An Investigation in Cause of High and Low Dissolved Arsenic in the Sonar Bangla Aquifer in the Eastern and Western Bank of the River Bhagirathi-Hoogly, West Bengal, India

Previous studies of dissolved arsenic (As) distribution in aquifers of the Bengal basin, India and Bangladesh, have shown spatial variability at various scales. However, the marked contrast in the elevated As in groundwater of the eastern bank and its near-absence in the western bank of the main Indian distributary of river Ganges (river Bhagirathi-Hoogly), has not been previously studied. Samples of groundwater (54), river water (4) and sediment (40) were collected from the Sonar Bangla aquifer at four sites, two on either bank at distances of ~5 and ~20 km from the river Bhagirathi-Hoogly in the Murshidabad district, West Bengal. Three of the four sites are located in recent alluvium, while the fourth site is in older sediments at a higher altitude.

Preliminary evaluation of the field and laboratory analyses shows that the groundwater at all sites (wells screened at 7 - 80 m below land surface) is predominantly circum-neutral, Ca-HCO₃ type, and show subtle differences in major solute chemistry. However, while 22 of the 27 eastern bank groundwater samples have As concentrations >10 ppb (WHO MCL) (median: 31 ppb, range: 1 - 160 ppb), all of the western bank samples have As < 10 ppb (median: 0.2 ppb, range: <0.1 - 6 ppb) and are significantly different from the eastern sites (P < 0.0001). Arsenic concentrations in stream water follow a similar trend: high As in eastern streams (28, 31 ppb) and low As in western streams (0.9, 1.2 ppb). Redox-sensitive parameters [e.g. Fe(II), NH₃, PO₄^3-] and HCO₃ are much higher and Eh is much lower in the eastern sites (median -159 mV, range: -1.9 to -204 mV) than in the western sites (median -61 mV, range: +45 to -144 mV), suggesting an obvious difference in water-sediment interaction and trace solute evolution pathway that is currently being evaluated.