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Iron (Fe) and dissolved organic matter (DOM) cycling have been implicated in arsenic mobilization via microbially mediated Fe oxide reduction. To evaluate the sources and multiple roles of DOM in Bangladesh aquifers, we conducted spectroscopic analyses on various types of surface water and groundwater samples from a site representative of aquifer chemistry and hydrology. Surface water contained humic substances with oxidized quinone-like moieties and high concentrations of labile microbially derived DOM. In contrast, in shallow groundwater where dissolved iron and arsenic concentrations were high, the quinone-like moieties of humic substances were more reduced, with less abundant labile DOM than that of surface water. Instead, DOM at these depths was characterized by terrestrial (plant/soil) signatures. A sediment microcosm experiment demonstrated that Fe(II) and terrestrially derived DOM were released from sediment over time. The results provide new evidence to support a dual role of natural DOM in Bangladesh aquifers (1) as a labile substrate for Fe- and humic-reducing bacteria and (2) as an electron shuttle via humic substances to enhance microbial iron reduction. Fluorescence index, amino acid-like fluorescence, and redox index may serve as useful indicators of the type of DOM likely to be involved in Fe solubilization and potentially As mobilization reactions.

A comparative study of arsenic enrichment in the Bengal Delta (BD) was carried out in three alluvial aquifers in south-central Bangladesh. Investigated sites included Sonargaon in Narayanganj, Chandina in Comilla and Sirajdikhan in Munshiganj districts. At all sites samples from different depths were collected, and water chemistry and redox status vs. depth trends were determined. The concentrations of DOC and HCO3 − were highest at Sirajdikhan site, while at the Sonargaon and Chandina sites the concentrations were lower. On the contrary, the NH4 + concentration was high at the Chandina site as compared to the other sites. There was a good match between dissolved As and Fe at the Sirajdikhan and Sonargaon sites, but not at the Chandina site. The dissolved aqueous concentration of Mn was low at the Chandina site, which suggested that the Mn(IV) redox buffering step was missing. Speciation modeling indicated a possibility of siderite precipitation at all sites, but precipitation of rhodochrosite only at the Sonargaon and Sirajdikhan sites. At the Sirajdikhan site, the log $$ {\text{P}}_{{{\text{CO}}_{ 2} }} $$ values were very high (−1.37), which revealed the production of CO2 in redox processes. Principal component analysis (PCA) indicated an impact of sea water and redox status of different samples. These results suggest that the dissolved As is de-coupled from dissolved Mn because when released, As is re-adsorbed onto the Fe(III) minerals in solid phase, as well as from dissolved Fe when precipitation of Fe(II) minerals controls the aqueous concentrations of Fe. In addition, several other concurrent redox processes may exert kinetic constraints depending on refractory characteristics of Fe(III) minerals.

Occurrences of arsenic (As) in the Bengal Basin of Bangladesh show close relationships with depositional environments and sediment textures. Hydrochemical data from three sites with varying physiog ...

Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most groundwater from the shallow alluvial aquifers (

Bacterial communities can exert significant influence on the biogeochemical cycling of arsenic (As). This has globally important implications since As in drinking water affects the health of over 100 million people worldwide, including in the Ganges–Brahmaputra Delta region of Bangladesh where geogenic arsenic in groundwater can reach concentrations of more than 10 times the World Health Organization’s limit. Thus, the goal of this research was to investigate patterns in bacterial community composition across gradients in sediment texture and chemistry in an aquifer with elevated groundwater As concentrations in Araihazar, Bangladesh. We characterized the bacterial community by pyrosequencing 16S rRNA genes from aquifer sediment samples collected at three locations along a groundwater flow path at a range of depths between 1.5 and 15 m. We identified significant differences in bacterial community composition between locations in the aquifer. In addition, we found that bacterial community structure was significantly related to sediment grain size, and sediment carbon (C), manganese (Mn), and iron (Fe) concentrations. Deltaproteobacteria and Chloroflexi were found in higher proportions in silty sediments with higher concentrations of C, Fe, and Mn. By contrast, Alphaproteobacteria and Betaproteobacteria were in higher proportions in sandy sediments with lower concentrations of C and metals. Based on the phylogenetic affiliations of these taxa, these results may indicate a shift to more Fe-, Mn-, and humic substance-reducers in the high C and metal sediments. It is well-documented that C, Mn, and Fe may influence the mobility of groundwater arsenic, and it is intriguing that these constituents may also structure the bacterial community.

International audience; We report osmium concentrations and isotopic compositions of 40 groundwater samples from the Bengal plain. Groundwaters have Os concentrations (16.9-191.5 pg/kg), about 5-10 times higher than those published for most rivers or seawater. 187Os/188Os varies widely (from 0.96 to 2.79) and is related to the isotopic signatures of the sediments constituting local aquifers. Os contents are correlated with those of soluble elements such as Sr, Mg, and Ca, suggesting that differing extents of solid-solution interaction explain most of the variation in measured Os concentrations. The covariation between Os and Sr allows us to estimate the mean Os content of Bengal groundwater (70 pg/kg). This concentration is too low to allow Bengal groundwater to significantly influence the marine Os isotopic composition, if likely fresh groundwater discharge rates to the Bay of Bengal are assumed. However, if Bengal groundwater Os concentrations are typical, the global Os groundwater flux would be expected to be around 180 kg/year, making it the second largest input of Os to the ocean after the river flux. Including this flux in the current Os marine budget, and assuming that this and other fluxes have remained constant with time, would decrease the calculated residence time of Os in the ocean by about 30%.