An Introduction to Environmental Chemistry

Nearly all cases of large-scale arsenic contamination in groundwater are caused
by reduction of iron oxides in aquifer sediments.

Microbial reduction of arsenic-bearing iron oxides

Reduction of iron oxide (FeOOH) is a potential source of arsenic to groundwa-
ter because it releases arsenic adsorbed to the oxide surfaces. A representation of
this process might be:

The reaction requires anoxic conditions, since iron is barely soluble in oxic water
(Box 5.4), and is fuelled by the microbial metabolism of organic matter (depicted
as CH 2 O in eqn. 5.26).
This mechanism of arsenic release to groundwater has recently been proposed
as the cause of high arsenic concentrations in millions of deep drinking water
wells across Asia, the example in the Ganges–Meghna–Bramaputra delta plain of
Bangladesh and West Bengal being much publicized. The original source of the
arsenic in these aquifer sediments is not known with much certainty, but proba-
bly comes from weathering of arsenic-rich coals and sulphide ores in the upstream
drainage basin. The arsenic was then transported downstream in solution, adsorb-
ing to clay-rich and organic-rich sediments that accumulated in the delta over
the last 2 million years or so.
Many of these drinking water wells, which provide 20 million people with
almost all their drinking water, exceed both the World Health Organization
(WHO) guideline value of 10mgl-^1 As and the Bangladesh drinking water
maximum of 50mgl-^1 As. The world’s press have recently picked up on this
problem, first discovered in the mid 1990s, with headlines claiming ‘the largest
mass poisoning of a population in history’. It is feared that up to 20 000 people
could soon die each year as a delayed result of accumulating arsenic in their bodies
from wells sunk up to 25 years ago.
The highest levels of contamination (> 250 mgl-^1 As) typically occur between
25 and 45 m depth—too deep to implicate sulphide oxidation (Section 5.4.2)—
and concentrations in excess of 50mgl-^1 As occur down to 150 m. The require-
ment for microbial metabolism of organic matter has recently led scientists to
link the presence of highly organic peat beds in the deltaic aquifer sediments as
the driver for equation 5.26. Clearly other sedimentary aquifers that host peats
or organic-rich muds might be vulnerable to arsenic contamination, including
other large deltas such as the Mekong and Irrawaddy.
As the release of arsenic requires reducing conditions, a logical application of
environmental chemical principles suggests that aeration of the water might
reverse the effects of equation 5.26, for example:

41 Fe()^2 aq+ ++O 22 ()gl 048 H O()ÆFe OH() 3 ()s + H+()aq eqn. 5.27

474 FeOOH()ss++CH O 223 () H CO()aqFe^2 ()aq+ ++ 8 HCO32l-()aq 6 H O()

The Chemistry of Continental Waters 179

eqn. 5.26