白石平野におけるクリーク水質汚濁要因と改善策の検討

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佐賀県西部に位置する白石平野は干拓によって形成された低平な水田地帯で,クリークが縦横に張り巡らされている.クリークの水質は,1960年頃までは清浄に保たれていたが,近年になって悪化が進んでいる.そこで,白石平野の窒素循環モデルを作成し,1960年頃と2000年頃におけるクリークへの窒素負荷量を解析した.その結果,クリークの水質変化要因として,生活雑排水の増加と処理方法の変化,クリークの維持管理方法の変化及び営農の変化に伴う農地からの窒素負荷の増加の影響が大きいことが分かった.特に,乾田化や水田転作により水稲作付面積が減少する一方,肥料を多投する畑作物の作付けが増加し,これがクリークへの窒素負荷量を増大させている大きな要因であることが明らかになった.また,これらの検討結果を踏まえて,嘉瀬川からの導水,クリークの泥土除去,生活雑排水処理の強化及び営農での施肥量削減によるクリークの水質浄化効果を評価した.その結果,嘉瀬川からの導水によりクリークの水質は確実に改善される見込みがあるものの,営農での施肥量削減を行わないと,大きな水質削減効果は期待できないことがわかった.The author has analyzed the creek water quality of the Shiroishi plain in two different times; the first analysis took place around 1960 when the creek water was reasonably kept clean with devoted efforts of the farmers, and the second analysis was conducted around 2000 at which point the farming crops and the living standards had been changed after the development and improvement of the farmland in this area. Following these analysis, the author formulated two typical nitrogen circulation models representing around 1960 and 2000 respectively in order to keep tracking changes of nitrogen circulation in the Shiroishi plain using the biomass recycling diagnosis model which had been contrived in an effort to enhance biomass utilization. These two typical nitrogen circulation models are characterized by three unique features: 1) placing creeks at key locations of the typical nitrogen circulation models; 2) irrigating paddies directly via the creeks irrespective of water sources i.e. irrigation ponds and groundwater, thereby irrigated water is returned to the creeks; and 3) a unique gray water treatment system so called "YUDONE", which was commonly accepted in this area around 1960, was taken into account in the nitrogen circulation. A paradigm in which changes of creek water quality has been hardly discussed from the view point of undergoing changes in the nitrogen cycles in this area. The author hopes that the two nitrogen circulation models of the Shiroishi plain discussed heretofore will not only help facilitate quantitative analysis of deteriorating factors of the creek water but also facilitate proactive approach to the improvement of the creek water quality.

白石平野におけるクリーク水質汚濁要因と改善策の検討

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