Groundwater chemistry and occurrence of arsenic in the Meghna floodplain aquifer, southeastern Bangladesh

Dissolved major ions and important heavy metals including total arsenic and iron were measured in groundwater from shallow (25–33 m) and deep (191–318 m) tube-wells in southeastern Bangladesh. These analyses are intended to help describe geochemical processes active in the aquifers and the source and release mechanism of arsenic in sediments for the Meghna Floodplain aquifer. The elevated Cl− and higher proportions of Na+ relative to Ca2+, Mg2+, and K+ in groundwater suggest the influence by a source of Na+ and Cl−. Use of chemical fertilizers may cause higher concentrations of NH 4 + and PO 4 3− in shallow well samples. In general, most ions are positively correlated with Cl−, with Na+ showing an especially strong correlation with Cl−, indicating that these ions are derived from the same source of saline waters. The relationship between Cl−/HCO 3 − ratios and Cl− also shows mixing of fresh groundwater and seawater. Concentrations of dissolved HCO 3 − reflect the degree of water–rock interaction in groundwater systems and integrated microbial degradation of organic matter. Mn and Fe-oxyhydroxides are prominent in the clayey subsurface sediment and well known to be strong adsorbents of heavy metals including arsenic. All five shallow well samples had high arsenic concentration that exceeded WHO recommended limit for drinking water. Very low concentrations of SO 4 2− and NO 3 − and high concentrations of dissolved Fe and PO 4 3− and NH 4 + ions support the reducing condition of subsurface aquifer. Arsenic concentrations demonstrate negative co-relation with the concentrations of SO 4 2− and NO 3 − but correlate weakly with Mo, Fe concentrations and positively with those of P, PO 4 3− and NH 4 + ions.