A. H.M.Selim Reza

Occurrence of arsenic in core sediments and groundwater in the Chapai-Nawabganj District, northwestern Bangladesh.

Groundwater and core sediments of two boreholes (to a depth of 50m) from the Chapai-Nawabganj area in northwestern Bangladesh were collected for arsenic concentration and geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10-40m of depth) range from 2.8microgL(-1) to 462.3microgL(-1). Groundwater geochemical conditions change from oxidized to successively more reduced, higher As concentration with depth. Higher sediment arsenic levels (55mgkg(-1)) were found within the upper 40m of the drilled core samples. X-ray absorption near-edge structure spectroscopy was employed to elucidate the arsenic speciation of sediments collected from two boreholes. Environmental scanning electron microscopy and transmission X-ray microscopy were used to investigate the characteristics of FeOOH in sediments which adsorb arsenic. In addition, a pH-Eh diagram was drawn using the Geochemists Workbench (GWB) software to elucidate the arsenic speciation in groundwater. The dominant groundwater type is Ca-HCO(3) with high concentrations of As, Fe and Mn but low levels of NO(3)(-) and SO(4)(2-). Sequential extraction analysis reveals that Mn and Fe hydroxides and organic matter are the major leachable solids carrying As. High levels of arsenic concentration in aquifers are associated with fine-grained sediments. Fluorescent intensities of humic substances indicate that both groundwater and sediments in this arsenic hotspot area contain less organic matter compared to other parts of Bengal basin. Statistical analysis clearly shows that As is closely associated with Fe and Mn in sediments while As is better correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing arsenic into the groundwater.

Implications of organic matter on arsenic mobilization into groundwater: Evidence from northwestern (Chapai-Nawabganj), central (Manikganj) and southeastern (Chandpur) Bangladesh

Boreholes (50 m depth) and piezometers (50 m depth) were drilled and installed for collecting As-rich sediments and groundwater in the Ganges, Brahmaputra, and Meghna flood plains for geochemical analyses. Forty-one groundwater samples were collected from the three areas for the analyses of cations (Ca(2+), Mg(2+), K(+), Na(+)), anions (Cl(-), NO(3)(-), SO(4)(2-)), total organic carbon (TOC), and trace elements (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb). X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) were performed to characterize the major mineral and chemical contents of aquifer sediments. In all three study areas, results of XRF analysis clearly show that fine-grained sediments contain higher amounts of trace element because of their high surface area for adsorption. Relative fluorescent intensity of humic substances in groundwater samples ranges from 30 to 102 (mean 58 ± 20, n = 20), 54-195 (mean 105 ± 48, n = 10), and 27-243 (mean 79 ± 71, n = 11) in the Ganges, Brahmaputra and Meghna flood plains, respectively. Arsenic concentration in groundwater (20-50 m of depth) ranges from 3 to 315 μg/L (mean 62.4 ± 93.1 μg/L, n = 20), 16.4-73.7 μg/L (mean 28.5 ± 22.4 μg/L, n = 10) and 4.6-215.4 μg/L (mean 30.7 ± 62.1 μg/L, n = 11) in the Ganges, Brahmaputra and Meghna flood plains, respectively. Specific ultra violet adsorption (SUVA) values (less than 3 m(-1) mg(-1) L) indicate that the groundwater in the Ganges flood plain has relatively low percentage of aromatic organic carbon compared to those in the Brahmaputra and Meghna flood plains. Arsenic content in sediments ranges from 1 to 11 mg/kg (mean 3.5 ± 2.7 mg/kg, n = 17) in the three flood plains. Total organic carbon content is 0.5-3.7 g/kg (mean 1.9 ± 1.1 g/kg) in the Ganges flood plain, 0.5-2.1 g/kg (mean: 1.1 ± 0.7 g/kg) in the Brahmaputra flood plain and 0.3-4.4 g/kg (mean 1.9 ± 1.9 g/kg) in the Meghna flood plain. Arsenic is positively correlated with TOC (R(2) = 0.50, 0.87, and 0.85) in sediments from the three areas. Fourier transform infrared (FT-IR) analysis of the sediments revealed that the functional groups of humic substances in three areas include amines, phenol, alkanes, and aromatic carbon. Arsenic and Fe speciation in sediments were determined using XANES and the results imply that As(V) and Fe(III) are the dominant species in most sediments. The results also imply that As (V) and Fe (III) in most of the sediment samples of the three areas are the dominant species. X-ray absorption fine structure (EXAFS) analysis shows that FeOOH is the main carrier of As in the sediments of three areas. In sediments, As is well correlated with Fe and Mn. However, there is no such correlation observed between As and Fe as well as As and Mn in groundwater, implying that mobilizations of Fe, Mn, and As are decoupled or their concentrations in groundwater have been affected by other geochemical processes following reductive dissolution of Fe or Mn-hydroxides. For example, dissolved Fe and Mn levels may be affected by precipitation of Fe- and Mn-carbonate minerals such as siderite, while liberated As remains in groundwater. The groundwaters of the Brahmaputra and Meghna flood plains contain higher humic substances in relative fluorescence intensity (or fluorescence index) and lower redox potential compared to the groundwater of Ganges flood plain. This leads to the release of arsenic and iron to groundwater of these three plains in considerable amounts, but their concentrations are distributed in spatial variations.

Factor and cluster analysis of water quality data of the groundwater wells of Kushtia, Bangladesh: Implication for arsenic enrichment and mobilization

The study area is located in the southwestern part of Bangladesh. Twenty-six groundwater samples were collected from both shallow and deep tube wells ranging in depth from 20 to 60 m. Multivariate statistical analyses including factor analysis, cluster analysis and multidimensional scaling were applied to the hydrogeochemical data. The results show that a few factors adequately represent the traits that define water chemistry. The first factor of Fe and HCO3 is strongly influenced by bacterial Fe (III) reduction which would raise both Fe and HCO3 concentrations in water. Na, Cl, Ca, Mg and PO4 are grouped under the second factor representing the salinity sources of waters. The third factor, represented by As, Mn, SO4 and K is related to As mobilization processes. Cluster analysis has been applied for the interpretation of the groundwater quality data. Initially Piper methods have been employed to obtain a first idea on the water types in the study area. Hierarchical cluster analysis was carried out for further classification of water types in the study area. Twelve components, namely, pH, Fe, Mn, As, Ca, Mg, Na, K, HCO3, Cl, SO4 and NO3 have been used for this purpose. With hierarchical clustering analysis the water samples have been classified into 3 clusters. They are very high, high and moderately As-enriched groundwater as well as groundwater with elevated SO4.

Trace metals pollution in seawater and groundwater in the ship breaking area of Sitakund Upazilla, Chittagong, Bangladesh

This study reveals potential accumulation of trace metals in the sea and groundwater due to ship breaking activities which take place along the Bay of Bengal in Sitakund Upazilla, Chittagong, Bangladesh. When compared with WHO and Bangladesh domestic standards for water quality, it is revealed that seawater was strongly polluted by Fe and Hg, moderately by Mn and Al, and slightly by Pb and Cd. Groundwater was strongly polluted by Fe, Pb and Hg, moderately by Mn and Al, and slightly by As. Trace element concentrations of all seawater samples exceeded the average concentration of elements in the Earths seawater. The application of Principal Components Analysis identified two sources of pollution-marine and ship breaking. The mechanism of groundwater pollution inferred that if seawater is polluted, nearby groundwater is also polluted with trace metals due to the influence of seawater intrusion.

Vertical distribution and mobilization of arsenic in shallow alluvial aquifers of Chapai-Nawabganj district, Northwestern Bangladesh

Core sediments from two boreholes and groundwater from fifty four As-contaminated well waters were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10 to 40 m of depth) range from 2.76–315.15 mg/l (average 48.81 mg/l). Arsenic concentration in sediments ranges from 3.26–10 mg/kg. Vertical distribution of arsenic in both groundwater and sediments shows that maximum As concentration (462 mg/l in groundwater and 10 mg/kg in sediments) occurs at a depth of 24 m. In January 2008, 2009 and 2010, maximum As concentration occurs at the same depth. Environmental scanning electron microscope (ESEM) with EDAX was used to investigate the presence of major and trace elements in the sediments. The dominant groundwater type is Ca-HCO3 with high concentrations of As and Fe, but with low levels of NO3− and SO3−2. Statistical analysis clearly shows that As is closely associated with Fe (R2 = 0.64) and Mn (R2 = 0.91) in sediments while As is not correlated with Fe and Mn in groundwater samples. Comparatively low Fe and Mn concentrations in some groundwater, suggest that probably siderite and/or rhodochrosite precipitated as secondary mineral on the surface of the sediment particles. The correlations along with results of sequential leaching experiments suggest that reductive dissolution of FeOOH and MnOOH mediated by anaerobic bacteria represents mechanism for releasing arsenic into the groundwater.