, 225:1792
Date: 29 Nov 2013

Geochemistry of Tungsten and Arsenic in Aquifer Systems: A Comparative Study of Groundwaters from West Bengal, India, and Nevada, USA

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Abstract

Tungsten (W) concentrations were measured along with arsenic (As) in groundwaters from the Murshidabad district of West Bengal, India. Tungsten concentrations range from 0.8 to ~8 nmol kg-1 (0.15–1.5 μg kg-1) in the circumneutral pH (average pH ~ 7.3) Murshidabad groundwaters, and attain concentrations as high as 14 nmol kg-1 (2.5 μg kg-1) in local ponds (n = 2). Total dissolved As concentrations (AsT) range from 0.013 to 53.9 μmol kg-1 (<1 to 4,032 μg kg-1), and As(III) predominates in Murshidabad groundwaters accounting for 70 %, on average, of As in solution. Tungsten concentrations in Murshidabad groundwaters are low compared to alkaline groundwaters (pH > 8) from the Carson Desert in Western Nevada, USA, where W concentrations are reported to reach as high as 4,036 nmol kg-1 (742 μg kg-1). Although W is positively correlated with As in groundwaters from the Carson Desert, it is not correlated with AsT or As(III) in Murshidabad groundwaters, but does exhibit a weak relationship with As(V) in these groundwaters. Surface complexation modeling indicates that pH related adsorption/desorption can explain the geochemical behavior of W in Murshidabad groundwaters. However, the model does not predict the high As concentrations observed in Murshidabad groundwaters. The high As and low W concentrations measured in Murshidabad groundwaters indicate that either As and W originate from different sources or are mobilized by different biogeochemical processes within the Murshidabad aquifers. Mobilization of As in Murshidabad groundwaters is presumed to reflect reductive dissolution of Fe(III) oxides/oxyhydroxides and release of sorbed and/or coprecipitated As to the groundwaters. Multivariate statistical analysis of groundwater composition data indicate that W is associated with Mn and Cl-, which may point to a Mn oxide/oxyhydroxide, clay mineral, and/or apatite source for W in the Murshidabad sediments.