ABSTRACT
Groundwater pollution due to presence of naturally occurring arsenic (As) is reported in sedimentary aquifer worldwide and health related issues due to drinking of As contaminated water have been documented in many parts of the world. In recent year, high concentration of As in groundwater in several parts of India has become a major cause of concern. Late Quaternary stratigraphy and sedimentation in the Ganga Alluvial Plain have influenced As contamination of groundwater, although other environmental factor such as water interactions, ion exchange, and shift in the climatic conditions, redox processes and anthropogenic activities are also responsible for elevated concentration of As in groundwater of Gangetic plain. Arsenic was transported from weathered rock and adsorbed on dispersed iron-oxyhydroxide in the Ganga alluvial plain. Arsenic was released later to groundwater mainly by reductive dissolution of hydrated-iron-oxide and corresponding oxidation of sediment organic matter. Elevated concentration of As in groundwater has been younger alluvium (dark and grey sediment) of Holocene age deposits. Older Alluvium, comprising Pleistocene brownish yellow sediment, extending as deeper aquifers in Newer Alluvium areas, is low in groundwater As. Inorganic arsenite {As(III)} is more dominated on arsenate in the groundwater. The concentration of As(III) in agricultural soil samples varies from not detectable to 40 μg/kg and As(V) was observed as the major species while the total As concentration varied from 3528 to 14,690 μg/kg in our study area. Results of X-ray diffraction (XRD) and environmental scanning electron microscope (ESEM) revealed the presence of hematite and goethite throughout the vertical section below while magnetite was observed only in the upper oxidized layer of the core collected from the study area. Alteration of Fe-oxides and presence of fibrous goethite indicating presence of diagenetic sediment. Siderite plays a crucial role as sinks to the As in subsurface sediments. The study also concluded that decomposition of organic matter trigger mobilization of As into the groundwater.
