Mechanism and source of arsenic contamination in groundwater in Bangladesh

Naturally occurring contamination of arsenic is of great concern in many Southeast Asian countries especially in Bangladesh. Because of the lack of understanding for the mechanism of As contamination, we have developed two studies through field surveys and laboratory experiments. First topic is focused on the underlying mechanisms controlling large spatial variation of As-contaminated groundwater based on the adsorption reaction. Because adsorption is kinetically first reaction, we hypothesized that groundwater As level for each household well water should controlled by adsorption equilibrium between sediment and water. To verify the hypothesis, we firstly verified adsorption equilibrium using the batch adsorption experiment for the sediment collected from As contaminated region in Bangladesh coupled with solid phase As speciation using X-ray absorption fine structure. The results indicate that dissolved As level is consistent with adsorption equilibrium. We further introduce a new insight to analyze contamination mechanism based on the adsorption equilibrium model. The second topic is the novel approach to identify the ultimate source of As using Sb stable isotope. Although there are enormous numbers of research about arsenic contamination in Bangladesh, the primordial source of arsenic is still unknown. Because arsenic is a monoisotopic element, we cannot discuss this issue using isotope geochemistry of arsenic, which can be a part of the reason why the source of arsenic contamination is unclear. Because of the similar geochemical behavior of As and Sb, we are trying to establish the antimony isotope analysis and verify its applicability to the source analysis of arsenic. Here, two topics: (i) determination method of antimony isotope in natural sample by multicollecter-type ICP-MS, and (ii) verification for the similarity of antimony and arsenic behavior in river water-sediment systems based on a model study around the Ichinokawa antimony mine, is introduced.