Dataset Information

Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge.

ABSTRACT: Riverbank sediment cores and pore waters, shallow well waters, seepage waters and river waters were collected along the Meghna Riverbank in Gazaria Upazila, Bangladesh in Jan. 2006 and Oct.-Nov. 2007 to investigate hydrogeochemical processes controlling the fate of groundwater As during discharge. Redox transition zones from suboxic (0-2 m depth) to reducing (2-5 m depth) then suboxic conditions (5-7 m depth) exist at sites with sandy surficial deposits, as evidenced by depth profiles of pore water (n=7) and sediment (n=11; diffuse reflectance, Fe(III)/Fe ratios and Fe(III) concentrations). The sediment As enrichment zone (up to ~700 mg kg-1) is associated with the suboxic zones mostly between 0-2 m depth and less frequently between 5-7 m depth. The As enriched zones consist of several 5 to 10 cm-thick dispersed layers and span a length of ~5-15 m horizontally from the river shore. Depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved Fe (11.6±11.7 mg L-1) and As (118±91 ?g L-1, mostly as arsenite) between 2-5 m depth, but lower concentrations between 0-2 m depth (0.13±0.19 mg L-1 Fe, 1±1 ?g L-1 As) and between 5-6 m depth (1.14±0.45 mg L-1 Fe, 28±17 ?g L-1 As). Because it would take more than a few hundred years of steady groundwater discharge (~10 m yr-1) to accumulate hundreds of mg kg-1 of As in the riverbank sediment, it is concluded that groundwater As must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s. Not only does this lend unequivocal support to the argument that As occurrence in the Ganges-Brahmaputra-Meghna Delta groundwater is of geogenic origin, it also calls attention to the fate of this As enriched sediment as it may recycle As into the aquifer.

SUBMITTER: Jung HB

PROVIDER: S-EPMC4740924 | biostudies-literature | 2015 Dec

REPOSITORIES: biostudies-literature

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Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge.

Jung Hun Bok HB Zheng Yan Y Rahman Mohammad W MW Rahman Mohammad M MM Ahmed Kazi M KM

Applied geochemistry : journal of the International Association of Geochemistry and Cosmochemistry 20151201

Riverbank sediment cores and pore waters, shallow well waters, seepage waters and river waters were collected along the Meghna Riverbank in Gazaria Upazila, Bangladesh in Jan. 2006 and Oct.-Nov. 2007 to investigate hydrogeochemical processes controlling the fate of groundwater As during discharge. Redox transition zones from suboxic (0-2 m depth) to reducing (2-5 m depth) then suboxic conditions (5-7 m depth) exist at sites with sandy surficial deposits, as evidenced by depth profiles of pore wa ...[more]

PMID: 26855475

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Project description:BackgroundMicrobe-mediated redox transformation of arsenic (As) leading to its mobilization has become a serious environmental concern in various subsurface ecosystems especially within the alluvial aquifers. However, detailed taxonomic and eco-physiological attributes of indigenous bacteria from As impacted aquifer of Brahmaputra river basin has remained under-studied.ResultsA newly isolated As-resistant and -transforming facultative anaerobic bacterium IIIJ3-1 from As-contaminated groundwater of Jorhat, Assam was characterized. Near complete 16S rRNA gene sequence affiliated the strain IIIJ3-1 to the genus Bacillus and phylogenetically placed within members of B. cereus sensu lato group with B. cereus ATCC 14579(T) as its closest relative with a low DNA-DNA relatedness (49.9%). Presence of iC17:0, iC15:0 fatty acids and menaquinone 7 corroborated its affiliation with B. cereus group, but differential hydroxy-fatty acids, C18:2 and menaquinones 5 & 6 marked its distinctiveness. High As resistance [Maximum Tolerable Concentration = 10 mM As3+, 350 mM As5+], aerobic As3+ (5 mM) oxidation, and near complete dissimilatory reduction of As 5+ (1 mM) within 15 h of growth designated its physiological novelty. Besides O2, cells were found to reduce As5+, Fe3+, SO42-, NO3-, and Se6+ as alternate terminal electron acceptors (TEAs), sustaining its anaerobic growth. Lactate was the preferred carbon source for anaerobic growth of the bacterium with As5+ as TEA. Genes encoding As5+ respiratory reductase (arr A), As3+ oxidase (aioB), and As3+ efflux systems (ars B, acr3) were detected. All these As homeostasis genes showed their close phylogenetic lineages to Bacillus spp. Reduction in cell size following As exposure exhibited the strain's morphological response to toxic As, while the formation of As-rich electron opaque dots as evident from SEM-EDX possibly indicated a sequestration based As resistance strategy of strain IIIJ3-1.ConclusionThis is the first report on molecular, taxonomic, and ecophysiological characterization of a highly As resistant, As3+ oxidizing, and dissimilatory As5+ reducing Bacillus sp. IIIJ3-1 from As contaminated sites of Brahmaputra river basin. The strain's ability to resist and transform As along with its capability to sequester As within the cells demonstrate its potential in designing bioremediation strategies for As contaminated groundwater and other ecosystems.

Dataset Information

Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge.

Publications

Redox Zonation and Oscillation in the Hyporheic Zone of the Ganges-Brahmaputra-Meghna Delta: Implications for the Fate of Groundwater Arsenic during Discharge.

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