Variability in Hydrogeochemical Characteristics in Regions with High Arsenic Groundwater at Matlab, Southeastern Bangladesh.

Abstract: Elevated levels of geogenic arsenic (As) in groundwater are regarded as the most formidable environmental crisis in the contemporary world and an estimated 30-70 million people in Bangladesh are at risk. Many of the provided options for mitigation have not been well accepted. In recent years, local drillers have been targeting As-low groundwater on the basis of the color of the sediments. A correlation between the color characteristics of the sediments and the groundwater redox conditions and thereby the risk for As mobilization has been established. It is possible to assess the relative possibility of occurrence of high As concentrations in groundwater if the color characteristics of the sediments are known. One main objective of the present study is to validate if targeting As-safe groundwater is a sustainable mitigation strategy. This has been done through monitoring groundwater from tube-wells installed in sediments with different color, depth and redox characteristics. Samples were collected yearly from a set of 17 tube-wells for the period of 2004 to 2008 in Matlab, Bangladesh. The wells were grouped into oxidized and reduced/non-oxidized wells. All of these wells tap water within a depth of 85 m from the surface. Major cations and trace elements including As were analysed by high resolution ICP-OES; and anions were measured by ion chromatography. Groundwater chemistry of the water abstracted from different sediments revealed that no major change had occurred over the period of 5 years. In reduced wells, concentration of major cations and anions varied by <10% over time, and no significant variation was observed in redox sensitive elements, such as Fe, Mn and S. There were some minor variations in the As concentrations in the wells. In the oxidized wells, except well 58, major cations, anions and redox sensitive elements varied by <5%, while the As concentrations were found quite low and stable (<5.2 μg/L). Time series trends thus suggest that the change in groundwater chemistry is insignificant over the period of 5 years from 2004 to 2008. Low As concentrations in the tubewells installed in the oxidized brownish sediments thus validate the mitigation strategy corresponding to the conceptual understanding of the groundwater system in Matlab in southeastern Bangladesh.