バングラデシュ,ガンジスデルタ堆積物の元素組成 ―ヒ素はピートから溶出する?―(<特集>アジア地下水ヒ素汚染の機構解明-とくにバングラデシュ調査について-)

Major and trace elements, and TOC (total organic carbon), TN (total nitrogen) and TS (total sulfur) of the sediments collected in 1999 by 18 boring operations (about 15m depth by PRC: Percussion Reverse Circulation method), in Samta and Deuli Villages (east of Samta Village), Bangladesh were analysed to reveal mechanisms of arsenic contamination in groundwater. Relatively higher concentration of arsenic (50ppm) from the organic black muds and 262ppm from the peat of the Deuli sample were significantly high compared with those of the Samta samples (20ppm for the mud samples). Arsenic concentration shows good correlation with those of vanadium and copper, which is suggestive of close correlation of arsenic to organic matter. P_2O_5 concentration gradually increases toward the ground surface samples (over 0.2wt%) and distribution of such higher P_2O_5 concentration suggests contamination by domestic waste. Sulfur is detected in some samples from both Samta and Deuli Villages. TS/TOC ratios, however, are lower than 0.36 of an average of normal marine sediments, suggesting non-marine or brackish conditions for their sedimentary environments. TOC/TN ratios have an average value of 11.1 suggesting organic matter derivated from both higher plants and marine planktons. Higher contents of arsenic in sediments correlated well with organic matter (OM), that may indicate contamination of groundwater under reducing condition of the aquifer. Decomposition of OM seems to be stimulated by increasing agricultural practice and domestic waste, which accelerated dissolution of arsenic from the sediments, especially from the peat layer.