イオンクロマトグラフィーによる灰塚ダム人工湿地の水質評価

摘要

Haizuka-dam lake located in Hiroshima prefecture of Southern Japan is facing to the eutrophication problem caused by influx of the nutrients such as nitrogen and phosphorus emitted from public and private sectors on upstream area of suburban-river watershed. This dam-lake is placed with a constructed wetland for improving water quality of incoming river at upstream of the dam lake. In this study, the water quality monitoring for evaluating the water quality of the constructed wetland in the Haizuka dam-lake was performed using several official analytical methods like JIS method (K-0102) to determine BOD, COD, total-P, total-N, SS, and Chlorophyll-a as well as the ion chromatography (IC) consisting of ion-exclusion/cation-exchange chromatography (IEC/CEC) with conductivity detection for determining simultaneously common anions such as SO_4~(2-), Cl~-, and NO_3~- and the cations such as Na~+, NH_4~+, K~+, Mg~(2+), and Ca~(2+), the IEC with spectrophotometric detection at 700nm for determining simultaneously phosphate and silicate ions as the nutrient component, and the IEC with enhanced conductivity detection for determining HCO~3~- (alkalinity) as a inorganic-carbon source. The water quality evaluation of the wetland waters taking at different sampling sites was carried out using the IC systems and the official analytical methods. As a result, it was found that the water quality of dam-lake is improved by the effect of various biological reactions taking place at the wetland.%ダム湖の上流に設置された人工湿地の水質浄化能を評価するために，各種の陰及び陽イオンの同時分離計測を可能にする導電率検出イオン排除/陽イオン交換型イオンクロマトグラフィー(IC)，リン酸及びケイ酸イオンの同時分離計測を可能にする可視吸光検出イオン排除型IC及び炭酸水素イオン（アルカリ度：無機炭素源）の高感度な導電率検出を可能にするイオン排除型ICを各々適用した。その結果，ダム湖水中の各種栄養塩や炭酸水素イオンの減少に伴うバイオマスの増大結果から，この人工湿地は，流入河川水中の窒素やリンをバイオマスとして固定する内部生産能力（自然浄化能）を有していることが分かった。また，下流の（仄）塚ダム湖の富栄養化を抑制する観点から，人工湿地の下流に設置された堰堤で滞留•沈降したバイオマスの嫌気的分解（バイオマスの無機化）の進行に伴う栄養塩の再溶出を抑制することが不可欠であることから，この定期的な浚渫（沈降バイオマスの抜き取り）が必要であると結論された。