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Lowering of Water Table and Possible Explanations of Groundwater Arsenic Contamination Problem in Bangladesh
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Paper number 022214, 2002 ASAE Annual Meeting . (doi: 10.13031/2013.9851) @2002Authors: Md. Nazrul Islam, Dr. R.D. von Bernuth, Dr. S. Davies
Keywords:
Large-scale irrigation withdrawal has caused lowering of water table and may
have triggered arsenic release into the groundwater of Bangladesh. Changes in
dissolved oxygen result in changes in redox potential and can trigger release of arsenic
into the groundwater. In order to understand the relation between lowering of water
table and groundwater arsenic problem, the changes in dissolved oxygen concentrations
in deeper layers before and after introduction of well fields in a contaminated aquifer was
computed using the Finite Element method. It appears from the result that lowering of water table provides atmospheric oxygen access to the unsaturated zone but it reduces
the diffusive oxygen supply in to the saturated zone with increasing depth. As lowering of
water table could not provide more oxygen access to the deeper saturated layers,
therefore, lowering of water table does not explain well the arsenic release process by
oxidation of arseno pyrite minerals.
However, the theory of arsenic release by reduction of Iron hydroxides is
supported. Because reduced supply of oxygen will lead to a reducing condition and
eventually arsenic is released from the surface of Iron coated sands in the aquifer.
Water quality test records about 3000 contaminated wells show that arsenic
concentration in the most contaminated wells is strongly correlated with the dissolved
Iron concentration in the groundwater. From the Dug-well water analysis records of 500
samples, none contained arsenic with more than 50 ppb and 86% of the water samples
have less than 10 ppb. This implies that groundwater in touch with atmospheric oxygen
was always oxic and rich in redox potential value (Eh).
Although, the intensive use of groundwater for irrigation in Bangladesh has been
found to have no direct relation with arsenic pollution distribution and depth to the water
table, however, lowering of water table influenced the arsenic release process by
decreasing oxygen supply to the deeper layers. Lowering of water table increases the
chances of arsenic release into the groundwater of Bangladesh. Distribution of arsenic
pollution as a function of depth indicates that the top most layer (0 to 15) of the aquifer
was arsenic free and could be kept reserved for drinking purpose. Deeper aquifer (20 to
100 m) contains maximum arsenic. Layers below the 150 m in the aquifer could be
used as a source of arsenic free water for the long-term irrigation development projects.