Arsenic enrichment and mobilization in the Holocene alluvial aquifers of the Chapai-Nawabganj district, Bangladesh: A geochemical and statistical study

Core sediments from three boreholes (to a depth of 50 m) and groundwater from 20 As-enriched water wells were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analyses. 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. Sodium, Cl, SO4, total dissolved solids (TDS), and electrical conductivity (EC) are grouped under the first factor representing the salinity sources of waters. The second factor, represented by As and Mn, is related to As mobilization processes. The third factor of Fe and alkalinity is strongly influenced by bacterial Fe(III) reduction which would raise both Fe and HCO3- concentrations in water. The fourth factor of Ca and Mg reflects the hardness of the Ca–HCO3 type of groundwater, which is confirmed by the hydrochemical facies analysis. Cluster analysis leads to the formulation of four water types including highly, moderately, and slightly As-enriched groundwater as well as groundwater with elevated SO42-, from anthropogenic sources. Multivariate analyses of the geochemical parameters suggest that Fe- and Mn-oxyhydroxides and mineral phases of phyllosilicates (e.g., biotite) are the main hosts of As in the sediments. Statistical analysis also shows that As is closely associated with Fe and Mn in sediments while As is positively 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 As into the groundwater.

Research highlights► Four factors (i.e., salinity factor, As-enrichment factor, reduction factor, and hardness factor) control groundwater geochemistry. ► Hydrochemical facies analysis showed that most of the water samples are dominated by Ca and HCO3 ions. ► Cluster 1 water types are highly enriched in As; an alternative water source is thus needed for domestic water supply. Possible solutions are to install tube wells in the deeper Pleistocene aquifers or use clean surface water sources such as reservoirs or rain water. ► Cluster 4 water types contain low concentration As, below the Bangladesh standard (<50 μg/L), and this type of water is generally suitable for extraction for domestic uses. ► Statistical analyses show that As, Fe and Mn are strongly correlated in sediments. By comparison, As is not correlated with Fe but is positively correlated with Mn in groundwater. ► Geochemical correlations along with results of sequential leaching experiments suggest that bacterial reductive dissolution of MnOOH and FeOOH is the primary mechanism for releasing As into the groundwater. ► Geochemical studies using sequential-extraction results and scanning electron microscopy on sediment samples demonstrate that Fe- and Mn-oxyhydroxides and Fe- and Mn-oxyhydroxide-coated quartz and feldspar, are the dominant carriers of As in the sediments.