Leaching of Iron, Controlling Factors and Implication to Arsenic Mobilization in an aquifer of the Brahmaputra Floodplain

Abstract

To understand the process of iron leaching and arsenic (As) mobilization, three bore wells were drilled in suspected iron and arsenic enriched areas of Jorhat, Assam, India, to study possible release and mobilization process in the aquifer. Sediments and groundwater samples, collected from different depths in these boreholes were analyzed for different parameters. Combined Eh-pH stability diagram of iron and arsenic indicated the presence of Fe (II) and As (III) species in the groundwater. BCR three step sequential extraction method indicated relative mobility of Fe and As in a similar trend of decreasing order as: Residual fraction > Reducible fraction > Exchangeable fraction > Oxidizable fraction. The digestion of aquifer sediments by D 3974-81, (ASTM) showed Fe (106 mg/kg to 26991 mg/kg), Mn (7 mg/kg to 1588 mg/kg), As (0.16 mg/kg to 18.6 mg/kg). The morphology and mineralogy of the aquifer sediments, studied by scanning electron microscope/energy dispersive x-ray (SEM/EDX) and x-ray diffraction (XRD) analysis indicated the levels of Fe and As concentrations in the aquifer sediments. Sequential heating in the muffle furnace indicated organic content (0.01 % to 13.7 %) and carbonate content (0.15 % to 9.19 %) of aquifer sediments. Laser particle size analyser was used to find the specific surface area of the sediments (0.02 m2/g to 0.19 m2/g). The lithology of the soil consisted of silt, very fine sand, fine sand, medium sand and fine gravel. The relationship between Fe, Mn, As, organic content, carbonate content and specific surface area of the sediments along the depth for different bore holes was interpreted separately including with the help of Principal Component Analysis (PCA). In multiple linear regression models, the experimentally found and predicted concentrations of iron correlated as 0.83 for B bore hole and 0.87 for E bore hole, whereas correlation coefficients for arsenic concentration were 0.88 in B bore hole and 0.80 in E bore hole. With the help of the linear regression models, the correlation between these elements and the factors controlling their concentrations were evaluated.