On the relation between fluvio-deltaic flood basin geomorphology and the wide-spread occurrence of arsenic pollution in shallow aquifers Marinus E. Donselaar a, ⁎, Ajay G. Bhatt a,b , Ashok K. Ghosh b a Department of Geoscience and Engineering, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands b Department of Environment and Water Management, Anugrah Narayan College (Magadh University), Patna, India HIGHLIGHTS • Point-bar and oxbow-lake/clay-plug geomorphological elements are pro- posed as the coupled source/sink of dis- solved arsenic. • A generic geomorphological model ex- plains the migration and accumulation of dissolved arsenic on entire flood-basin scale. • Anoxic hypolimnion oxbow-lake water and clay-plug sediments are the loci of reactive organic carbon. • Released arsenic is trapped in perme- able point-bar sands surrounded by low-permeable clay plugs. • Permeability contrasts in the point-bar geomorphological element cause spatial arsenic concentration differences. GRAPHICAL ABSTRACT abstract article info Article history: Received 28 April 2016 Received in revised form 7 September 2016 Accepted 9 September 2016 Available online xxxx Editor: D. Barcelo Pollution of groundwater with natural (geogenic) arsenic occurs on an enormous, world-wide scale, and causes wide-spread, serious health risks for an estimated more than hundred million people who depend on the use of shallow aquifers for drinking and irrigation water. A literature review of key studies on arsenic concentration levels yields that Holocene fluvial and deltaic flood basins are the hotspots of arsenic pollution, and that the dominant geo- morphological setting of the arsenic-polluted areas consists of shallow-depth meandering-river deposits with sand- prone fluvial point-bar deposits surrounded by clay-filled (clay plug) abandoned meander bends (oxbow lakes). Analysis of the lithofacies distribution and related permeability contrasts of the geomorphological elements in two cored wells in a point bar and adjacent clay plug along the Ganges River, in combination with data of arsenic concentrations and organic matter content reveals that the low-permeable clay-plug deposits have a high organic matter content and the adjacent permeable point-bar sands show high but spatially very variable arsenic concentra- tions. On the basis of the geomorphological juxtaposition, the analysis of fluvial depositional processes and lithofacies characteristics, inherent permeability distribution and the omnipresence of the two geomorphological el- ements in Holocene flood basins around the world, a generic model is presented for the wide-spread arsenic occur- rence. The anoxic deeper part (hypolimnion) of the oxbow lake, and the clay plugs are identified as the loci of reactive organic carbon and microbial respiration in an anoxic environment that triggers the reductive dissolution of iron oxy-hydroxides and the release of arsenic on the scale of entire fluvial floodplains and deltaic basins. The Keywords: Aquifer flushing efficiency Arsenic pollution Holocene flood basin Microbial action Permeability heterogeneity Point-bar sand Science of the Total Environment 574 (2017) 901–913 ⁎ Corresponding author. E-mail address: m.e.donselaar@tudelft.nl (M.E. Donselaar). http://dx.doi.org/10.1016/j.scitotenv.2016.09.074 0048-9697/© 2016 Published by Elsevier B.V. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv