Geology Reference
In-Depth Information
3.3.2 Metals and Metalloids in Water
3.3.2.1 Arsenic in Groundwater. In what has been described as the
largest arsenic poisoning epidemic in the world, hundreds of thousands
of people in West Bengal (India) have been seriously affected by arsenic
poisoning resulting from the consumption of water drawn from tube
wells sunk some 20 to 150 m below the ground into aquifers.
67
In West
Bengal, Bangladesh, and other parts of the world (e.g. Chile, Mexico,
Argentina, Ghana, Mongolia, and Taiwan) where inorganic arsenic
concentrations in drinking water are elevated (mg L
1
vs. WHO
recommended limits of 10 mgL
1
), exposure has resulted in hyperpig-
mentation, hyperkeratosis, and cancer of the skin and various internal
organs.
68-72
Sulfide minerals are one of the most important natural sources of
arsenic in groundwater. Oxidation of arsenopyrite (FeAsS), in analo-
gous fashion to pyrite (FeS
2
), may release high concentrations of arsenic
into solution.
73,74
4Fe
21
þ
4AsO
3
4
þ
4SO
2
4
þ
12H
1
(3.88)
4FeAsS
(s)
þ
13O
2(g)
þ
6H
2
O
"
In Bengal and Bangladesh, it has been suggested that large-scale with-
drawal of groundwater for irrigation produces seasonal fluctuation of the
water table, which in turn results in intake of oxygen into the pore waters
of sediments that are arsenic-rich in the form of arsenopyrite.
73,74
The
exact speciation of soluble inorganic arsenic, e.g. as the undissociated
forms or different oxyanions of the acids H
3
As
III
O
3
and H
3
As
V
O
4
,willbe
dependent upon the prevailing pe and pH. Sorption of arsenic, especially
pentavalent arsenate (e.g. AsO
4
3
,HAsO
4
2
,H
2
AsO
4
), onto ferric
hydroxide (Fe(OH)
3
) produced under oxidizing conditions may, however,
restrict its mobility and availability, although fertilizer phosphate present
in groundwater could perhaps displace the arsenate. Arsenite, especially
as H
3
As
III
O
3
(pK
1
¼
9.2), the predominant form under reducing condi-
tions at pH
o
9.2, is much less strongly sorbed.
As an alternative to the theory of arsenopyrite oxidation, however,
especially where there is an absence of elevated concentrations of sulfate,
it has been proposed that elevated arsenic concentrations in Bengal and
Bangladesh groundwater could result from the release of adsorbed
arsenate during the dissolution of hydrous iron oxides (similar to ferric
hydroxide) under reducing conditions (see Section 3.3.3.2).
73-76
This
phenomenon can be seen in those parts of Asia where sub-strata consist
of recent alluvial sediments, rich in decaying organic matter, that were
deposited and buried during the post-glacial period. Indeed, McArthur
