title: Geochemical characterisation of shallow aquifer sediments of Matlab Upazila, Southeastern Bangladesh - implications for targeting low-As aquifers.
abstract: High arsenic (As) concentrations in groundwater pose a serious threat to the health of millions of people in Bangladesh. Reductive dissolution of Fe(III)-oxyhydroxides and release of its adsorbed As is considered to be the principal mechanism responsible for mobilisation of As. The distribution of As is extremely heterogeneous both laterally and vertically. Groundwater abstracted from oxidised reddish sediments, in contrast to greyish reducing sediments, contains significantly lower amount of dissolved arsenic and can be a source of safe water. In order to study the sustainability of that mitigation option, this study describes the lithofacies and genesis of the sediments within 60 m depth and establishes a relationship between aqueous and solid phase geochemistry. Oxalate extractable Fe and Mn contents are higher in the reduced unit than in the oxidised unit, where Fe and Mn are present in more crystalline mineral phases. Equilibrium modelling of saturation indices suggest that the concentrations of dissolved Fe, Mn and PO(4)(3-)-tot in groundwater is influenced by secondary mineral phases in addition to redox processes. Simulating As(III) adsorption on hydroferric oxides using the Diffuse Layer Model and analytical data gave realistic concentrations of dissolved and adsorbed As(III) for the reducing aquifer and we speculate that the presence of high PO(4)(3-)-tot in combination with reductive dissolution results in the high-As groundwater. The study confirms high mobility of As in reducing aquifers with typically dark colour of sediments found in previous studies and thus validates the approach for location of wells used by local drillers based on sediment colour. A more systematic and standardised colour description and similar studies at more locations are necessary for wider application of the approach.
authors.affiliation: Department of Land and Water Resources Engineering, Royal Institute of Technology, Teknikringen 76, SE-100 44 Stockholm, Sweden. mattias.bromssen@ramboll.se
authors.forename: Mattias
authors.initials: M
authors.lastname: von Brömssen
authors.forename: Sara
authors.initials: S
authors.lastname: Häller Larsson
authors.forename: Prosun
authors.initials: P
authors.lastname: Bhattacharya
authors.forename: M Aziz
authors.initials: MA
authors.lastname: Hasan
authors.forename: Kazi Matin
authors.initials: KM
authors.lastname: Ahmed
authors.forename: M
authors.initials: M
authors.lastname: Jakariya
authors.forename: Mohiuddin A
authors.initials: MA
authors.lastname: Sikder
authors.forename: Ondra
authors.initials: O
authors.lastname: Sracek
authors.forename: Annelie
authors.initials: A
authors.lastname: Bivén
authors.forename: Barbora
authors.initials: B
authors.lastname: Dousová
authors.forename: Claudio
authors.initials: C
authors.lastname: Patriarca
authors.forename: Roger
authors.initials: R
authors.lastname: Thunvik
authors.forename: Gunnar
authors.initials: G
authors.lastname: Jacks
mesh_terms: D001151:Arsenic / Q000032:analysis*; D001459:Bangladesh; D004784:Environmental Monitoring; D005618:Fresh Water / Q000032:analysis*; D019015:Geologic Sediments / Q000032:analysis*; D055593:Geological Phenomena; D014874:Water Pollutants, Chemical / Q000032:analysis*; D053833:Wetlands
chemical_list: D014874:Water Pollutants, Chemical; D001151:Arsenic