Geology Reference
In-Depth Information
and co-workers
77
believe that the proximity of soluble organic matter to
drive FeOOH reduction is the key and that this can only occur at the
margins of buried peat basins. There is an alternative theory, pro-
pounded by Harvey and co-workers,
78
that the arsenic mobilization is
associated with the inflow of younger carbon, probably untreated hu-
man waste in water that has been drawn into the aquifers of Bangladesh
as a consequence of irrigation pumping.
Although trivalent inorganic arsenic, with its propensity for binding
to the SH group of enzymes, is acknowledged to be more toxic to
humans than pentavalent inorganic arsenic, it must be recognized that
As
V
can be converted to As
III
in the human body as part of the
reduction/biomethylation pathway of excretion
As
V
As
III
-
-
MMAA
-
DMAA
(3.89)
where MMAA (CH
3
AsO(OH)
2
, monomethylarsonic acid) and DMAA
((CH
3
)
2
AsO(OH), dimethylarsinic acid) are less toxic metabolites, the
latter predominating as the major form of arsenic excreted in urine.
68-72
There is evidence to suggest, however, that trivalent metabolic inter-
mediates of MMAA and DMAA may be highly implicated in the
initiation of cancer.
70-72
Methylation could also occur naturally by
microbial action in freshwaters, although reported occurrences suggest
that the effect is small.
79
3.3.2.2 Lead in Drinking Water. The naturally soft, slightly acidic,
plumbosolvent water of the Loch Katrine water supply for the Glasgow
area was recognized many years ago to release lead from the lead pipes
and tanks in the domestic plumbing of the Victorian and subsequent
(even post-World War II) eras.
80
O
2(g)
þ
4H
1
þ
4e
E
0
H
¼
1.229 V (3.90)
"
2H
2
O
Pb
21
þ
2e
E
0
H
¼
0.126 V (3.91)
"
Pb
(s)
O
2(g)
þ
4H
1
þ
2Pb
(s)
2Pb
21
þ
2H
2
O
E
0
H
¼
1.355 V (3.92)
"
Essentially, elemental lead becomes soluble in acidic conditions due
to its oxidation by dioxygen. Furthermore, compounds such as carbo-
nate and hydroxycarbonate compounds of lead, i.e. PbCO
3
and Pb
3
(CO
3
)
2
(OH)
2
, that may coat the pipes, will dissolve under acid condi-
tions.
In view of the concern over detrimental effects of exposure to lead
upon human health, in particular the possible impact upon intelligence
