琵琶湖北部の森林流域から流出する硫酸イオンの動態と起源解析

摘要

We aimed to determine the sources of sulfur ions in precipitation in forest watersheds and runoff from the watersheds by collecting and analyzing bulk deposition and streamwater samples from the small Kutsuki and Surumi forest watersheds, which are on the northern side of the Lake Biwa basin in central Japan. The major ions and the δ~(34)S values in the samples were analyzed. Samples were continuously collected from 1990 to 2010, and the average ± standard deviation (minimum, maximum) SO_4~(2-) concentrations were 1.60 ± 0.31 (0.76, 3.58) mg L~(-1) in the streamwater samples from Kutsuki and 6.58 ±1.54 (3.68, 16.1)mg L~(-1) in the stream water samples from Surumi. The concentrations in the stream water samples from the different watersheds were significantly different (t-test, p ＜ 0.01). However, the average SO_4~(2-) concentrations in the bulk deposition samples from the two watersheds were similar, being 2.16 ± 1.30 (0.20, 10.2) mg L~(-1) in Kutsuki and 2.27 ± 1.27 (0.35, 6.07) mg L~(-1) in Surumi (t-test, p ＞ 0.05). The average non-sea-salt (nss-) δ~(34)S values in the bulk deposition samples were +5.28 ‰ in Kutsuki and +5.89 ‰ in Surumi. The average nss-δ~(34)S values in the streamwater samples from Kutsuki and Surumi were significantly different (t-test, p ＜ 0.01), at +2.14 and -3.97 ‰, respectively. It has been found that the δ~(34)S values in coal products from northern China vary between -3 and -1 ‰ and that the δ~(34)S values in Japanese oil vary between +5 and +18 ‰. Our findings suggest that the significant differences between the nss-δ~(34)S values in the streamwater samples from the different forests were caused by differences in the abilities of the forest canopies to catch dry SO_4~(2-)deposition. The Kutsuki watershed forest consists of young conifers and deciduous broadleaf trees and has a relatively open canopy, whereas the Surumi watershed forest consists of mature conifer trees packed tightly together and has a less open canopy. There are no domestic sources of air pollution to the north of the Kutsuki watershed, and it appears that the Kutsuki watershed was only affected by SO_4~(2-) deposition from the Asian continent. In contrast, the dense canopy of the Surumi watershed was affected by local SO_4~(2-) pollution from national roads (which are 1 km northwest of Kutsuki) rather than continental sources. Overall, our results suggest that the differences in the SO_4~(2-) concentrations in the streamwater from the two watersheds are caused by the differences in the forest canopies in the two watersheds.%近年,越境大気汚染が大きな社会問題となっているが,森林生態系における物質循環,渓流水質形成過程を解明するには,その発生源の推定が重要な課題の一つである。ここでは,硫黄酸化物について中国大陸に近い琵琶湖北部の2森林流域を選定し流域に沈着し流出する硫酸イオン（SO_4~(2-))の起源を検討するために降水と渓流水のSO_4~(2-)と硫黄安定同位体比（δ~(34)S)を測定した。1990年以降の降水中のSO_4~(2-),nss-δ~(34)Sは,朽木（2.16mg L~(-1);+5.28‰）,摺墨（2.27 mg L~(-1);+5.89‰）共に差はなかったが,渓流水中のSO_4~(2-),nss-δ~(34)Sは朽木（1.60 mg L~(-1);+2.14‰）と摺墨（6.58 mg L~(-1);-3.97‰）では大きく異なった。これは,森林流域では大気降下物中の成分が溪流水中に至る間には樹冠と土壌があるが,そこに捕捉される乾性降下物の多寡により渓流水質に差が生じた結果と考えられた。樹冠の乏しい朽木渓流群では中国大陸からの汚染物質,針葉樹の密な樹冠をもつ摺墨溪流群では中国大陸からの汚染物質の影響よりも近傍を通過する道路からの影響を強く受けた渓流水中nss-δ~(34)Sの値となったと考えられた。