HYDROGEOCHEMICAL CONTRAST BETWEEN TWO ARSENIC AFFECTED AREAS NEAR THE EASTERN AND WESTERN MARGINS OF BENGAL BASIN: SOME PRELIMINARY RESULTS

In order to understand the control of groundwater dynamics, hydrostratigraphy, hydrochemistry and anthropogenic influences on the worst arsenic contaminated area of the world, a detailed basin-wide study has been initiated in the Bengal basin (India and Bangladesh). Realizing the significance of the regional-scale heterogeneities along with local-scale variations in geology, redox regimes and land-use pattern, a transect extending from the western to eastern margin of the Bengal basin has been selected. Two study sites have been selected for comparison, located along this transect: (A) parts of Nadia district (West Bengal, India) on the western margin and (B) Brahmanbaria district (Bangladesh) on the eastern margin.

At both the locations the main water type is Ca²⁺-HCO₃^- type with few isolated Na⁺-Cl^- waters. However, mean depths of isolated aquifers are more in (A) than (B). The Ca²⁺-HCO₃^- waters at both areas have circum-neutral pH, similar concentrations of Na, Mn and SO₄^2- and observed Eh in disagreement with calculated Eh from redox couples. The mean concentrations of Ca²⁺, HCO₃^-, Cl^- and As(III)/As(V) are higher at (A) than (B), while (B) has higher NH₄⁺ and DOC. Also, both maximum and mean concentrations of As(tot) and Fe(tot) are higher at (B) than (A). PO₄³⁺ or NO₃^- were detected at (B), while none were found in (A). Bivariate comparison of DOC with As(III), Fe(tot), HCO₃^- and NH₄⁺ yielded moderate to good correlations at (B), while such values were not observed at (A). SI calculation of mineral phases in groundwater shows that Fe(III) oxides (e.g. ferrihydride and goethite) are supersaturated at (A) in contrast to near equilibrium to slightly undersaturated values for (B), while Fe(II) minerals (e.g. siderite) and carbonates (calcite and dolomite) are close to equilibrium. These comparisons suggest that although the main processes controlling groundwater quality across the Bengal basin are similar, significant differences exist in redox and microbial processes between the eastern and western margin.