山地小流域におけるBHCの挙動

Abstract

森林におけるBHCの挙動を知るために, 過去にBHCが散布されたことがなく, 水文観測のデータを得ることのできる山地小流域おいて, 水を媒体とするBHCの流入および流出挙動を調査した。その結果, 降水, 樹冠通過雨および樹幹流などの流入水中には常にBHCが検出され, それらの濃度は本調査地における9年前の調査時の濃度と比較して低下していない。これに対して, 流出水中にはBHCは痕跡程度しか検出されず, 森林土壌にBHCに対する強い浄化機能があることがわかった。また, 流入水中のBHC組成は, 9年前の調査では, αよりも環境中での分解が困難と考えられるβおよびγ異性体の比率が著しく高くなっていたが, 今回の調査では, より工業原体の組成に近くなっていた。これらのことから, 今回の調査結果は, 過去の日本国内の局所的な汚染の影響にとどまらず, よりグローバルな汚染の影響を反映しているものと考えられる。とくに, dry falloutの影響を最も強く受けていると考えられる樹冠通過雨中のBHC濃度が, 降水中のそれと比べて著しく高い値を示す時期は, 黄砂の降下量の多い時期に対応することから, 中国大陸からの汚染の影響が示唆される。

Behavior of BHC in a small wooded watershed was investigated over a year (from Oct.31, 1985 to Oct. 30, 1986) in terms of BHC concentrations in input water to the watershed (precipitation, throughfall, stemflow) and output water from the watershed (run-off stream water). The watershed is located in Kiryu, southeast part of Shiga prefecture, where various hydrological measurements have been done. As the results, comparatively large amount of BHC was always detected in the input waters, while only trace amount was found in the output water. This indicates that the soil of the forest plays a very efficient purification role of water from BHC contamination. The levels of BHC concentrations in the input water have not decreased since the time of the previous study1) in 1976. With respect to BHC composition, β and γ isomers, which may be less degradative than α isomer in the environment, were much more abundant in the previous study than those in the technical BHC. However, BHC compositions in the present study were comparable to that of the technical one. Thus, the present study shows that recent BHC contamination must be affected by the global contamination rather than the high level of contamination occured in Japan. In particular, the contamination from mainland China is suggested, since BHC concentration in throughfall, which thought to be most influenced by dry fallout among the input waters, was much higher than that in precipitation in the season when large amount of "Kosa"(a kind of dry fallout originally composed of yellow colored fine sand in the north part of China) is conveyed into Japan.