Arsenic enrichment in groundwater of the alluvial aquifers in Bangladesh: an overview

Arsenic in the groundwater of Bangladesh is a serious natural calamity and a public health hazard. Most groundwater from the shallow alluvial aquifers (<150 m), particularly in the Holocene plain lands, are vulnerable to As-enrichment. Delta plains and flood plains of the Ganges–Brahmaputra river system are moderately to severely enriched and more than 60% of the tube wells are affected. Shallow aquifers in the Meghna river basin and coastal plains are extremely enriched with more than 80% of the tube wells affected. Aquifers in the Pleistocene uplands and Tertiary hills are low in As. The vertical lithofacies sequence of the sediments from highly enriched areas of the country show two distinct lithofacies associations—a dominantly sandy channel-fill association and a fine-grained over bank association. The sediments can be grouped into 4 distinct lithofacies, viz. clay, silty clay, silty sand and sand. Thin section petrography of the As-enriched aquifer sands shows that the sands are of quartzolithic type and derived from the collision suture and fold thrust belt of the recycled orogen provenance. Groundwater is characterized by circum-neutral pH with a moderate to strong reducing nature. The waters are generally of Ca–Mg–HCO3 or Ca–Na–HCO3 type, with HCO3− as the principal anion. Low SO42− and NO3−, and high dissolved organic C (DOC) and NH4+ concentrations are typical chemical characteristics of groundwater. The presence of dissolved sulfides in these groundwaters indicates reduction of SO4. Total As concentration in the analyzed wells vary between 2.5 and 846 μg l−1 with a dominance of As(III) species (67–99%). Arsenic(III) concentrations were fairly consistent with the DOC and NH4+ contents. The HNO3 extractable concentrations of As image in the sediments (0.5–17.7 mg kg−1), indicate a significant positive correlation with FeNO3, MnNO3, AlNO3 and PNO3. The concentrations of SNO3 (816–1306 mg kg−1) peaked in the clay sediments with high organic matter (up to 4.5 wt.%). Amounts of oxalate extractable As (Asox) and Fe (Feox) ranged between 0.1–8.6 mg kg−1and 0.4–5.9 g kg−1, respectively. Arsenicox was positively correlated with Feox, Mnox and Alox in these sediments. Insignificant amounts of opaque minerals (including pyrite/arsenopyrite) and the presence of high As contents in finer sediments suggests that some As is incorporated in the authigenically precipitated sulfides in the reducing sediments. Moreover, the chemical extractions suggest the presence of siderite and vivianite as solid phases, which may control the aqueous chemistry of Fe and PO43−. Reductive dissolution of Fe oxyhydroxide present as coatings on sand grains as well as altered mica (biotite) is envisaged as the main mechanism for the release of As into groundwater in the sandy aquifer sediments.