ARSENIC HOTSPOT ALONG AN ABANDONED CHANNEL IN MIDDLE GANGA PLAIN AND ITS POSSIBLE LINKAGES WITH GROUNDWATER CHEMISTRY AND AQUIFER LITHOLOGY

The recent finding of very high arsenic (As) in groundwater in MGP has patchy occurrences in shallow aquifers of MGP. An integrated study of lithology, hydrology at micro scale along the bank of an abandoned channel revel several arsenic hotspots, which is surrounded by moderate or uncontaminated aquifers. Groundwater quality for 1.5 year survey shown increasing trend of Aqueous As, as average concentration shift from 74.52 µgl^1- to 94.06 µgl^1-(n=52). As (III) shows dominance ranging from 72% to 99% in both moderate (<50 µgl^1- dissolved As) and highly contaminated groundwater in negative Eh condition. Lithologmap of area suggests separating middle clay around 100m between newer contaminated and safe older alluvium and presence of clay lances at upper layers. A core of 32m depth drilled in study area, in which a distinctive dark coloured fine clay lances with high organic (≈6%) and carbonate content (≈10%) are found at 32m bgl.Arsenic content (5.1 mg.kg^1-)in sediment is found to around background concentration. The same dark fine grained layer has comparatively high iron, manganese and arsenic content. Presence of clay lances at shallow depth may provide semi confined condition for possible contamination of lower aquifers and vice-versa. Distinctive lithology and geochemistry of shallow aquifers in these regions possibly provide pre-condition for As release.