My Account: Log In | Join | Renew
Table of Contents
Select All Chapters
Selenium (Se) is both beneficial and toxic to animals, plants, and humans. Consequently, it is imperative to know its concentration in the environment and to understand the processes controlling its distribution. Determinations of Se concentrations in a variety of materials indicate that Se is widely distributed throughout the environment. The processes responsible for its distribution include volcanic activity, the burning of fossil fuels, the weathering of rocks and soils, groundwater transport, precipitation of minerals, adsorption, chemical or bacterial reduction and oxidation, and metabolic uptake and release by plants and animals. The importance of a particular process in controlling the distribution of Se is intimately linked to the speciation of Se, which is controlled by the pH and redox conditions of the environment. Selenium can exist as selenide (Se2−), elemental Se (Se0), selenite (SeO2−3), and selenate (SeO2−4). Each oxidation state exhibits different chemical behavior. Selenide and elemental Se occur in acidic, reducing, and organic-rich environments. Metal selenides, Se-sulfides, and elemental Se are insoluble, and therefore biologically unavailable. For the pH and redox conditions of most soil and aquatic environments, SeO2−3 and SeO2−4; should be the dominant forms of Se. Selenite is immobilized by adsorption onto particles, particularly Fe oxyhydroxides. Selenate is highly mobile and biologically available because of the solubility of its salts and its weak adsorption by particles. Microbial action can change the speciation of Se through oxidation or reduction, or through the formation of organic Se compounds.
Selenium is biologically important because (i) it is essential in animal and possibly plant metabolism, (ii) in many areas diets do not contain sufficient Se to meet animals' needs, and (iii) in other areas it is toxic to animals when it occurrs in high concentrations in soil, water, plants, fly ash, or in aerosols. Animals require 0.05 to 0.1 mg Se/kg in their diets to prevent Se deficiency but suffer Se toxicosis when dietary levels exceed 5 to 15 mg Se/kg. The earth's crustal materials generally contain <0.1 mg Se/kg. Higher concentrations are found in Cretaceous shales. The Se-accumulator plants growing on the seleniferous soils may contain hundreds or even thousands of mg Se/kg. However, the nonaccumulator grasses and forbs seldom accumulate > 50 mg Se/kg and more often contain < 5 mg Se/kg. Soils and plants may discharge volatile forms of Se into the atmosphere. However, plants may also absorb measurable amounts of gaseous Se from the atmosphere. Anthropogenic activities impact the amount of Se entering our nation's lakes, rivers, and the atmosphere. Combustion of coal and incineration of municipal waste exhaust Se into the environment. In addition, crop-fallow and irrigation practices that allow leaching waters to pass through seleniferous strata prior to intersecting with surface flow, augment the Se levels encountered by plant and animal life.
Important selenium (Se) data in the literature are discussed with reference to the solubility relationships of Se minerals and soluble species in cultivated soils. Redox potential and pH are the most important parameters controlling the solubility and chemical speciation of Se in cultivated soils. Soils can be classified into three categories. Soils of high redox in arid regions: Selenate (SeO2−4) is the major species in soil solution. No selenate minerals are expected to precipitate. Adsorption mechanism may control Se concentration in soil solution. Soils of moderate redox in humid regions: Biselenite (HSeO−3) is predominant at low pH, whereas SeO2−3 is the major species at high pH. Apart from MnSeO3, none of the known selenite minerals are stable in soils. Adsorption mechanism may control Se concentration in soil solution. Soils of low redox (i.e., gley and wetland): Monohydrogen selenide (HSe−) is the most important species in soil solution. Only under strongly acidic environment does H2Se0 contribute to Se in solution. Elemental Se and metal-selenide minerals can form. Either elemental Se or a metal selenide (i.e., Cu2Se, PbSe, etc.) may control Se concentration in soil solution. Furthermore, in this chapter, transformation of Se in soils and effect of soil amendments on Se solubility and plant uptake were discussed with reference to chemical equilibria of Se in soils.
The accumulation of Se by plants is of concern worldwide. Many regions grow crops that contain insufficient Se to meet animal nutritional requirements. In these locations with low Se, efforts have been made to increase tissue Se concentrations. Other areas have problems with excessive Se found in vegetation grown on seleniferous soils. Plant species vary in their ability to accumulate and tolerate Se. Selenium accumulator plants can accumulate extremely high Se concentrations (several thousand mg Se kg−1) when grown in seleniferous soils, whereas typical agricultural crops have a much lower Se tolerance (<50 mg Se kg −1). It has been suggested that Se may have an essential role in plant growth, but this has not yet been substantiated. In soil, the phytoavailability of Se6+ is generally many times greater than Se4+ Elemental Se is largely unavailable to plants. Soil chemical and physical factors such as pH, soil texture, organic matter content, and the presence of ions such as SO2−4 and PO3−4 also influence Se uptake by plants. Techniques to increase Se tissue concentrations include Se applications to soil, to seed, and to plant foliage. Regardless of the application methods, Se6+ additions generally result in higher plant Se concentrations than do additions of Se4+.
Two Greenhouse studies were conducted in which fourwing saltbush [Atriplex canescens (Pursh) Nutt.] and (Astragalus bisucatus L.) were grown in pots containing 3 kg of 18 different soils and mine spoil materials collected from locations in Wyoming with documented histories of selenium (Se) toxicity in livestock. Plant Se concentration was compared to total soil Se and Se extracted by ammonium bicarbonate-DTPA (AB-DTPA), hot water, saturated paste extracts, DTPA (2 h), and 0.5 mol Na2CO3 L−1. Data from saturation extracts were found to be the most useful in predicting Se concentration in both fourwing saltbush (r2 = 0.66) and two-grooved milkvetch (r2 = 0.78). The results suggest that soil or mine spoil materials that yield < 0.1 mg L−1 in a saturation extract may produce Se-toxic fourwing salt-bush plants (> 5 mg Se kg−1). Sodium carbonate extracted the largest fraction of the total soil Se among the five soil tests. Values obtained from all five extractants were highly correlated.
Problems associated with Se toxicities have been reported since 1856 for many areas of the western USA. Most of the occurrences of Se toxicity in the western states are associated with known seleniferous geological formations or areas of historic U exploration and mining. The development of large-scale surface mining operations has raised additional concerns regarding the potential for Se toxicities. The mining of coal, bentonite, and U in the western USA has increased the potential for Se contamination of soils, surface water, and groundwater. Extremely high Se, As, and Mo values have been reported in areas of U enrichment in Utah, North Dakota, South Dakota, Colorado, Wyoming, New Mexico, and Arizona. Many of these seleniferous areas are watersheds for reservoirs and irrigation units.
Selenium in certain soils may be taken up by plants in amounts to render them toxic. Seleniferous forage can be found in most of the western states. Intoxication of livestock by seleniferous plants has been classified as acute and chronic. Acute poisoning results from consumption of plants having high levels of Se; chronic Se poisoning has been described in two forms—alkali disease and blind staggers. Alkali disease is said to result from the consumption of seleniferous grains and grasses, and is manifest by loss of hair, lameness, and loss of weight. Blind staggers is said to result from the consumption of Se indicator plants and is manifest by wandering, circling, loss of ability to swallow, and blindness. Some research casts doubt on the above classification of Se poisoning. Research using pigs (Sus scrofa domesticus) indicates that the source of Se does not alter the type of lesion or signs of poisoning observed. There are data available that suggest that blind staggers is not related to Se poisoning.
Selenium is a naturally occurring trace element that is essential for animal nutrition, but the range between dietary requirements and toxic levels is relatively narrow. The greatest abundance of Se is in igneous rocks (although bioavailability may be low), but high concentrations are found in some sedimentary rocks and fossil fuels. Human activities affecting biological availability of Se from these various sources can increase the potential for adverse effects on wildlife. Two examples include disposal of Se-containing drainage water from agricultural fields and fly ash from coal-fired power plants. This chapter provides a general review of characteristics and environmental occurrence of Se, procedures for collection and analysis of biological samples, and the occurrence and significance of Se (e.g., toxic effects) in plants and animals. The primary focus is on potential adverse effects of elevated Se in wildlife, particularly those animals that may be affected by disposal of subsurface agricultural drainage waters, and on their food chains. This problem was first documented in the U.S. Fish and Wildlife Service's studies at Kesterson Reservoir in the San Joaquin Valley of California, where Se from agricultural drainage water bioaccumulated to high enough levels in plants and animals to cause mortality and impair reproduction of aquatic birds. Research results from Kesterson Reservoir are summarized as a case history. Briefly, aquatic plants, invertebrates, fish, frogs, snakes, birds, and mammals at Kesterson Reservoir contained much higher Se concentrations than those at the reference sites, often averaging 100 times greater. The most pronounced effects in wildlife species were found in birds that fed regularly in Kesterson Reservoir. These effects included high incidences of embryonic mortality and deformity as well as mortality of adult birds attributed to Se toxicity. Similar effects of Se toxicosis may occur as a result of agricultural drainage in other areas, such as the southern San Joaquin Valley where embryonic deformities similar to those at Kesterson have been found in recent studies.
Total chemical analyses of soils on the alluvial fans of Panoche and Cantua Creeks in western Fresno County, California, were studied to identify associations among elements in the soils. Some elements, particularly Se, have been identified as pollutants in agricultural drainage water from this area. Samples of the C-horizon from 168 to 183 cm were analyzed for total concentrations of 43 elements. An R-mode factor analysis of data for 26 elements suggested five element associations in the soils. Factor 1, predominantly Al, Ti, gallium (Ga), yttrium (Y), Cerium (Ce), scandium (Sc), Fe, and Li, is interpreted as a felsic-sediment factor. Factor 2, predominantly Ni, Mg, Cr, Co, Mn, and Fe, is related to serpentine. Factor 3, Se and S, reflects the similar chemical behavior of these two elements; Na is negatively related to this factor. Selenium is probably hosted in sedimentary rocks that border the valley. Factor 4, Ca, Sr, C, and S, is an alkaline-earth factor related to carbonate and sulfate precipitation. Factor 5, C and Hg, reflects common alluvial sedimentation.
A study was undertaken to investigate the distribution of selenium (Se) and salinity in shallow groundwater and soil in three drained agricultural fields in the western San Joaquin Valley, California. Groundwater and soil samples were collected along transects in fields that differed in age of the drainage systems (15, 6, and 1.5 yr) and in the Se concentrations of the drain water (430, 58, and 3700 µg/L, respectively). Isotopic enrichment and chemical composition of groundwater samples indicated that saline- and Se-enriched water has evolved due to evaporation of shallow groundwater. This evaporated, isotopically enriched water containing elevated concentrations of Se is being displaced toward the drains by less saline irrigation water that percolates through the soil. Soluble Se, mainly selenate (SeO2−4), is also being leached from the soil and moved toward the groundwater by irrigation water. For the field drained for 15 yr, 5% or less of the total Se in one soil profile was soluble or adsorbed. Analyses of one soil profile in the field drained for only 1.5 yr indicated that soluble plus adsorbed Se in the soil accounted for < 15% of the total Se in the top 1.2 m but increased to 80% of the total Se at a depth of 2.7 m. Soil leaching, especially in fields with recently installed drainage systems, can contribute substantial quantities of Se to the groundwater.
A preliminary screening study was conducted of the hydrologic implications of three alternatives to Se management in the Westlands Water District using a simple flow model and existing data. Results from a 15-yr simulation of the partially calibrated model show that reduction of contributions to the regional groundwater from excessive irrigation applications may be more important than upslope contributions in accounting for rising water tables. Selenium migration may be most rapid in the area of water district within the Panoche Creek alluvial fan opposite the town of Tranquillity, where a strongly sloping hydraulic gradient combines with high Se concentrations in soils and groundwater. A number of management strategies exist, which can be adopted by growers to address the current Se drainage problem.