Distribution of arsenic in core sediments and groundwater in the Chapai Nawabganj district, Bangladesh

Arsenic (As) concentration and geochemical analysis of groundwater as well as core sediments of 20 tube wells revealed the uppermost aquifer (20-40m of depth) groundwater As concentrations ranging from 2.34 to 586.96gL(,)(-1) and groundwater geochemical conditions change from oxidized to gradually more reduced as higher concentrations of As increased with depth. The highest sediment As levels (25mgkg(-1)) were found within the shallow depth (15m). The groundwater of the study area is of Ca-HCO3 type, where Ca is the dominant metal with high concentrations of As, Fe, and Mn but low levels of NO3- and SO42-. The trace elements Fe, Mn, and As exceed the WHO and DoE standard in groundwater which clearly indicated As contamination except some few areas where less Fe and Mn are released in aquifer water from sediment. The contaminated As zone also corroborated with the GIS study. Considerable variability with depth and location is noted in the levels of As, Fe, and Mn in groundwater, and from these levels it can be seen that the groundwater of Rajarampur village of Chapai Nawabganj Paurasava is more reduced than the groundwater of Baragharia. Groundwater in Baragharia village contains low concentration of As below the WHO standard (<10gL(-1)), and this type of water is generally suitable for extraction for domestic uses. Some places have been detected as having the most favorable conditions for reducing the environment where higher concentrations of As are observed. Fine-grained sediments are also associated with high levels of As concentration in aquifers. Statistical analysis clearly shows that As is closely related with Fe (R-2=0.557) and Mn (R-2=0.627) in sediments. However, As is not well correlated with Mn (R-2=0.2175) in groundwater. These correlations indicate that the reductive destruction of Mn- and Fe-rich compounds (e.g., MnOOH, FeOOH) is controlled by anaerobic bacteria states for releasing As into the aquifer. Notwithstanding, in spite of the poor correlation between As and Fe, As and Mn bare that Fe and Mn are assumed to be precipitated onto the sediments as siderite and rhodochrosite.