Distribution of Arsenic and its Mobility in Shallow Aquifer Sediments from Ambikanagar, West Bengal, India - 2011

Geochemistry & Hydrochemistry

Routh and Hjelmquist reported in 2011 that sediments from a core retrieved during installation of a shallow drinking water well in Ambikanagar (West Bengal, India) were analyzed for various physical and chemical parameters. The geochemical analyses included: 

1) a 4-step sequential extraction scheme to determine the distribution of As between different fractions,

2) As speciation (As³⁺ vs. As⁵⁺), and 

3) C, N and S isotopes.

The sediments have a low percentage of organic C and N (0.10–0.56% and 0.01–0.05%, respectively). Arsenic concentration is between 2 and 7 mg kg^-1, and it is mainly associated with the residual fraction, less susceptible to chemical weathering. The proportion of As³⁺ in these sediments is high and ranges from 24% to 74%. Arsenic in the second fraction (reducible) correlates well with Mn, and in the residual fraction As correlates well with several transition elements. The stable isotope results indicate microbial oxidation of organic matter involving SO₄ reduction. Oxidation of primary sulfide minerals and release of As from reduction of Fe- (oxy)hydroxides do not seem important mechanisms in As mobilization. Instead, the dominance of As³⁺ and presence of As⁵⁺ reducing microorganisms in this shallow aquifer imply As remobilization involving microbial processes that needs further investigations.

The authors concluded that grain size and redox conditions play an important role in the distribution of As and other elements in Ambikanagar sediments.
Fine-grained sediments have high concentrations of most elements. Total As concentration in aquifer sediments ranges from 2 to 7 mg kg^-1; a considerable part of this occurs as As³⁺ species. It has been shown that As mostly occurs in the residual fraction on the basis of stepwise sequential extraction from different geochemical fractions. The residual fraction is less susceptible to geochemical weathering and release of As in groundwater. The stable isotope records indicate microbial oxidation of organic matter, e.g., by SO₄ reduction. Biogeochemical i.e., bacterially mediated, processes are likely to play a role in regulating As (re)mobilization at this site.