Abstract
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.