Hydrochemistry and Arsenic Contamination of Shallow Aquifers in Bidyananda and Nazimkhan Unions, Rajarhat Upazilla, Kurigram, Bangladesh

Abstract

In this study, the spatial distribution and hydrochemistry of arsenic (As) in shallow alluvial aquifers of Bidyananda and Nazimkhan Union of Rajarhat opazilla, Kurigram district of Bangladesh have been evaluated. Nineteen groundwater samples (depth: 10 to 50 m) were collected from the studied area for physical and chemical analysis. Based on analytical data of chemical analysis, a Piper trilinear diagram have been prepared which revealed that the groundwater of the studied area is CaHCO3 type, where Ca is the dominant metal. Physical data and permeability index values indicate good quality of the groundwater for drinking and irrigation. The concentrations of Fe, Mn and As in almost all samples exceed the WHO and Bangladesh national standard indicating significant pollutions. Groundwater As concentrations in the aquifer (10–50 m of depth) ranged from <0.005 to 0.323 mgL−1 and geochemical conditions changed from oxidized to successively more reduced, higher As concentrations with depth. Correlation studies revealed a good correlation between As and Mn (R2=0.20) in groundwater implying that natural reductive dissolution of MnO(OH) and FeO(OH) may be responsible for As release into the groundwater. However, a poor correlation between As and Fe (R2 = 0.0084) in groundwater is probably due to the precipitation of dissolved Fe and Mn as siderite (FeCO3) and rhodochrosite (MnCO3), respectively, under reducing conditions. The poor correlations among dissolved As, Fe and SO 4 2− concentrations (As vs. SO 4 2− R2 = 0.0674; Fe vs. SO 4 2− , R2=0.1542) in groundwater suggest that pyrite/sulfide oxidation is not the dominant process for As release. A high concentration of As, Fe and Mn indicate that the natural reductive dissolution of iron oxyhydroxides is probably the dominant process for As mobilization in the groundwater of the studied area.