Distribution of Arsenic In Core Sediments as Well as Groundwater In Arsenic Hot Spot Area In Bangladesh

Core sediments (to a depth of 50 m) collected from two boreholes by wash boring method in arsenic hot spot area, Bangladesh were characterized for their chemical composition, geochemistry and distribution of arsenic in sediments. Groundwater was also collected from 10 wells in a piezometer nest at different depth as well as from different part of hot spot area. Dissolved oxygen (DO), temperature, pH, electrical conductivity (EC), total dissolved solids (TDS), salinity and electrical potential (Eh) were measured in a flow-through cell every 5 min during well purging in the field. Arsenic concentrations in uppermost aquifer between 10 to 40m in 37 wells including piezometer wells in hot spot area range from 10 mg/l to 2300 mg/l. As subsurface conditions changes from oxidized to successively more reduced, arsenic concentration increases with depth. Higher arsenic concentrations were found within upper 40m in the drilled core samples. Statistical analysis clearly shows that As, Fe, Mn and Ca are strongly correlated in sediments but less correlated in groundwater. From the piper diagram, the dominant water type is Ca-HCO₃^- with high concentrations of As, Fe, Mn but low level of NO₃^-. High levels of arsenic concentration in aquifers are associated with fine-grained sediments, which are deposited mainly by meandering channels during Holocene period. Fluorescent intensity values indicate that both groundwater and sediments in this arsenic hot spot area contain less organic matter compared to the other parts of Bengal basin. Based on lithology, core sediments are grouped into 4 distinct lithofacies viz. clay and silty clay, silty sand, medium to coarse sand and clay with gravel bed. Sediments in the upper most aquifer comprises of gray to dark gray silty sand which contain high arsenic and deeper aquifer mainly consists of yellow to brownish yellow medium to coarse sand which is free from arsenic.