LINKING SEDIMENT GEOCHEMISTRY TO ARSENIC AND MANGANESE MOBILITY IN AQUIFERS IN MURSHIDABAD, WEST BENGAL, INDIA

Groundwater and shallow aquifer sediments (2m-40m) were collected from areas in Murshidabad, West Bengal, India, to study their hydrochemical and geochemical properties and examine relationships between groundwater arsenic (As) and solid-phase As and Mn concentrations and speciation. Murshidabad was chosen for this study because local groundwaters are highly As-affected (up to ~4600 μg/l). Mn, which has a potential neurotoxic effect, has also been reported along with high-As groundwaters from the Bengal Basin, complicating occurrence and distribution of As in the region. Issues addressed in our study were: characterize sediment cores and groundwaters in areas with contrasting As concentrations—west (low-As) and east (high-As) of the river Bhagirathi and examine the relation with Mn occurrence; describe the spatial variability of dissolved As concentrations in shallow (< 60m) aquifers, comparing sediment core chemistry to water chemistry; identify source(s) of manganese and evaluate the bioavailability and mobilization of As and Mn from sediments to groundwaters. Mineralogical differences between (grey) sediments bearing high-As groundwaters and (orange-brown) sediments bearing low-As groundwaters included greater amounts of micas, Fe- and Mg-rich clays, amphiboles, carbonates, and apatite in high-As sediments; these were virtually absent from low-As sediments. In high-As areas, sequential extractions showed an association of As and Mn with amorphous and poorly-crystalline Fe(III)-oxides/oxyhydroxide phases and labile phases, especially where Fe(II):Fe_T was high in the sediments. Total Mn in the same sediments [range: 97-678 ppm] was chiefly concentrated in the upper 20m of sediments and was well correlated (r²=0.92) to solid-phase As in one area. Poor Correlation was found between dissolved Mn and As in the tubewell waters. The data suggest that both microbially-mediated reduction of amorphous Fe and Mn oxides and oxyhydroxide phases and competitive ion-exchange processes occurring in shallow aquifers of Murshidabad drive As mobilization and sequestration by aquifer sediments, indicating that additional detailed study of the geochemical behavior of Mn in such reduced environments may be fruitful.