GRAIN SIZE, MINERALOGY AND SEDIMENT COMPOSITION AS FACTORS CONTROLLING RELEASE AND MOBILIZATION OF ARSENIC IN PARTS OF THE BRAHMAPUTRA FLOODPLAINS, NORTHEASTERN INDIA

An exploratory borehole drilled in an established arsenic contaminated aquifer regime in a non-industrial area was studied in detail based upon soil samples collected at an interval of 5 ft upto a depth of 200 ft. Samples collected were analysed and results were examined vis-a-vis hypothesis and computer programs. The soil samples from selective depths investigated through XRD, FTIR, SEM-EDS and TEM analysis showed association of arsenic with secondary minerals that are responsible for elevated arsenic levels in groundwater. The oxidation and reduction of sulfide minerals along with iron, manganese, aluminium in the aquifer appeared to be possible source for arsenic mobilisation in the study area. The water chemistry data suggested a reducing condition in the deeper aquifer. The sequential extraction results confirmed the results of the analytical methods employed for both the deeper aquifer and the shallow aquifer contaminated by arsenic bearing minerals. Particle size analyses provided evidence of alluvial nature of the soil deposited by sediment transported by the rivers originating from upstream Bhutan hills. The results suggested that the deeper aquifers (beyond 90 ft) were contaminated by secondary arsenic bearing minerals, mainly reducible forms of orpiment and realgar. A numerical modelling done for finding flow profile by MODFLOW for ground water flow simulation in the study area could predict the effect of the source well on the surrounding area. Shepherd empirical equation was used to calculate the hydraulic conductivity using particle size results. The mineralogical studies revealed the dominating arsenic mobilizing minerals at different depths of the aquifer while the hydraulic conductivity provided idea about contaminant movement rate in the aquifers. The obtained data were used to build a subsurface scenario by modelling with GMS 6.5 software. The results of possible contamination flow and transport mostly conformed to the field investigations. With the help of the simulation results, possible safe groundwater zones in the area could be predicted.