Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh

International audienceBackground The origin of the spatial variability of dissolved As concentrations in shallow aquifers of the Bengal Basin remains poorly understood. To address this, we compare here transects of simultaneously-collected groundwater and aquifer solids perpendicular to the banks of the Hooghly River in Chakdaha, India, and the Old Brahmaputra River in Araihazar, Bangladesh. Results Variations in surface geomorphology mapped by electromagnetic conductivity indicate that permeable sandy soils are associated with underlying aquifers that are moderately reducing to a depth of 10–30 m, as indicated by acid-leachable Fe(II)/Fe ratios 5 mg L-1. More reducing aquifers are typically capped with finer-grained soils. The patterns suggest that vertical recharge through permeable soils is associated with a flux of oxidants on the banks of the Hooghly River and, further inland, in both Chakdaha and Araihazar. Moderately reducing conditions maintained by local recharge are generally associated with low As concentrations in Araihazar, but not systematically so in Chakdaha. Unlike Araihazar, there is also little correspondence in Chakdaha between dissolved As concentrations in groundwater and the P-extractable As content of aquifer particles, averaging 191 ± 122 ug As/L, 1.1 ± 1.5 mg As kg-1 (n = 43) and 108 ± 31 ug As/L, 3.1 ± 6.5 mg As kg-1 (n = 60), respectively. We tentatively attribute these differences to a combination of younger floodplain sediments, and therefore possibly more than one mechanism of As release, as well as less reducing conditions in Chakdaha compared to Araihazar. Conclusion Systematic dating of groundwater and sediment, combined with detailed mapping of the composition of aquifer solids and groundwater, will be needed to identify the various mechanisms underlying the complex distribution of As in aquifers of the Bengal Basin

Arsenic sequestration by organic sulphur in peat

Wetlands cover more than 6% of the global ice-free land area, and have been recognized as important sinks for arsenic. Wetland soils and sediments are subject to frequent changes in redox conditions, driven by fluctuations in the water table and shifts in biological activity. Under oxic conditions, natural organic matter promotes arsenic release from metal-(hydr)oxides, thereby enhancing arsenic mobility. Under strongly reducing conditions, however, organic matter triggers the formation of arsenic-sequestering sulphides, leading to a reduction in arsenic mobility. Furthermore, the sorption of arsenic to natural organic matter is increasingly thought to suppress arsenic mobility, but the binding mechanisms have remained elusive. Here we use X-ray absorption spectroscopy to analyse the speciation of solid-phase arsenic in peat samples collected from a naturally arsenic-enriched peatland in Switzerland. We show that natural organic matter can completely sequester arsenic through the formation of covalent bonds between trivalent arsenic and organic sulphur groups, which have an average arsenic–sulphur bond distance of 2.26 Å. We suggest that by binding arsenic in this way, natural organic matter plays an active role in arsenic immobilization in sulphur-enriched, anoxic wetlands