Environmental Engineering Reference
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far in excess of allowable limits. Tests on these wells at time of installation showed arsenic
concentrations, if any, well below threat levels. The present levels of arsenic concentra-
tion indicate arsenic poisoning developed several years after installation of the wells. By
switching to groundwater resources to avoid waterborne diseases from ingestion of sur-
face water supplies, the population now faces considerable risk of arsenic poisoning as a
result of ingesting arsenic-contaminated water obtained from tube wells tapping into the
groundwater.
If the aquifers were not contaminated by arsenic before extensive harvesting of the
aquifer resource, how did the arsenic get there? Is the arsenic contamination due to natu-
ral causes? Or indirectly due to a man-made cause? Arsenic occurrence in the hard rock
and sedimentary rock aquifers have been reported in Argentina, Nepal, Nigeria, Czech
Republic, and many other countries. Although it is not uncommon to ind vanishingly
small concentrations of arsenic in the groundwater in many parts of the world, the arsenic
concentrations in the samples obtained from the aquifers in Bangladesh and West Bengal
for instance are too high to be attributable to natural processes for arsenic release from
arsenopyrites. In fact, the arsenic concentrations in the Bangladesh and West Bengal aqui-
fers are also too high to be accounted for by direct human activities such as those associ-
ated with metalliferous mining. There appears to be no doubt that the release of arsenic
into the aquifers is from geological source materials. Why is arsenic being released from
the source rocks? Is this a natural process or is the release of arsenic triggered by some
man-made event?
In the Bangladesh case, two factors appear prominently: (1) presence of arsenopyrites
and arseniferrous iron oxyhydroxides in the substrate material and (2) the use of tube
Arsenopyrites and arseniferrous iron oxyhydroxides present
in subsurface geologic material
Reductive dissolution of arseniferrous
iron oxyhydroxides
Oxidation of arsenopyrites
Tube well
Aquitard
Aquifer
FIGURE 2.5
Schematic of speculative models for release of arsenic from arseniferrous iron oxyhydroxides and arsenopyrites
into aquifers.
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