LEACHING OF IRON AS A CONTROLLING FACTOR FOR ARSENIC MOBILIZATION IN GROUNDWATER OF ASSAM FLOODPLAIN, INDIA

MAHANTA, Chandan¹, BAVISKAR, Shirish M.², CHOUDHURI, Runti² and BHATTACHARYA, Prosun³, (1)Department of Civil Engineering, Indian Institute of Technology, Guwahati, 781039, India, (2)Department of Civil Engineering, Indian Institute of Technology, Guwahati 781039, India, (3)KTH-International Groundwater Arsenic Research Group, Department of Land and Water Resources Engineering, Royal Institute of Technology (KTH), Teknikringen 76, Stockholm, SE-10044, Sweden, mahantaiit@gmail.com

A comprehensive investigation on groundwater arsenic (As) occurrences in the Brahmaputra Floodplain in India revealed that the elevated As concentrations are prevalent at isolated zones of the floodplain located in the seismically active regions. In order to understand the spatial and vertical variability of the distribution of As, multi-level aquifer based study was carried out for exploring mechanisms of As mobilization in groundwater in Jorhat district of Assam, India. Nine bore wells were drilled from established Fe- and As-enriched areas where sediment and groundwater samples were collected at different depths. Plots of Eh and pH in the combined Eh-pH stability diagram of Fe and As indicated the presence of Fe(II) and As(III) species in groundwater. The lithology of the aquifer sediments were characterized by silt, very fine, fine and medium sand with occasional fine gravel. Three step sequential extraction revealed concentrations of Fe and As in the order: residual fraction > reducible fraction > exchangeable fraction > oxidizable fraction. Geochemistry of aquifer sediments following the ASTM method (D 3974-81) revealed considerable variability in the concentrations of Fe (328 - 33660 mg/kg), Mn (10.6 - 5085 mg/kg), and As (0.002 -8.6 mg/kg). Morphology and mineralogy of the aquifer sediments studied using SEM/EDX and XRD analysis indicated the presence of As and Fe in the aquifer sediments. The contents of organic matter (OM, 0.14 - 69.6 %) and carbonate (0.08 -12.5%) in the sediments were variable. Specific surface area (SSA) of the sediments ranged between 0.002 and 0.51 m2/g. Principle Component Analysis (PCA) revealed the relationship between Fe, Mn, As, OM content, carbonate content and SSA of the sediments for each bore hole. Results of the hydrogeochemical study of the aquifer sediments indicated that the reductive dissolution of Fe-oxy-hydroxide was the key mechanism of As release along with Fe, Mn concentrations, OM and SSA of the sediments playing an important role in mobilizing As in groundwater.

Session No. 280

T12. Arsenic in Geologic Systems II

Wednesday, 3 November 2010: 1:30 PM-5:30 PM

Room 612 (Colorado Convention Center)

Geological Society of America Abstracts with Programs. Vol. 42, No. 5, p.652

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