Selenium [Se, 34]
285
take up elevated amounts of Se and/or on stimulation of its volatilization. There are
calculations that up to about 20% of Se contents of soils can be removed by harvested
plants (Kabata-Pendias 2011). Activities of rhizobacteria, as well as other microbial
activities, have a significant impact on the Se phytoremediation. Brassica plants reveal
great capability to remove Se from soils, which may take up 740–950 g Se/ha/yr (Dhillon
and Dhillon 2009). These practices are applied mainly in two regions, northwest
India and west-central California.
42.3 WATERS
Mean Se concentration in seawaters is given within the range of 0.1–0.35 μg/L
(Table 42.1), and its median value is estimated to be 0.2  μg/L (Reimann and de
Caritat 1998). According to other sources, Se mean level in ocean waters is 0.09 μg/L
(ATSDR 2002b), and in the North Pacific is 0.1 μg/L (Nozaki 2005).
The global Se average concentration in river waters is given at the value of
0.07 μg/L, with the range of 0.05–22.0 μg/L (Table 42.1), and its world average
riverine flux is estimated to be 2.6 kt/yr (Gaillardet et al. 2003). Colorado River in
the United States contain Se from 1 to 4 μg/L, however, in some places much higher
amounts, up to 400 μg/L, are reported (ATSDR 2002b). Much of this Se is derived
from industrial sources, for example, oil refineries contribute up to 75% of its load
to the San Francisco Bay (Plant et al. vide Kabata-Pendias and Mukherjee 2007).
Waters of coal mines may also contain elevated Se amounts, mainly as dimethyl
selenide, (CH 3 ) 2 Se.
The common range of Se in rainwaters is 0.04–1.7  μg/L, and in polar ice its
content averages 0.02 μg/L. Median Se concentration in rainwater from the remote
region of the Kola Peninsula is 0.5 μg/L, and increases up to 0.9 μg/L in the polluted
area (Reimann and de Caritat 1998). The average Se content of rainwater of Sweden,
in 1999 was 0.15 μg/L, with the maximum value of 0.26 μg/L (Eriksson 2001a). The
main source of Se in rainwater is fossil-fuel combustion, but in some regions it is
also volcanic eruption. Also sewage sludge, containing Se up to 280 μg/L, may be
its source (ATSDR 2002b).
Groundwaters contain usually more Se than surface waters. Selenium contents
of groundwater, in the region close to chemical plant, vary from 0.25 to 1.8 μg/L,
and the proportion of Se 4+ and Se 6+ is also varied (Siepak et al. 2003). Especially
elevated Se concentrations, up to 1000 μg/L, are in groundwaters in areas with seleniferous bedrocks. In some arid regions (e.g., the United States, China, Pakistan,
Venezuela), Se concentrations in waters are elevated to about 2000 μg/L (Plant
et  al. vide Kabata-Pendias and Mukherjee 2007). High Se levels in soils resulted
in its higher concentration in stream and river waters and bottom sediments. After
Se fertilization of soils in Finland, its concentration in river waters increased up to
180 μg/L, and in bottom sediments up to about 4  mg/kg (Haygarth vide KabataPendias and Mukherjee 2007).
Aquatic biota is relatively sensitive to Se excess in waters. Acute hazard to these
organisms are established for Se at 20 μg/L in freshwater, and up to 300 μg/L in
marine water (ATSDR 2002b). In general, Se is toxic to fish. The most resistant to
elevated Se concentrations is salmon, but its content of about 1000 μg/kg is toxic.