Hydrology and Arsenic Distribution in Shallow Aquifers of Bangladesh.

摘要

The research conducted under this thesis explores the interactions between recharge, groundwater flow and geochemical processes to understand the distribution of As concentrations in the shallow (20 m deep) aquifers of Bangladesh. All the field work was carried out in Araihazar upazilla, a region where As concentrations in shallow groundwater are spatially highly variable.;Taken as a whole, the research carried out under this thesis has established a new connection between surface hydrology and concentrations of As in shallow aquifers while, at the same time, showing that biogeochemical processes provide significant buffering of As concentrations against natural or induced variations in surface forcing.;In Chapter 2, systematic relationships between the permeability of surface sediments types, the age of groundwater since recharge, and dissolved As concentrations in shallow aquifers were documented over a 25 km2 area. The associations indicate that local groundwater recharge through permeable sandy soils prevents As concentrations from rising in shallow reducing groundwater. A significant portion of the variability of As concentrations in shallow aquifers is therefore determined by the impact of different depositional environments on local hydrology. The linkage between surface forcing and groundwater As in shallow aquifers is supplemented in Chapter 3 with multiple evidence, from the solid and from the dissolved phase, of sequestration of As in sediment in the form of As(III)-sulfides. This mechanism requires reducing conditions as a well as a supply of sulfur and could be a key factor in the relationships observed in the Chapter 2 because the permeability of surface soils probably regulates the flux of sulfate into shallow groundwater. The impact of a human perturbation in the form of irrigation pumping on the supply of sulfate and, therefore, increased As retention in the sediment, is explored in Chapter 4 documenting temporal variability in groundwater As concentrations in a few very shallow wells. These observations suggest that, contrary to expactations, irrigation pumping may lower rather than raise As concentrations in shallow aquifers. The 3-dimensional groundwater flow and transport model presented in Chapter 5 merges the concepts presented in the preceding chapters in the context of the As distribution of a specific village in Bangladesh. The simulations show that the spatially variable distribution of As in that village can be explained to a large extent as the result of flushing, the combined effects of groundwater flow and sorptive exchange of As, for several thousand years following deposition of the aquifer sands. By simulating the installation of a pair of irrigation wells, the model is also used to show that the impact of irrigation pumping on the composition of groundwater is strongly dampened by retardation.