Geology Reference
In-Depth Information
significant increasing trend in acid-neutralizing capacity of 1.60 meq L
1
per year from 1992 to 2000 for 60% of the monitored lakes.
46
Further-
more, there was a growing concentration of dissolved organic carbon in
15% of the lakes, helping to shift the aluminium from its toxic inorganic
form towards less toxic organic forms although likely to delay recovery
of acid-neutralizing capacity. Overall, however, by 2000 both USA and
European surface waters appeared to be on the way to long-term
recovery from acidification, with decreases in SO
4
2
concentrations,
increases in pH and acid-neutralizing capacity, and decreases in con-
centrations of labile toxic forms of aluminium.
47-49
With respect to human exposure to aluminium in drinking water, it
must be remembered that aluminium has often been deliberately added,
in the form of Al
2
(SO
4
)
3
, to water supplies at treatment works to remove
coloration by organic compounds in reservoirs in upland catchments.
This it achieves by hydrolyzing to a gelatinous, high surface area,
precipitate of Al(OH)
3
, which helps to remove the coloured organic
colloids. It has been suggested, partially because of observed water
aluminium-related dialysis dementia in some chronic renal patients on
artificial kidney dialysis machines in the 1960s and 1970s, that exposure
to low concentrations of aluminium in drinking water, for which there is
a current EC Maximum Admissible Concentration of 200 mgL
1
, might
be implicated in Alzheimer's Disease.
50
There is still no proof of this at
present,
51
although it is known that the incidence of senile dementia on
the remote Pacific island of Guam, where bauxite is mined and the local
water is elevated in aluminium, is a hundred-fold greater than in the
USA. Ten years after an accident at Camelford in south-west England in
1988, when 20 tonnes of Al
2
(SO
4
)
3
were emptied directly into the main
water supply, there did appear to be a lasting effect on the cerebral
function of those who had been exposed to greatly enhanced aluminium
concentrations in the tap water.
52
It has been mooted that, for humans,
a high intake of silicic acid (H
4
SiO
4
) could reduce the bioavailability of
aluminium by two orders of magnitude for, at slightly alkaline intestinal
pH, hydroxyaluminosilicates are stable and unavailable for absorp-
tion.
53
This would mimic the situation in nature, where, in the absence
of a strong acid input, the bioavailability of aluminium is kept low by
the formation of hydroxyaluminosilicate species. The latter has been
described as a 'geochemical brake on an aluminium juggernaut which
would otherwise career into the biotic cycle'.
54
More generally, in
addition to aluminium, the role of other metals, including zinc, copper,
iron, and manganese, in neurodegenerative disease is now under scru-
tiny, there being suggestions of adverse effects upon protein folding and
subsequent oxidative stress.
55
