論文内容の要旨

(1)

論文内容の要旨

For understanding the element cycles under subaerial environment, to know chemical forms and the variation of chemical forms of elements in river water is important. The aim of this study was to assess the relevance of dissolved (<4 nm), colloidal (4 nm-0.2 µm) and particulate (>0.2 µm) forms for the transportation and accumulation of elements in the river water of Tamagawa-Omonogawa River System in Akita Prefecture, Japan. This river system contains acidic thermal water due to the discharge of the Obuki Cl-SO4 type thermal water to Tamagawa River and Kawarage Cl-SO4 type thermal water to Omonogawa River at the upstream of both rivers. Obuki Hot Spring discharges 9000 L/min of acidic thermal water (pH 1.2) to Tamagawa River while Kawarage discharges 7300 L/min thermal water of pH 1.4 to Omonogawa River. The environmental impact of thermal water of Obuki Hot Spring is larger than that of thermal water of Kawarage Hot Spring. Tamagawa River runs through the area mainly composed of Quaternary and Neogene volcanic rocks and has 4 lakes and dam lakes along the river, while Omonogawa River runs through the area mainly composed of Neogene strata and Quaternary strata covering Neogene sedimentary rocks and has no

氏名（本籍） Pham Minh Quyen（ベトナム）

専攻分野の名称博士（工学）

学位記番号工博第241号学位授与の日付平成30年3月22日学位授与の要件学位規則第4条第1項該当研究科・専攻工学資源学研究科・資源学専攻

学位論文題目（英文）秋田県の酸性温泉水を含む玉川－雄物川水系の河川水中の元素の挙動：河川におけるダム湖の役割

（Behavior of elements in river watar of Tama-Omono River System containing acidic thermal watar in Akita Prefecture: Roles of dam lakes along the River System）

論文審査委員（主査）教授石山大三

（副査）教授柴山敦

（副査）教授大場司

（副査）教授佐藤時幸

Akita University

(2)

dam lakes.

The characteristics of major elements are similar in Tamagawa and Omonogawa Rivers. All of the major elements present in the dissolved form in the river water. Inflow of acidic thermal water into Tamagawa and Omonogawa Rivers changes the pH of the river waters to acidic in upstream. Toward downstream, due to the addition of neutral river water from tributaries, the river water of Tamagawa and Omonogawa River become more neutral. However, the change of pH value of river water of Tamagawa-Omonogawa River System would not give effect to the state of chemical forms of major elements.

According to acidic river water at upstream of Tamagawa River, concentrations of dissolved metals are also high except Fe and As. Dissolved Fe and As easily change into colloidal and/or particulate forms in river water in upstream of Tamagawa River. As a result of the process, Fe-rich layered sediment is present in Tamagawa Dam Lake.A large influence of hydrothermal activity from Obuki Hot Spring was recorded in the sediment at Tamagawa Dam Lake, resulting in high Pb content within sediment layers at 5 to 8 cm from bottom of the dam. At the downstream site, the influence of acidic thermal water becomes small due to addition of neutral river water from tributary and supply of particulate material from the erosion of Neogene and Quaternary sedimentary rocks. The pH of river water of Tamagawa River changes from 4.6 at Tamagawa Dam Lake in upstream to 7.1 in downstream at Omagari in Daisen City. The particulate forms of Al, Fe and trace metals in river water contribute to increase of concentration of these elements in lower Tamagawa River in Omagari. Based on observation and analyses by SEM-EDX and XRD, the particulate materials are thought to be clay minerals such as chlorite and illite/montmorillonite.

In Omonogawa River, sizable proportion of Al and Fe were found within the particulate form that is similar to Tamagawa River. The small proportions of Cu and As were observed as dissolved form and the proportion of these elements in colloidal form increases toward downstream, which is a different tendency with the distribution of these elements in Tamagawa River. The clay minerals derived from the sedimentary rocks along Omonogawa River could be the host of elements as particulate form and caused the increasing concentration of Al, Fe, Mn and trace metals in river water at downstream site. Concentrations of Al, Fe, Mn and trace metals in particulate form of river water of Omonogawa River are higher than those of Al, Fe, Mn and trace metals in particulate form of river water of Tamagawa River.

The amounts of Al, Fe, Cu and As that are transported by river water of Tamagawa River in winter season decreased from upstream to downstream (118 to 18 ton/month, 8

Akita University

(3)

to 6 ton/month, 49 to 26 kg/month and 37 to 23 kg/month, respectively). On the other hand, the amounts of Fe, Cu and As that are transported by river water of Omonogawa River in winter season increase from upstrearn to downstream (15-148 ton/month, 22-225 kg/month and 112-308 kg/month, respectively) while the amount of Al in river water of Omonogawa River is constant (27 ton/month). The amounts of Al, Fe, Cu and As in river water of Omonogawa River, however, are 18, 5, 13 and 2 times larger than those are transported by river water of Tamagawa River to the Sea of Japan, respectively. In other seasons, the amounts of elements transported by Omonogawa River are also several times larger than the amounts of elements transported by Tamagawa River. These differences accord with (l) the numbers of presence of dam lakes along Omonogawa (no dam lake) and Tamagawa rivers (4 dam lakes) and (2) difference in dominance of Quaternary and Miocene sedimentary rocks between Omonogawa and Tamagawa Rivers. The dam lake could play an important role as a sink for particulate materials and reducing the environmental impact.

論文審査結果の要旨

秋田県の玉川および雄物川の上流部にはCl-SO₄型の酸性温泉水（玉川温泉，川原毛温泉）

が湧出し，河川に流出しており，このような温泉は河川水に影響を与えていると考えられる．しかしながら，これまで河川水中の成分を化学形態（溶存態，コロイド態，懸濁物態）

毎に区分し，元素の運搬機構を解析した例は少なく，河川水による物質運搬機構には不明な点が多かった．

本研究では，主に玉川水系の玉川ダムから大仙市大曲までと湯沢市須川の高松川と雄物川の合流点付近から大曲までの区間の河川水を研究対象として，河川水中の溶存態，コロイド態，懸濁物態の各成分濃度変化，野外での観測結果，XRDとSEMにより同定された河川水中の物質の特徴に基づき，河川水による物質運搬機構について検討を行った．その結果，

以下のような結論が得られた．

１）河川水中の成分の溶存形態

河川水中のNa⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-等の主成分元素は，ほぼ溶存態として存在していることが示された．微量成分元素については，元素により存在しやすい化学形態が異なり，溶存態として存在する元素としてはSr，懸濁物態として存在しやすい元素にはFe, As などがあることが示された．コロイド態として存在する元素には，Alなどがあるが，多くの元素においてコロイド態として存在する割合は，溶存態や懸濁物態に比較して多くはないことが明らかにされた．

２）化学形態と河川水水質との関連性

Akita University

(4)

FeやAsなど河川水中で一般的には低濃度でしかも懸濁物態で存在しやすい元素では，

上流域の酸性のpHから下流域での中性へのpH変化に伴い，溶存態から懸濁物態に変化し，

下流域に懸濁物態として運搬される．この運搬プロセスの中でFeの場合には，溶存態Fe が懸濁物態の水酸化鉄に変化し，河床に沈殿したり懸濁物態粒子として下流に運搬されることが示された．主要河川中流部から下流部では水酸化鉄懸濁物態物質の減少と入れ替わるように支流域から供給されるFe含有粘土鉱物が河川水中の主要懸濁物態になることが示され，上流域から下流域にかけて河川水中の懸濁物態の種類が変化することが明らかにされた．

３）河川システムにおけるダム湖での元素の挙動と河川水の化学組成への影響

今回検討した玉川水系には田沢湖や多くのダム湖が存在する．一方，雄物川水系には湖やダム湖はほとんど存在しない．Srなど溶存態で存在する割合の大きい元素では，河川の下流域に運搬される元素量は，温泉から放出される元素量よりも集水地域の地下水からの供給量が多く，温泉水の影響よりも集水地域の面積比に比例することが示された．また，

溶存態で存在する元素の場合には，ダム湖等が存在する玉川水系においても，ダム湖における沈殿は認められず，溶存態で存在する元素の運搬量については上流から下流までほぼ一定量であることが示された．一方，Al, Fe, Asなど河川水中で一般的には低濃度でしかも懸濁物態で存在しやすい元素では，ダム湖等が多く存在する玉川水系では，これらの懸濁物態がダム湖にトラップされ，下流域に運搬される懸濁物態の元素量は小さくなる．このことは，河川におけるダム湖等が有害元素等の貯蔵庫の役割をしており，下流域の環境への影響を軽減していることが示された．

本研究では，秋田県の玉川および雄物川の河川水の温度，pH等の観測データ，河川水の化学形態別の化学組成データ，懸濁物質の同定結果に基づき，河川水中の元素や物質の運搬機構を推定し，河川におけるダム湖の役割を明らかにした．得られた成果は，これまでの河川水による物質運搬機構に関して河川水中の元素の化学形態変化を考慮したモデルを提案するとともに環境リスク評価にも重要な知見を与えた．

従って，本論文は博士（工学）の学位論文として十分価値のあるものと判断した．