Arsenic in groundwater of the Bengal Delta Plain: a detailed case study on a local hotspot

This study aims on the detailed characterization of the hydro- and geochemical environment of a local As hot-spot, in order to put constrains on the source, availability and mechanism of As release into the groundwater. A small area (40 km2) was selected in the Malda district (West Bengal, India) to carry out two sampling campaigns in March 2002 and 2003. Sampling comprises groundwater and sediment samples up to 60 m depth.
With arsenic concentrations of more than 800 µg/L, the groundwater in the working area can be ranked as highly polluted. The comparison of the sampling in 2002 and 2003 has shown both, constant and slightly increased As-concentrations. Distribution of anorganic As(III) and As(V) species shows domination of the more toxic As(III), representing in average 76% (2002) and 86% (2003) of the total As content. This is in agreement with a gradual release of As under reducing conditions.
The main hot spot is close to two old river channels, which may favor the development of a reducing environment due to possibly increased content of clay minerals and organic matter in the aquifer. Correlation of As and Fe in water suggests the dissolution of As-bearing iron phases as one possible mobilisation mechanism. ... mehrAdditionally, phosphate ions may compete with arsenate. This is supported by the good correlation of As with phosphate (r=0.63) and ammonia (r=0.73), suggesting a possible local input of nutrient rich anthropogenically polluted water. Such an input may be enhanced by both, leaking well drillings and/or intense groundwater exploitation.
The As content in the aquifer sediments up to 60 m depth is quite low (3-12 mg/kg) and diffusely distributed. The good correlation of As with Fe and Mn in sediments (As-Fe: r=0.88; As-Mn: r=0.79) suggests the association with Fe- or Mn-rich mineral phases. Measurements with µ-XRF on the aquifer material confirmed the diffuse distribution of arsenic on µm-scale. Arsenic contents reached up to 100 mg/kg only in a few grains.
The main As-bearing phases are represented by Fe and Mn minerals, but simultanously increased concentrations of Zn and Pb may suggest also an association with sulfidic ore minerals.
Further investigations like XANES-studies and leaching experiments will be taken to explain the massive release of As into the groundwater, despite of the low background concentrations in the aquifer.