Geochemical study of arsenic concentrations in groundwater of the Meghna River Delta, Bangladesh

The geochemistry of groundwaters from the Meghna River Delta of Bangladesh was studied to determine the distribution of arsenic, and to evaluate the potential mobility of As from the host sediments to the groundwater. Groundwaters from tube wells are characterized by pH of 6.5 to 7.5, oxidation reduction potential (ORP) of +52 to -2 mV, Ca contents of 7 to 25 mg/l, and Fe contents of 0.01 to 22.2 mg/l. Arsenic concentrations ranged from 6 to 934 mu g/l, with an average value of 347 mu g/l. A moderate positive correlation exists between As and Fe (r(2)=0.514; p<0.0001) concentrations in groundwater. Leaching tests under both acidic (I M HQ and alkaline (0.5M NaOH) conditions were carried out on core samples collected from the Meghna Plain, Brahmaputra. Plain and Madhupur Tract. The results show that As concentrations in acid leachates from silt, clays and peat were 0.021 to 0.212 mg/kg, compared to 0.009 to 0.140 mg/kg from sands. Extracts from alkali treatment contained 0.015 to 0.458 mg/ kg and 0.007 to 0.054 mg/kg, respectively. Average amount of As leached under acidic and alkaline conditions were 0.79% and 0.64% from silt, clays and peat, and 0.72% and 0.45% from sands, respectively. Greater amounts of solid phase Fe and Ca were extracted under acidic conditions, probably due to dissolution of oxide coatings on sediment particles and of carbonate phases. Significant amounts of As were extracted by both extraction media, but Fe and Ca concentrations in the alkaline extracts were low. When alkaline conditions prevail, As can be released from organic fractions in sediments even if dissolution of Fe and Ca is low. (c) 2007 Elsevier B.V. All rights reserved.