Arsenic and groundwater properties of Araihazar, Bangladesh

Arsenic is a highly toxic and ubiquitous metalloid dispersed throughout the earth's crust. It can be mobilized in the environment through a combination of natural processes. The mobilization of arsenic from aquifer materials usually occurs under reducing conditions and is microbially mediated. This was observed in aquifers of Araihazar, Bangladesh. There, elevated dissolved As concentrations in groundwater were found mostly in the form of inorganic arsenic as arsenite, using several analytical methods including one that uses a modified colorimetric method. However, time series data demonstrate that variations in groundwater As concentrations were considerably less in comparison to other redox-sensitive elements such as Fe, Mn and S, suggesting that there are significant decoupling between As mobilization and redox state of the aquifer. To resolve the uncertainty regarding whether mobilization of As from sediment requires reduction or not at crustal abundances, a series of laboratory microcosm experiments using a representative natural sediment sample collected from Araihazar, Bangladesh were conducted. The results show that mobilization of As does not require reduction. The mobilization and reduction of Fe and Mn are predominantly limited by availability of electron donors such as lactate. However, the mobilization and reduction of As appears to have more limiting factors such as the type of microbes present in the system. This study also reinforced the notion that although reducing conditions are key to the mobilization of As in Bangladesh shallow aquifer, As concentrations can be significantly decoupled from Fe and P. This difference, in part, is because Fe and P concentrations are regulated by several mineral precipitation reactions. In addition to potential differences in microbial communities, there is a remarkable difference in several nutrients between the Holocene shallow aquifer and the Pleistocene deep aquifer. Future studies of the implication of the nutrient and microbial contrast on As mobility in both aquifers and viability of deep aquifer as a drinking water source are much needed.