles sulfur. Consequently, sulfides of bismuth, iron, mercury, silver, copper, lead, and zinc have been found to contain selenium (Shamberger 1981). Selenium is concentrated in the sulfide minerals galena, chalcopyrite, arsenopyrite, sphalerite, pyrite, marcasite, and pyrrhotite (Coleman and Delevaux 1957). Jarosite and barite have also been found to contain selenium at low levels. The sulfides containing the highest selenium concentrations are those associated with uranium ores in sandstone-type deposits in the western United States. In the immediate vicinity of sandstone-type uranium deposits, selenium concentrations as high as 1,000 mg/kg have been found (Shamberger 1981). Hydrothermal ore is also known to contain high concentrations of selenium. The best known are epithermal gold, silver, antimony, and mercury deposits (Shamberger 1981). Selenium has been found in volcanic rocks in the western United States at concentrations as high as 120 mg/kg (Glover et al. 1979).

Various studies estimated selenium concentration of most soils to be between 0.01 and 0.2 mg/kg (Sindeeva 1964). One study analyzed several thousand soil samples in the United States and found that most seleniferous soils contained >2 mg/kg, with a maximum concentration of >100 mg/kg (Rosenfeld and Beath 1964). The highest U.S. soil levels of selenium are found in areas of the West and Midwest.

Atmospheric deposition of selenium from mining and smelting activities also appears to be a source to soils and plants (Glooschenko and Arafa 1988). In this study, an indirect relationship between distance from smelters and selenium concentration was shown using Sphagnum fuscum as an indicator. Washout of atmospheric selenium by precipitation appeared to be the primary mechanism for accumulating selenium in soils and plants in the vicinity of smelters (Glooschenko and Arafa 1988).

Sandstone has been found to contain selenium in varying concentrations, but most probably contains >1 mg/kg (Rosenfeld and Beath 1964). However, sandstone in Wyoming has been found to contain >100 mg/kg (Beath et al. 1946). Generally, the selenium concentration of limestone is low; however, shales of the Niobrara formation in South Dakota have been found to contain over 40 mg/kg. The range of selenium concentrations in phosphate rocks is >1–300 mg/kg (NAS 1976a). Shales appear to contain consistently higher concentrations of selenium than limestone or sandstone. Despite the fact that shales vary so widely in their selenium concentration, they are fairly reliable indicators of soils high in selenium (NAS 1976a).

The disposal of selenium contaminated waste water has resulted in elevated selenium levels in sediments of Lake Belews, North Carolina. The concentration of selenium in sediments ranged from 4 to 12 µg/g (pre-1986), but has dropped to 1–4 µg/g (1996) due to the discontinued release of selenium laden waste water from a local coal fired power plant (Lemly 1997). Selenium was measured in 445 surface soil samples from Florida with a concentration range of 0.01–4.62 µg/g and an arithmetic mean of 0.25 µg/g (Chen et al. 1999). Selenium was detected in soils and bed sediment from the South Platter River Basin at concentrations of 0.30–3.80 µg/g (Heiny and Tate 1997). The highest levels were observed in areas consisting of a high degree of Precambrian rock formation.

6.4.4. Other Environmental Media

Coal and Oil. Petroleum has been found to contain 500–950 mg/kg crude petroleum and 500–1,650 mg/kg heavy petroleum (Hashimoto et al. 1970). An average of 2.8 mg/kg coal has been reported for 138 samples from U.S. deposits (Pillay et al. 1969).

Plants. Several species of grasses and herbaceous plants accumulate selenium, and some of these are endemic to the western United States. Primary accumulators are Astragalus, Oonopsis, Stanelya, Xylorhiza, and Machaeranthera. Secondary accumulators are Astor, Gatierreaia, Atriplex, Grindelia, Castillaja, and Comandra. Primary accumulators can contain 100–100,000 mg/kg of plant tissue, whereas secondary accumulators contain 25–100 mg/kg of plant tissue (dry weight). Nonaccumulator plants generally contain less than 25 mg of selenium/kg of plant tissue (dry weight) (Rosenfeld and Beath 1964). In some plants, including the leaves of beets and cabbage, and in garlic, as much as 40–50% of the selenium may be in the form of selenate (Cappon 1981).

A study by Arthur et al. (1992) showed an increased uptake of selenium by terrestrial plants growing on soil-capped fly ash landfill sites. Selenium concentrations rarely exceeded 5 mg/kg, and there were no signs of selenium toxicity to plants. A similar study by Shane et al. (1988) on greenhouse vegetables established that the uptake of selenium by these vegetables is proportional to the percentage of selenium in the growth medium. Another greenhouse study showed that four floating aquatic plants, Azolla caroliniana, Eichjornia crassipes, Salvinia rotundifolia, and Lemna minor, absorbed selenium quickly upon exposure (Horne 1991).

Animals. Aquatic animals accumulate selenium from lakes and rivers high in selenium content. Fish in the Kesterson National Wildlife Refuge in California had selenium concentrations up to 96 mg/kg, and aquatic birds had levels up to 130 mg/kg (Barceloux 1999). Selenium was detected in fish from three sites of the Pigeon River and Pigeon Lake in Michigan (Besser et al. 1996). It was determined that selenium concentrations in fish at sites receiving seepage and effluents from a coal fly ash disposal facility were considerably higher than for fish upstream from the facility. Mean concentrations of selenium in white sucker and northern pike ranged from 0.46 to 0.88 µg/g in an uncontaminated portion of the river, while concentrations in a contaminated portion of the river and lake were 1.1–2.4 µg/g (Besser et al. 1996). The mean concentrations of selenium in the feathers of five species of birds at Clear Lake, California were 3.20 µg/g (osprey), 1.38 µg/g (western grebe), 2.51 µg/g (great blue heron), 0.94 µg/g (turkey vulture), and 1.05 µg/g (mallard) (Cahill et al. 1998). Ospreys (which consume large mature fish) had the highest selenium levels, while turkey vultures (which rarely interact with the contaminated aquatic system) and mallards (which are semi-domesticated) had the lowest levels. Selenium was observed in 24 of 24 black-crowned night herons from the Delaware Bay at concentrations of 2.84–5.95 µg/g (Rattner et al. 2000). The highest levels were observed in herons from Pea Patch Island, an island adjacent to a shipping channel for the petrochemical industry. Selenium was observed in the liver of 70 out of 70 redheads (Athya americana) in Louisiana and Texas at concentrations of 1.56–5.86 µg/g (Michot et al. 1994). The selenium concentration in moose liver from 12 areas of Sweden ranged from 0.0027 to 3.054 µg/g (Galgan and Frank 1995). The highest levels were observed in areas with a high degree of selenium deposition from industrial sources.

Food. In a review of the foods that contribute the highest proportion of the daily selenium intake of human populations in the United States, Schubert et al. (1987) estimated selenium concentrations in over 100 food items on the basis of 65 articles published after 1960. Table 6-3 presents the selenium concentrations for some of the food items analyzed. The quality of the data was evaluated on the basis of sample size, analytic method, sample handling, sampling plan, and analytic quality control. Schubert et al. (1987) chose not to present standard deviations or standard errors of the samples because of the different sampling biases present in the studies.

Table 6-3

Selenium Concentrations in Foods in the United States (mg selenium/kg, wet weight).

In general, fruits and vegetables were found to contain >0.01 mg/kg, whereas root vegetables contained higher concentrations of selenium (Table 6-3). Beale et al. (1990) found milk and meat to have the same range of selenium concentrations as Schubert et al. (1987). In another study, no apparent correlation existed between the selenium concentration of canned versus fresh fruits and vegetables (Morris and Levander 1970).

Grain products varied greatly in their selenium concentration. Wheat bread and flour were high in selenium, whereas white bread and white flour contained considerably less selenium. Very low levels of selenium were found in certain processed cereals, such as corn flakes, but not in others, such as oat cereal (Morris and Levander 1970; Schubert et al. 1987).

Dairy products contained variable concentrations of selenium as well, but, in general, contained lower levels than meat products. Organ meats (e.g., liver and kidney) and seafoods contained higher levels of selenium than poultry or beef (Morris and Levander 1970; Schubert et al. 1987). The U.S. Fish and Wildlife Service collected 315 whole fish samples from 109 stations nationwide and analyzed them for selenium. Selenium concentrations were as follows (wet weight): geometric mean of 0.42 µg/g, maximum of 2.3 µg/g, and 85^th percentile concentration of 0.73 µg/g (Schmitt and Brumbagh 1990). Consumption of the foods with higher selenium levels contributes to the daily intake of adequate amounts of selenium.

Analysis of commercial baby foods indicated that processing may reduce selenium levels of the food (Morris and Levander 1970).

A recent survey conducted by the U.S. Food and Drug Administration (FDA), which analyzed foods consumed in the United States during the period of 1991–1999, detected selenium in 3,654 out of 6,679 food samples analyzed (FDA 2000). The results of this survey are summarized in Table 6-4.

Table 6-4

U.S. Food and Drug Administration—Total Diet Study (TDS)—Market Baskets 91-3 through 99-1.

6.8.1. Identification of Data Needs

Physical and Chemical Properties. The physical and chemical properties of elemental selenium and most of the common environmental forms of selenium have been characterized (Budavari et al. 1996; Lide 2000) and no further data are needed (see Chapter 4).

Production, Import/Export, Use, Release, and Disposal. Knowledge of a chemical’s production volume is important because it often correlates with possible environmental contamination and human exposure. Current data regarding the import (USGS 2002), export (USGS 2002), and use (Hoffmann and King 1997) of selenium are available. No statistics regarding the U.S. production of selenium have been reported since 1996 (USGS 2002). Current information on the U.S. production of selenium would assist in identifying potential exposures, particularly in regions of the country where environmental exposure to selenium through food and drinking water is already relatively high.

According to the Emergency Planning and Community Right-to-Know Act of 1986, 42 U.S.C. Section 11023, industries are required to submit substance release and off-site transfer information to EPA. The Toxics Release Inventory (TRI), which contains this information for 2000, became available in May of 2002. This database will be updated yearly and should provide a list of industrial production facilities and emissions.

Environmental Fate. Information is available to permit assessment of the environmental fate and transport of selenium in air (NAS 1976a), water (Chau and Riley 1965; NAS 1980b; Ohlendorf et al. 1986a; Rudd and Turner 1983a; Saiki and Lowe 1987), and soil (Kabatas-Pendias and Pendias 1984, NAS 1976b). Selenium released to the air will be removed by wet and dry deposition. The forms of selenium expected to be found in surface water and the water contained in soils are the salts of selenic and selenious acids. Selenic acid (H₂SeO₄) is a strong acid. The soluble selenate salts of this acid are expected to occur in alkaline waters. Sodium selenate is one of the most mobile selenium compounds in the environment because of its high solubility and inability to adsorb onto soil particles (NAS 1976a). Selenious acid (H₂SeO₃) is a weak acid, and the diselenite ion predominates in waters between pH 3.5 and 9. Most selenites are less soluble in water than the corresponding selenates (NAS 1980b).

It has been suggested that a biological cycle exists for selenium (Shrift 1964), but certain components of the cycle remain uncharacterized. The biological transformation of selenide to elemental selenium has not been well described in the literature (see Maier et al. 1988). Further research on the biological selenium cycle might help to identify “hot spots” of selenium in the environment. For example, further investigation of parameters that influence the tendency of selenium to move from one medium to another (e.g., from soil to water) would improve fate and transport modeling efforts.

Bioavailability from Environmental Media. The available monitoring data indicate that selenium is present in samples of air (Dams et al. 1970; Harrison et al. 1971; John et al. 1973; Peirson et al. 1973; Pillay et al. 1971), water (Besser et al. 1996; CRWQCB 1988; Cutter 1989; Glover et al. 1979; Lakin and Davidson 1967; Lewis 1988; Maier et al. 1988; NCDNR 1986; Oster et al. 1988a; Schutz and Turekiam 1965; Smith and Westfall 1937), soil/sediment (Glover et al. 1979; Lemly 1997; Sindeeva 1964), human tissues (Li et al. 1999; Orszczyn et al. 1996; Yang et al. 1989a, 1989b), fish (Besser et al. 1996; Lowe et al. 1985; May and McKinney 1981; Ohlendorf et al. 1986b), and food (Beale et al. 1990; FDA 2000; Schubert et al. 1987). Thus, it can be concluded that selenium is bioavailable from the environmental media.

Food Chain Bioaccumulation. Selenium in food contributed to the highest proportion of the daily selenium intake for human populations in the United States. Fruits, vegetables, milk, meat, and grains contain very low levels of selenium. However, selenium is bioaccumulated by aquatic organisms (Chau and Riley 1965; Ohlendorf et al. 1986a; Rudd and Turner 1983a). Based on reported BCFs and BAFs (Lemly 1982, 1985), selenium is expected to bioaccumulate in fish. Some evidence indicates that under natural conditions, selenium might also biomagnify in aquatic organisms (Lemly 1985; Maier et al. 1988; NCDNR 1986; Sandholm et al. 1973).

Exposure Levels in Environmental Media. Selenium has been detected in air (Dams et al. 1970; Harrison et al. 1971; John et al. 1973; Peirson et al. 1973; Pillay et al. 1971), water (CRWQCB 1988; Cutter 1989; Glover et al. 1979; Lakin and Davidson 1967; Lewis 1988; Maier et al. 1988; NCDNR 1986; Oster et al. 1988a; Schutz and Turekiam 1965; Smith and Westfall 1937), soil and sediment (Beath et al. 1946; Coleman and Delevaux 1957; Glooschenko and Arafat 1988; Glover et al. 1979; Lemly 1997; NAS 1976a; Rosenfeld and Beath 1964; Shamberger 1981; Sindeeva 1964), coal and oil (Hashimoto et al. 1970; Pillay et al. 1969), plants (Arthur et al. 1992; Cappon 1981; Horne 1991; Rosenfeld and Beath 1964; Shane et al. 1988), and food (Beale et al. 1990; FDA 2000; Schubert et al. 1987). Continued monitoring data of selenium levels in the environment are necessary to understand current exposure levels.

Reliable monitoring data for the levels of selenium and selenium compounds in contaminated media at hazardous waste sites are needed. This information can be used in combination with the known body burden of selenium and selenium compounds to assess the potential risk of adverse health effects in populations living in the vicinity of hazardous waste sites.

Exposure Levels in Humans. Selenium has been detected in the blood (Barceloux 1999; Orszczyn et al. 1996), urine (Gromadzinska et al. 1996), hair (Raie 1996; Yang et al. 1989a, 1989b), and nails (Yang et al. 1989a, 1989b) of exposed individuals. Various estimates of selenium intake for the U.S. populations have been reported (FDA 1982a; Levander 1987; Pennington et al. 1989; Schrauzer and White 1978; Schubert et al. 1987; Welsh et al. 1981). The largest numbers of exposed workers were heavy equipment mechanics, painters, mechanics, and special trade contractors (NOHS 1976). Preliminary data from another workplace study indicate that workplace exposure decreased from 1976 to 1984 (NIOSH 1989). Continued monitoring data are necessary to understand and evaluate human exposures to selenium in both occupational and nonoccupational settings.

Exposures of Children. Data are available regarding the exposure and body burdens of children to selenium. Children, like adults, are primarily exposed to selenium through the diet. In areas containing low (0.42 mg/kg), medium (3.09 mg/kg), and high (9.54 mg/kg) seleniferous soils, the mean whole blood selenium levels of school children (7–14 years of age) were 0.13, 0.37, and 1.57 mg/L, respectively (Yang et al. 1989b). Selenium was detected in postmortem liver, lung, and spleen samples of infants in Glasgow, Scotland at mean concentrations of 2.24, 0.76, and 0.099 ppm, respectively (Raie 1996). Children can be exposed to selenium from breast feeding mothers. Selenium was identified in the postpartum breast milk of women at different lactation stages at concentrations of 6.1–53.4 µg/L (Li et al. 1999). Using these concentrations, the daily intake of selenium for fully breast fed infants was estimated to range from 5.2 to 17.9 µg/day. Others have reported the estimated daily dietary intake of selenium for infants as 20 µg/day, while the daily intake for adult males was estimated as 120 µg/day (Pennington 1989). Since selenium is found in soil surfaces and children ingest soil either intentionally through pica or unintentionally through hand-to-mouth activity, pica is a unique exposure pathway for children. While selenium is found in some home products like shampoos (IARC 1975a) and dietary supplements (Goodman et al. 1990), this exposure route should be low and will not disproportionally affect children. Continued monitoring data are necessary to understand potentially dangerous routes of childhood exposure.

Child health data needs relating to susceptibility are discussed in 3.12.2 Identification of Data Needs: Children’s Susceptibility.

Exposure Registries. No exposure registries for selenium or selenium compounds were located. This substance is not currently one of the compounds for which a subregistry has been established in the National Exposure Registry. The substance will be considered in the future when chemical selection is made for subregistries to be established. The information that is amassed in the National Exposure Registry facilitates the epidemiological research needed to assess adverse health outcomes that may be related to exposure to this substance.

The development of a registry of exposures would provide a useful reference tool in assessing exposure levels and frequencies. In addition, a registry developed on the basis of exposure sources would allow an assessment of the variations in exposure levels from one source to another and of the effect of geographical, seasonal, or regulatory actions on the level of exposure from a certain source. These assessments, in turn, would provide a better understanding of the needs for research or data acquisition based on the current exposure levels.

6.8.2. Ongoing Studies

A summary of some pertinent ongoing research related to selenium is reported. Federally sponsored research reported in the Federal Research in Progress (FEDRIP 2002) databases is shown in Table 6-7.

Table 6-7

Ongoing Studies on the Environmental Effects of Selenium.