Geology Reference
In-Depth Information
subjected the sediments to weathering under oxidizing conditions. With
resultant acid neutralized by the carbonate component of the sediments, the
predominant form of selenium produced from oxidation of selenide, Se
II
,
under alkaline conditions, is selenate, Se
VI
O
2
,inpreferencetoselenite,
Se
IV
O
2
, or biselenite, HSe
IV
O
3
.
87
Thus the soils on the Coast Ranges,
which are alkaline, contain significant amounts of soluble mineral salts
(e.g. Na
2
SO
4
.10H
2
O, Na
2
Mg(SO
4
)
2
.4H
2
O), including selenates (e.g. Na
2
SeO
4
.10H
2
O, Na
2
Mg(SeO
4
)
2
.4H
2
O).
The chemistry and mobility of selenite and selenate differ greatly in
soils. Selenite is adsorbed by specific adsorption processes (e.g. on clays
and hydrous metal oxides) and to a much greater extent than selenate,
which is adsorbed, like sulfate, by comparatively weak non-specific
processes. Thus, in aerated alkaline soils, such as those of semi-arid
regions like the west-central San Joaquin Valley, mobile Se
VI
O
2
will be
the dominant form and, through runoff, be capable of entering the
groundwaters and sub-surface drainage waters, ultimately being re-
moved to Kesterson Reservoir. There, it is taken up by the biota, with
such devastating effects, and also converted to a range of selenium-
containing species (e.g. Se
IV
O
2
,Se
0
,H
2
Se, Se-rich protein), with much
ultimately being deposited in the sediments. There, over geological time,
natural diagenetic processes would presumably lead again to the for-
mation of reduced seleno-sulfide species.
Considerable effort has been devoted to the clean-up of Kesterson
Reservoir, which was drained in 1987, one year after the input of drainage
water was stopped. Natural processes, stimulated by the addition of
nutrients, of summertime microbial conversion and volatilization to the
atmosphere of methylated forms such as dimethylselenide, (CH
3
)
2
Se, and
downwards leaching of Se
VI
O
2
by percolating water in winter helped to
dissipate 68-88% of total selenium from the topsoil (0-15 cm) over a
period of eight years.
88
Phytoremediation techniques were also tested.
Similarly, pilot bioreactors were set up to convert drainage water SeO
2
into commercially useful selenium-containing products. Although Kes-
terson Reservoir has been closed, selenium loading in the Central Valley
of California, the San Joaquin River, the San Joaquin-Sacramento Delta,
and San Francisco Bay still occurs and an emerging selenium contamina-
tion issue is developing in south-eastern Idaho, USA, in streams draining
areas with phosphate mining activities.
89
3.3.2.5 Chromium in Groundwater. Millions of tonnes of high-pH
chromite ore processing residue (COPR) were deposited (e.g. as landfill
material) in the past in urban areas such as Glasgow, Scotland,
90
and
Hudson County, New Jersey, USA.
91
Even now the high-lime process
