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-)

抄録
1999年にバングラデシュのシャムタ村と東となりのデウリ村で18本のボーリングによって採集した堆積物試料(パーカッションリバースサーキュレーション工法による,一本の深さは約15m)をもちいて,地下水ヒ素汚染のメカニズムを解明するために,主元素,微量元素および全有機炭素,窒素,イオウの定量を行った.デウリ村の有機質泥ではAsが50ppmを超えるものがあり,ピート層では262ppmに達する.これらはシャムタ村の一般的な泥質試料(As=20ppm)に比べ高い値を示す.As濃度はVやCuの濃度と良い相関をもち,有機物の濃縮に関係する.P_2O_5も地表の試料で高いものがあり,人間活動に関連するといえる.試料のいくつかからはイオウが検出され,海成や汽水環境で堆積したことを示唆する.しかし,TS/TOC比は一般の海成層の値(0.36)よりも低く,淡水もしくは汽水環境で堆積した可能性がある.TOC/TN=9.2〜14.2(平均値11.1)であり,プランクトンおよび高等植物の両者に起源すると判断される.有機物に関連する高濃度のヒ素は帯水層での還元的環境下で地下水汚染を引き起こすと考えられる.農業や家庭排水の増加に伴って有機物の分解は促進され,堆積物,特にピート層からのヒ素溶出の速度を増していると考えられる.

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.