IMMOBILIZATION OF GROUNDWATER ARSENIC IN RIVER BANK SEDIMENT FROM THE GANGES-BRAHMAPUTRA-MEGHNA RIVER DELTA

In the Ganges-Brahmaputra-Meghna River Delta (GBMD), elevated concentrations of As prevalent in the Holocene shallow aquifers has been attributed to mobilization of natural As present in the aquifer sediments under reducing conditions. This leads to the question about the fate of groundwater As when the reducing groundwater reaches the point of discharge at a hydrologic boundary such as the river bank. There, the sediment is likely to be more oxic and may act as a natural reactive barrier to immobilize discharging As. We report geochemical data in support of the redox trapping of discharging groundwater As.

In January 2006, we collected 33 sediment cores down to 7 m depth with 102 sediment and 20 pore water samples, 18 shallow well water and 6 river water samples from 18 sites along Meghna and Brahmaputra Rivers. The first area is located at ~ 23.4 N on the Meghna River where most existing shallow wells (< 25 m) contained >50 ug/L As. The second area is located at ~ 24.6 N on the Brahmaputra River where nearly all shallow wells contained < 10 ug/L As. The concentrations of 1.2M HCl extractable As, PO4, Fe in sediment samples were measured in the field by colorimetry and re-analyzed in the laboratory by high resolution inductively plasma mass spectrometry (HR ICP-MS). Water samples were also analyzed by HR ICP-MS for major cations, As, Fe, Mn, S and P. The speciation of As sorbed on sediment was determined by voltammetry after an anaerobic extraction by 1 M phosphate with 0.1 M L-ascorbic acid.

Several sediment cores from the Meghna River site displayed high As much greater than the crustal value of sediment of ~ 10 mg/kg, with the maximum of HCl-leachable As detected as 709 mg/kg at 0.3 m. Most of the P-extractable As were in the form of arsenite. None of the 5 cores from the Brahmaputra site had elevated sedimentary As. Concentrations of Fe, As, P displayed a stronger decreasing trend from existing shallow wells to river bank sediment pore water, and finally to the river water than that of the major cations such as Ca, Mg, K and Na. Our results illustrate that elevated sediment As resulted from groundwater-river water interaction in a natural reactive barrier most likely consisted of Fe minerals, preventing As from leaving the system. Recycling of this As to the Ganges-Brahmaputra-Meghna Delta (GBMD) aquifer provides a potential source of As.