Mechanism of As Pollution of Groundwater, Ganges Plain and Viet Nam
Groundwater in the Ganges Plain and in northern Viet Nam is polluted by naturally-occurring arsenic, a
toxic carcinogen. The London Arsenic Group (LAG) has shown that dissolution of arsenical iron-oxides
causes the arsenic pollution in the Ganges Plain and so probably in Viet Nam. It is using that knowledge
to predict where else in the World arsenic pollution might occur via the operation of this mechanism.
2.
3a.
In Bangladesh, 25% of
wells exceed the Bangladesh
limit for drinking water,
which is 50 µg/L of arsenic.
1.
Groundwater wells in the Ganges Plain provide more
than 100 million people with household water - but much
of it is highly polluted with arsenic. Mr. Fakir’s small well
has 360 µg/L of arsenic and was his family supply for
seven years until replaced by the bigger, arsenic-free, well.
Pollution occurs only in
Holocene aquifers and its
severity differs from place
to place. In Chandpur, in SE
Bangladesh, 90% of wells
exceed this limit and 25%
exceed 300 µg/L.
The extent and degree of
pollution in wells of West
Bengal is less known; little
data has been published
although much exists.
to hydrous iron oxides (FeOOH). Fermentation of
peat in the subsurface releases organic molecules
(such as acetate) to drive reductive dissolution of
FeOOH and so the release of sorbed arsenic to
groundwater. The process of degradation is the
same as that which occurs in garden compost.
From McArthur et al. (2001). Water Resources
Research, 37, 109-117.
8FeOOH + CH3COOH + 14H2CO3
(+ sorbed As)
–
→ 8Fe2+ + 16HCO3 + 12H2O
(+ dissolved As)
5a.
6. Groundwater in both the Ganges
7.
In the Ganges Plain and
Viet Nam, install wells
where peat is absent i.e. in
Pre-Holocene sediments.
5b. Arsenic in the Red River
5c. Peat in the Red River
Basin, Vietnam.
Basin, Vietnam.
Plain and Viet Nam carries chemical
signs of peat degradation (high P
and N concentrations); that, plus the
high amounts of iron in the
groundwaters, confirms LAG’s view
that reduction of FeOOH causes
arsenic pollution in both areas.
The LAG model predicted that arsenic pollution would occur in aquifers in
the north and south of Vietnam. In 2001, Berg et al. found arsenic in lethal
amounts in the aquifer supplying Hanoi city’s 11 million people with drinking
water (Fig. 5b). Pollution is worst to the south of Hanoi, where peat is common
(Fig. 5c). Previous water-quality surveys missed the arsenic pollution.
8
6
-1
4. The degree of arsenic pollution in a well depends
upon the distance of its screen from buried peat.
Well A. Low concentrations of organic moieties from
distant peat cause little FeOOH reduction and so little
arsenic release from FeOOH and little As pollution.
Well B. High concentrations of organic moieties from
nearby peat cause much FeOOH reduction, release of
much arsenic, and so substantial arsenic pollution.
Well C. Arsenic pollution above a peat layer caused by
migration of arsenic in response to strong pumping and
also migration of organic moieties upwards to cause
FeOOH reduction and further arsenic release.
Well D. A shallow well that is uncontaminated. Because
it is oxic, it has a low chance of being arsenical.
Well E. Uncontaminated: the chance of contamination
depends on how far organic moieties move laterally before
being consumed by FeOOH reduction, and on the rate of
movement of dissolved arsenic.
3b. In sediments, much arsenic occurs adsorbed
P mg l
From Ravenscroft et al. (2001). In: Arsenic Exposure and Health Effects IV.
W.R. Chappell, C.O. et al. (Eds), 53-77, Elsevier Science Ltd., Oxford.
The LAG model:
The arsenic pollution is caused by
subsurface peat, which drives a
microbiological reaction that releases
arsenic from sedimentary iron oxide,
so it can accumulate in groundwater.
4
2
Viet Nam
Bangladesh
0
0
10
20
-1
NH4 mg l
30
What To Do Now?
Elsewhere, gaining an
ability to predict arsenic
pollution
in
aquifers
requires we understand the
distribution of subsurface
peat - which means we
must
understand
how
vegetation is controlled by
climate, sea-level, and
sedimentology.
J.M. McArthur, London Arsenic Group, UCL.
From Berg, M. et al. 2001. Arsenic contamination of
ground and drinking water in Vietnam: a human health
threat. Environ. Sci. Technol., 35, 2621-2626.
From ‘BGS Technical Report WC/96/22 (1996).
The effect of urbanization on the groundwater
quality beneath the city of Hanoi, Vietnam.
j.mcarthur@ucl.ac.uk
www.es.ucl.ac.uk/research/lag/as