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

Reply to the comment on “Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part I: Comparative hydrochemical and hydrogeological characteristics” by Subhrangsu K. Acharyya

International audienc

Comparison of sediment Fe(II)/Fe ratios (a-b) and dissolved Fe (c-d) concentrations as a function of sulphate for needle-sampler samples collected in India (left) and Bangladesh (right)

<p><b>Copyright information:</b></p><p>Taken from "Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh"</p><p>http://www.geochemicaltransactions.com/content/9/1/1</p><p>Geochemical Transactions 2008;9():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2246114.</p><p></p> The symbols are color-coded in three ranges according to groundwater As concentrations. Dashed lines show the sulfate concentration in the Hooghly and Old Brahmaputra rivers

Histogram of EM conductivities around (a) Chakdaha, India and (b) Balia Para, Bangladesh

<p><b>Copyright information:</b></p><p>Taken from "Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh"</p><p>http://www.geochemicaltransactions.com/content/9/1/1</p><p>Geochemical Transactions 2008;9():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2246114.</p><p></p> The location of the measurements is shown by black symbols on a contour map of EM conductivity, color-coded in the same ranges as in the histogram (c-d). Also shown are the locations of needle-sampler profiles using the same symbols of Fig. 2. (e-f) Lithological cross-sections for the two sites showing the distribution of clay/silt (black) and sand (white). (g-h) Contoured section of major cation concentrations for groundwater collected with the needle-sampler. Sections drawn with Ocean Data View [47]

Comparison of P-extractable As concentrations with the acid-leachable As content of the sediment collected with from (a) Chakdaha, India, and (b) Balia, Para, Bangladesh

<p><b>Copyright information:</b></p><p>Taken from "Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh"</p><p>http://www.geochemicaltransactions.com/content/9/1/1</p><p>Geochemical Transactions 2008;9():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2246114.</p><p></p> The two dashed lines in each panel correspond to a one-to-one correspondence and 10-fold higher HCl-extractable As concentrations, respectively. Also shown are dissolved As concentrations as a function of P-extractable As (c-d). Symbols identifying the profiles are the same as in Figure 2