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Paper No. 2
Presentation Time: 1:45 PM
CHANGES IN GROUNDWATER CHEMICAL CHARACTERISTICS IN REGIONS WITH HIGH-ARSENIC GROUNDWATER AT MATLAB, SOUTH-EASTERN BANGLADESH
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 have not been well accepted. In recent years, local drillers have been targeting As-low groundwater on the basis of the colour of the sediments. A correlation between the colour characteristics of the sediments and the groundwater redox conditions and risk for As mobilisation has been established and it is possible to assess the relative risk of high concentrations of As if the colour characteristics of the sediments are known.One main research interest 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 colour, 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 shallow (<85m) wells. Major cations and trace elements were analysed by high resolution ICPOES with better precision; and anions were measured by ion chromatography.Groundwater chemistry of the water tapped from the different sediments showed that no major change had occurred over a period of 5 years. In reduced wells, concentration of major cations and anions varied by <10% over time, but the redox sensitive elements (Fe, Mn and S) did not vary significantly. There were some minor variations in the As concentrations in these wells. In the oxidized wells, major cations, anions and redox sensitive elements (Fe, Mn and S) varied by <5%, while the As concentrations were low and stable (<5.2 ug/l) for the 5 years. Time series trends of the hydrochemical characteristics over a period of 5 years (2004-2008), thus suggest that the change in groundwater chemstry is insignificant over the period. Low As concentrations in the oxidised wells thus validates the mitigation strategy corresponding to the conceptual understanding of the groundwater system in Matlab, southeast Bangladesh .