Role of shallow alluvial stratigraphy and Holocene geomorphology on groundwater arsenic contamination in the Middle Ganga Plain, India

Floodplain geomorphology and Late Quaternary alluvial stratigraphy influence groundwater arsenic contamination in the Gangetic Plains in India. Elevated concentrations of arsenic (>50 µg/L) in the Middle Ganga Plain (MGP) are spatially related with dark grey to black coloured organic-rich clay sediments found both in the Active (AFP, T0-surface) and Older Floodplains (OFP, T1-surface) of the Ganga River. The floodplains comprise Newer Alluvium of Holocene age, mainly derived from Himalaya. Abandoned/palaeochannel cut-offs, either filled (often in OFP) or in the process of filling under fluvio-lacustrine environments (often in AFP) host such clay deposits. The settlements on anthropogenic fills (to avoid flood inundation) over such palaeochannels exhibit higher concentrations of arsenic in groundwater (max. 987 µg/L). The organic carbon from the clay bodies when released to groundwater promotes reducing environment that causes release of arsenic entrapped in the sediments. The sandy areas, such as scroll bar ridges, levees and point bar platforms, exhibit low concentrations of arsenic. The model on groundwater arsenic distribution and Holocene geomorphic units can be used for micro-level delineation of arsenic-free shallow aquifers for rural water supply in the arsenic-contaminated areas.