Click on “Download PDF” for the PDF version or on the title for the HTML version.

If you are not an ASABE member or if your employer has not arranged for access to the full-text, Click here for options.

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

Citation:  Paper number  022214,  2002 ASAE Annual Meeting . (doi: 10.13031/2013.9851) @2002
Authors:   Md. Nazrul Islam, Dr. R.D. von Bernuth, Dr. S. Davies

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.

(Download PDF)    (Export to EndNotes)