Establishment of the evaluation method of trade-off among various environmental aspects in the improving countermeasures of water environment

采用条件价值法对苏和湖的水环境改善效益进行了评估，预计每年可达到7.3亿日元。另一方面，产生的二氧化碳等二次环境负荷的损失为2300万日元。这个比较证明了这个湖的水质改善是合理的。选择实验表明，人们对野生动物保护的重视程度较低，对水环境改善的重视程度高于对降低死亡风险的重视程度。人类活动的分析表明，苏瓦湖的氮负荷在过去30年中减少了一半。频繁的水质调查表明，像农田这样的非点源产生了高负荷的降雨。这说明防治水土流失是减少湖泊养分负荷的有效方法。分析了道路多环芳烃浓度与交通流量的相关性。对广岛湾生态系统和饮用水中化学物质的风险进行了评价。分析了多环芳烃和丁基锡的盆地装载量与湖泊沉积量之间的关系。这两个人比较一致。苏瓦湖近年来藻华爆发减少，蚊虫存储量明显下降。此外，事实证明，捕鱼量的减少正在加强，在20世纪70年代达到了污染最严重的峰值。虽然苏瓦湖污水渠的建设使水质有所改善，但非点源负荷的贡献比例有所增加。

Benefit of water environmental improvement of Lake Suwa was evaluated to be 730 million Yen per year by Contingent Valuation Method. On the other hand, the damage of secondary environmental loads, such as CO2 generated was 23 million Yen. This comparison justifies the water improvement of this lake. Choice experiments showed that people give low value to protection of wildlife, and the water environmental improvement was esteemed be higher than mortal risk reduction. The analysis or human activities showed that the nitrogen load to Lake Suwa was reduced by half in the past 30 years. Frequent survey of water quality revealed that non-point source like the cropland generates high loading with rainfall. This shows that prevention of soil erosion is effective method to reduce nutrient loading to this lake. The correlation between concentration of PAHs in roadside and amount of traffic was analyzed. The risk of chemical substances in the ecosystem and drinking water in Hiroshima Bay was evaluated. The relation between the amount of loading from the basin, and the amount of deposition in the lake was analyzed for poly aromatic hydrocarbon and butyl tin. These two were comparatively well in agreement. In Lake Suwa, outbreak of algal bloom decreased in recent years and the amount of existing of mosquito fell remarkably. Moreover, it turns out that reduction of a fish catch is being enhanced with a peak of the 1970s which were the severest as for contamination. Although water quality improvement progressed by construction of a sewer in Lake Suwa, the percentage of contribution of the load from non-point source increased.