3 Environmental Occurrences and Concentrations

3.1 Sources

3.1.1 Natural Sources

Selenium is inconsistently dispersed in geological materials; however, it is widely distributed in the environment and ranks 68th in earth crust abundance among elements (Adriano 1986).

Volcanic emanations and metallic sulphides associated with igneous activities are primary sources of Se in nature. Secondary sources include biological pools in which Se has accumulated. In general, shales have the highest concentrations of Se that is partly attributed to biological activity. Fifty Se minerals are known, but Se is commonly associated with heavy metal sulphides where it occurs as selenide (Se^-2), or as a substitute ion for sulphur in the crystal lattice (Adriano 1986).

`Seleniferous' soils or soils containing Se well above the normal levels are found in the arid and semi-arid areas of the world, including the western areas of Canada and the United States. In general, high-Se soils do not occur in humid regions (Adriano 1986).

Mosher and Duce (1987) estimated that 6 000 000- 13 000 000 kg of Se are annually released to the atmosphere from natural sources with 60-80% of the emissions being of marine biogenic origin. Nriagu (1989) suggested that natural sources contribute 700 000- 18 100 000 kg/a of Se to the atmosphere. Selenium contributions from individual natural sources were as follows: 0- 400 000 kg/a from wind-borne soil particles, 0-1 100 000 kg/a from sea salt spray,
100 000 -1 800 000 kg/a from volcanoes, 0-500 000 kg/a from wild forest fires, and 600 000-
14 300 000 kg/a from biogenic processes.

3.1.2 Anthropogenic Sources

Anthropogenic sources of selenium include fuel (coal and oil) combustion, primary and secondary non-ferrous metal industries, waste (domestic, municipal, and industrial) disposal and incineration, manufacturing processes, mining, smelting, refining, and steam electric¹.

Combustion of coal and other fossil fuels is the primary source of airborne selenium compounds (USDHHS 1994). Incineration of rubber tires, paper, and municipal waste also contribute to atmospheric selenium. Hashimoto et al. (1970) reported that rubber tires contained 1.3 mg Se/kg. Seventy different kinds of paper were found to contain selenium (West 1967). Other sources of atmospheric Se include microbial action within the soil, plants and animals, fly ash ponds (through fugitive dust emissions), and emissions from manufacturing and processing facilities (USDHHS 1994). Nriagu and Pacyna (1988) estimated that the worldwide emission of anthropogenic Se to the atmosphere was 1 700 000-5 800 000 kg/a. The major sources of emissions included coal combustion (900 000-2 800 000 kg/a), oil combustion (100 000-800 000 kg/a), primary non-ferrous metal industry (e.g., mining, and Pb, Cu-Ni, and Zn-Cd production) (700 000-2 100 000 kg/a) and waste incineration (0-100 000 kg/a) (Nriagu and Pacyna 1988).

Surface water can receive selenium from the atmosphere by dry and wet deposition, from adjacent water, from surface runoff, and from subsurface drainage. Selenium is released during coal mining because of the oxidation of selenium-bearing pyrite (Dreher and Finkelman 1992). The total loading of Se to aquatic ecosystems (including rivers, lakes, and oceans) was estimated to be 10 100 000-71 900 000 kg/a. Of the five sources, atmospheric deposition (500 000-1 100 000 kg/a) contributed the least amount to the aquatic ecosystems. The contribution from other sources was determined to be as follow: 6 000 000-30 000 000 kg/a from steam electric; 3 300 000-21 000 000 kg/a from mining, smelting, and refining; 300 000-11 300 000 kg/a from waste disposal; and 0-8 500 000 kg/a from manufacturing processes (Nriagu and Pacyna 1988).

Nriagu and Pacyna (1988) estimated that the worldwide emission of selenium into soils was 6 000 000 to 76 500 000 kg/a. The individual sources, in decreasing order of importance, included bottom fly ash (4 100 000-60 000 000 kg/a), disposal of industrial waste (500 000- 13 000 000 kg/a), atmospheric deposition (1 300 000-2 600 000 kg/a), peat (agricultural and fuel uses- (up to 400 000 kg/a), wastage of chemicals (100 000 to 200 000 kg/a), and fertilizers (up to 100 000 kg/a).

3.2 Residues

3.2.1 Water

Selenium concentrations in aquatic environments are generally low, but they can vary widely. In seleniferous areas, selenium levels are naturally high.

Concentrations ranging from 0.00012 to 0.00044 mg Se/L have been reported in drinking water from worldwide sources (Eisler 1985). However, Yang et al. (1983) reported that the average selenium content of drinking water taken from selenium-rich area in China with a history of disease reported as chronic selenosis was 0.054 mg Se/L.

Highly variable levels of selenium were reported in wells of seleniferous areas in the USA, ranging from non-detectable levels to 0.330 mg Se/L (Smith and Westfall 1937). Selenium concentrations in 107 irrigation and 44 livestock well waters in the San Joaquin area of California exceeded 0.01 mg Se/L in 26 wells, and 0.02 mg Se/L in 11 wells. The maximum concentration was 0.272 mg Se/L (Oster et al. 1988).

Under the Canada-British Columbia (BC) agreement on water quality monitoring, trace metal concentrations were measured in various rivers of British Columbia. The total Se concentration in the BC rivers was generally low and ranged up to 0.001 mg Se/L in unmineralized to 0.009 mg Se/L in highly mineralized ambient environments. The concentration in a coal mining area ranged up to 0.0025 mg Se/L. The following table summarizes the results of the BC data.

Anderson et al. (1994) reported trace metal levels from six sites in the Battle River, located in East-Central Alberta and West-Central Saskatchewan. The total Se concentrations were below the detection limit (0.0001 mg/L) at two of the six sites. Measurable concentrations at <0.0002 mg Se/L were reported for other sites. The highest concentration measured in the basin was 0.001 mg/L at a site that was mainly influenced by agriculture and oil and gas exploitation.

Adams and Johnson (1977) reported selenium concentrations ranging from 0.000001 to 0.000036 mg Se/L in the Great Lakes. In the lake water, in the vicinity of the nickel-copper smelter in Sudbury, Ontario, Nriagu and Wong (1983) reported concentrations of 0.0001 to 0.0004 mg Se/L.

In the U.S.A., Canton and Van Derveer (1997) reported average selenium concentrations of 0.018 mg/L and 0.0235 mg/L in some tributaries of the Arkansas River, Colorado. The high Se concentrations in these tributaries were primarily a result of natural weathering of Cretaceous marine shales and shale-derived soils.

In seawater, concentrations of selenium were reported to range from 0.000009 to 0.00045 mg Se/L (Ebens and Shacklette 1982, Whittle et al. 1977).

3.2.2 Sediments and Soils

All concentrations in this section are reported on a dry-weight (dw) basis.

Selenium concentration in soils is highly variable. The average concentration of Se in soils based on data collected from various countries was quoted to range up to 2 mg/kg (Kabata-Pendias and Pendias 1984). Background levels of Se in some Canadian soils were reported to range from 0.03 to 2 mg/kg (McKeague and Wolynetz 1980). However, concentrations ranging from 80 to 90 mg/kg have been measured in seleniferous soils (Adriano 1986). Seleniferous soils of North America extend into Alberta, Manitoba, and Saskatchewan. In these soils, levels ranging from 0.1 to 6 mg Se/kg have been reported (Byers and Lakin 1939). More recently, Mermut et al. (1996) reported Se levels in Saskatchewan surface soils that ranged from 7.1 to 10.3 mg/kg.

Anderson et al. (1994) measured trace metal levels in the sediments of the Battle River, located in East-Central Alberta and West-Central Saskatchewan. The mean selenium concentrations at the six sites ranged from 0.023 to 0.402 mg/kg. The highest value of 0.402 mg Se/kg was reported for the site that was located near the headwaters; this site was least affected by human activity, although there is substantial oil and gas exploitation in this area.

Nriagu and Wong (1983) reported that in lake sediments, in the vicinity of a nickel copper smelter (Sudbury, Ontario), the selenium concentration ranged from 2 to 6 mg/kg. The lake sediment 240-km south from Sudbury recorded lower concentrations: 1 to 3 mg Se/kg.

3.2.3 Biota

Anderson et al. (1994) measured trace metal levels in invertebrates and aquatic plants of the Battle River of Alberta and Saskatchewan. The mean selenium concentration in the invertebrates (Amphipoda, Sphaeriidae, Unioidae, and Gastropoda) ranged from 0.2 to 1.6 mg Se/kg dry-weight. The highest concentration of 4 mg Se/kg dry-weight was recorded for unioidae at the site that was located near the mouth of the river and was mainly influenced by agriculture. The results for macrophytes (Potamogeton richardsonii and filamentous green algae) produced selenium concentrations that ranged from 1.3 to 25.3 mg Se/kg dry-weight in the algae, 0.9 to 3.5 mg Se/kg dry-weight in the stems and leaves of P. richardsonii, and 7.7 to 243 mg Se/kg dry-weight in the rhizome of P. richardsonii. The maximum concentration of 243 mg Se/kg dry-weight in the P. richardsonii rhizome was measured at the site located downstream from an area with active surface coal mining and a coal-fired power generating plant. These investigators also reported selenium levels in white sucker and northern pike caught from the Battle River. The concentration in the muscle tissue of these fish ranged from <0.007 (method detection limit) to 0.023 mg Se/kg wet-weight.

3.2.4 Analytical methods for water

Several techniques are in use to measure selenium in water. They include (a) atomic absorption methods (e.g., hydride generation/atomic absorption spectrometric and electrothermal atomic absorption spectrometric), atomic emission methods (e.g., inductively coupled plasma), and (c) chemical methods that involve conversion of selenite and determination of the organic derivative by colorimetry or fluorometry (Standard Methods 1992). The hydride generation or electrothermal atomization, colorimetric, and fluorometric are among the most sensitive methods, and the continuous hydride generation method is the method of choice for its simplicity, reproducibility, and low detection limit (Standard Methods 1992).

The estimated detection limit for the atomic absorption methods is 0.002 mg Se/L. However, an increase in sensitivity may be achieved by using a larger sample volume, by reducing the flow rate of the purge gas, or by using gas interrupt during atomization. Quartz atomization cells provide for the most sensitive selenium hydride determination and minimize background noise associated with the argon-air entrained-hydrogen flame. The modified hydride- generation method (#02-2200) employed by the National Laboratory of Environmental Testing in Burlington, Ontario has produced a detection limit of 0.0001 mg/L for selenium in water (Manual of Analytical Methods1994).

1 Electric utilities that employ steam produced by burning fossil fuels for power generation.