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Biogeochemical Role of Surficial Diatomaceous Biomats in Groundwater Purification: A Key Revelation from the Gangetic Alluvial Floodplain, South 24 Parganas, India

A multidisciplinary study involving field geology, mineralogy and geochemistry of sediments and waters (surface and underground) and microbiology in two representative and juxtaposed areas having contrasting shallow aquifer conditions in West Bengal (India) has helped resolve the problem of high As-Fe incidence in Gangetic Delta and provided a new insight into the biogeochemical role of surface diatomaceous biomats (multilayered sheets of micro-organisms dominated by diatoms held together and bound to the moist substrate by secreted slimy extracellular polymeric substances) in groundwater purification. Regional geological investigation provides geoenvironmental clues to the development of profuse diatomaceous biomats (maiden report) selectively in clay-silt-rich moist geomorphic depressions (floodplains) conditioned by regular supply of nutrients, rain and flood waters and sunlight and unveils a correlation between the surface biomats and subsurface safe aquifers and vice versa. Lateral confinement and predominant monsoonal recharge pattern of the aquifers in this region restrict subsurface lateral saturation across the aquifers and allow maintenance of their respective geochemical status. This finding allows ready assessment of groundwater quality and helps develop a new bioengineering technique for improving groundwater conditions by growing artificial biomats. The present study provides the first natural evidence of arsenic groundwater purification through diatomaceous biomats.