Lowering of the water table, irrigation policy change and its influence on the groundwater arsenic contamination problem in Bangladesh.

摘要

This study is based on the hypothesis that shortage of dissolved oxygen at and below the water table and extraction of groundwater by irrigation wells at a rate greater than the aquifer recharge rate are the main causes of arsenic release in the groundwater of Bangladesh. The main purpose of this study was to identify the processes that produce high arsenic concentrations in the groundwater system and to formulate a quantitative irrigation development policy for Bangladesh. This dissertation evaluated the currently existing oxidation and reduction theories of arsenic release from the context of dissolved oxygen shortage in recharging groundwater. This study also conducted both numerical and thermodynamic analyses to demonstrate how the oxidation theory of arsenic release was not adequate to explain the cause of arsenic release in the groundwater system and how shortage of dissolved oxygen in recharging groundwater at and below the water table produces high arsenic concentrations.;This study focused on quantitative analysis of the oxygen diffusion rate in deeper layers of the arsenic-contaminated aquifer and the variations in redox potentials over the aquifer's depths in order to evaluate the oxidation and reduction theory of arsenic release. This study also computed the amount of shallow aquifer recharge (SAR) available and the amount of groundwater flow passing through the deeper layers of the aquifer in order to establish a relation between lowering of the water table, the tube well extraction rate, and the likelihood of arsenic migration from the currently contaminated layers to uncontaminated deeper layers of the aquifer.;Based on the dissertation's main findings, it can be concluded that variable management decisions such as the tube well extraction rate, increased irrigation system efficiencies, shifted and adjusted cropping patterns and seasons, rainwater harvesting, and avoidance of pumping from the top layers of the aquifer can improve the control of the system and reduce the risk of arsenic migration.