Artificial Infiltration facilities were introduced to urban areas with the aim of reducing urban flooding from stormwater runoff. In Japan the "Tokyo Metropolitan Sewage Works" constructed infiltration facilities that constitute of infiltration soakaways and trenches at Nerima Ward in Tokyo during early 1980's as part of 'Experimental Sewer System' (ESS).

Among the micropollutants in runoff that are directed to subsoil below infiltration facilities heavy metals are the most problematic due to their persistent (non-biodegradable) nature and relatively easy change in speciation with changing environment. Significant accumulation of heavy metals was observed within infiltration facilities in Nerima by recent studies. This poses threat of migration downward and contamination of subsoil and groundwater. The major volume of dissolved metals in runoff is also reaching the soil system below the facilities. The retention behaviour of the heavy metals reaching the subsoil is essential for assessing its fate and devise better management technologies.

The fate of the heavy metals in soil system depends on metals sorption dynamics in the soil system which is dependent on soil characteristics, environmental conditions, heavy metals preference for certain chemical binding sites and the nature of competition among heavy metals, etc. The binding sites for heavy metals in soil are limited, hence competition is becomes intense as it becomes increasingly occupied. The competition and environmental factors (e.g., soil character, chemical forms, etc.) influence the speciation of metals.

This study aimed at clarifying the processes dictating the sorption behaviour of heavy metals below infiltration facilities with special notion given to competition among metals and speciation. Speciation of heavy metals determines the fate of the metals under a variety of environmental scenarios, and hence was exclusively covered throughout this study. Speciation analysis of metals bound in soil and sediments had been carried out in this study following 'BCR (Community Bureau of Reference) sequential extraction method'.

The analysis of the soil characteristics at site is essential to predict changes in metal retention characteristics with depth which is crucial in case infiltration of runoff water. A soil core of approximately 5 metre depth was collected from nearby the infiltration facilities at Nerima. There was two distinct layer of soil existing at the site. The first one metre sample was of dark brown colour organic soil while from one metre below the soil type was typical of loam soil in the 'Kanto' area, i.e., 'Kanto loam' soil. The significant apparent difference prompted inclusion of the surface organic rich soil together with the kanto loam soil for analysis.

The soil core was analyzed for generating vertical profile of soil properties and to observe the soil background heavy metal speciation patterns. The soil properties like pH, cation exchange capacity (CEC), organic content, humic matter content as well as fractional distribution of humic acid and fulvic acid, and humic matter functional group analyses were performed on the soil core along with metal content and metal speciation analyses. From analyses of soil core segments differences in soil chemical and mineralogical characteristics were observed with depth.

The two layers, surface soil and underlying soil, differed significantly in their properties. The kanto loam soil had lower pH, cation exchange capacity and organic content than the surface soil. The difference in cation exchange capacity and soil humic substance was very significant. A decreasing trend was observed for humic substance and also oxide contents in the underlying soil that indicated lesser adsorption potential for underlying soil at greater depth. The lower functional groups presence in the underlying soil indicated possibility of greater competition in underlying soil compared to surface soil. Preliminary adsorption experiments indicated lesser adsorption capacity for the underlying soil.

Analysis of composite or bulk soil samples from the site representing surface soil and underlying Kanto loam soil analysis was in line with the observations form soil core analyses. Surface soil samples from different locations outside the Kanto area were compared with the surface soil and underlying (i.e. below the infiltration facility at Nerima, Tokyo) 'Kanto loam' soil from the study site. The properties of the soil were much different from surface soil and underlying soil at site. The redundant nature of the observed properties for the soils stressed for the development of an integrated index for evaluating the metal retention characteristics of different soils.

The adsorption dynamics of heavy metals in subsoil is of concern for its speciation and fate. Even though the surface soil is not expected to receive runoff water at the current setup, the significant variation in character of the surface soil and underlying soil led to the use of both soil types for analyzing competitive adsorption dynamics in soil. This was useful for making a comparative evaluation of the metal adsorption dynamics in the soils with regard to the difference in soil character.

The adsorption dynamics were evaluated with single metal and combination of two metal scenarios that helped to clarify the one to one interaction between the heavy metals in soil during adsorption. The differences in interaction in different soil types (i.e., surface soil and underlying soil) were analyzed in a batch system. The speciation in soil after the adsorption test, i.e., the distribution of the adsorbed metals within soil, were analysed with sequential extraction method, namely BCR sequential extraction scheme. The analyses involved adsorption isotherm analyses, comparing the adsorptivity and site selectivity of metals revealed through distribution to put more light on the adsorption phenomenon and eventually the fate of the adsorbed metals.

The competition effect on adsorption of heavy metals in general was found less pronounced in surface soil when compared to the underlying sub soil below infiltration facilities. It concurred with the observations from the preliminary experiments with soil core sample.

It was found that the competition affect the adsorption a heavy metal differently. Zinc (Zn), nickel (Ni) and cadmium (Cd) were observed to be more affected by competition, either among themselves or by more competitive heavy metals (e.g., Cr, Cu and Pb). Chromium (Cr) and lead (Pb) seemed to be very little affected by competition. The effect of competition was also very specific for the metals, for example Zn was most affected by Cu, while Ni was most affected by presence of Zn.

The interaction of heavy metal produced different change to the binding sites in surface soil and underlying soil in different ways. In some cases improvement of adsorption for a metal was observed for surface soil in presence of another metal cation. In case of underlying soil the effect was always negative.

In the underlying soil ('kanto loam soil') the distribution of Cu, Pb and Cr were more to the mobile exchangeable or carbonate bound phases in contrast to the surface soil where organic and oxides sites seem to be selective of Cr, Cu and Pb. The more mobile metals, Zn, Cd and Ni, preferred the exchange sites. The organic binding sites in underlying soil seemed less capable of retaining metals as a whole and the surface soil organics showed little preference for the metals Zn, Cd and Ni. This is in contrast to numerous literatures that predict and hypothesize role of organic matter in the removal of Zn and Ni.

The selectivity of the organic matter to heavy metals seemed to be specific to the type of organic matter in that soil and no general conclusion can be made on that. It was also observed that the more competitive metals prefer the organic binding sites at low metal load. But as the metal load increases and the binding sites become occupied more of these metals move to the more mobile fractions (i.e., acid exchangeable fraction). The more mobile metals (Zn, Cd and Ni) are limited to the exchange sites for adsorption. Thus the presence of other more competitive heavy metals dictated the speciation of the mobile metals.

The effect of infiltration of runoff water on change in soil character and speciation of heavy metals were investigated. The effect of flow condition on the extent and the mode of such change were assessed since it has influence on the metal retention and fate of heavy metals resulting from changes in soil character. Post analysis of a laboratory soil column simulating impact of infiltration of runoff water was performed.

It was found that infiltration of runoff water introduced significant changes to underlying soil character and metal speciation. Two different flow conditions were evaluated, continuous flow and intermittent flow conditions (i.e., having dry periods in between). It was observed that the changes to soil character, especially soil organic matter were more significant under intermittent flow condition. It seemed that dry weather before a wet weather even has the potential to enhance mobility of metals by introducing changes in soil organic matter and other binding sites.

The leaching behaviour of soakaways sediment bound heavy metals was of importance as they constitute a significant metal load to underlying soil. The soakaways sediment samples at Shakuji area in Nerima, Tokyo had high accumulation of heavy metals. The heavy metals speciation patterns indicated potential for release. The sediments potential to leach heavy metals under different environmental scenario (e.g., acid rain situation, anoxic condition and leaching through organic chelating agents were evaluated in this study. The action of organic chelates appeared to have the greatest influence on initiating heavy metals migration from soakaways sediments to soil system below followed by acid rain condition. The leaching in anoxic environment was significant only for Zn and Cu.

The speciation analyses revealed significant changes due to organic chelating agent and at low ORP condition. The chelating action affected the heavy metals having strong complex with organic matter or soil oxides (e.g., Pb, Cu) as well as for known mobile metals like Zn and Cd. Migration of metals from sediment organic fraction revealed the vulnerability of the organic matter to organic chelates as well as anaerobic microbial activities. Zn was found the most mobile among the heavy metals. While in the soil system Cu seemed to be less mobile, sediment leaching experiments indicated potential for Cu migration to soil as organic labile complexed form.

The experiments conducted in this study provided better understanding of the competitive adsorption dynamics and the speciation of different heavy metals in soil system below rainwater infiltration facilities by clarifying the by clarifying the site selectivity of the metals and the role of soil properties in dictating the speciation. It also gave insight on the expected changes in speciation resulting from long term infiltration of runoff water and the effects of weather conditions on the speciation and long term fate of incoming metals, the vulnerability of the sediment bound heavy metals to different environmental stress. These are expected to aid in better management of the facilities.

本研究は、「Competitive adsorption dynamics and speciation of heavy metals in soil system below rainwater infiltration facilities」と題して、8つの章から論文を構成している。

第1章では、研究の背景と目的、および論文の構成を述べている。

第2章では、雨水浸透施設、都市域における重金属の発生起源、土壌による重金属の競合吸着特性や存在形態に関する過去に知見について詳細に整理している。

第3章では、実験に用いた土壌の採取場所やその方法、前処理を含めた土壌中の重金属分析方法、BCR逐次抽出法による存在形態別の重金属定量法などを整理している。

第4章では、雨水浸透施設が20年以上前から導入されている練馬区地域から柱状の土壌コアを入手して、その深さ方法の物性値や存在状態別の重金属分布を調査している。約1m程度の表層土壌と下層の関東ローム土壌の特性値の違いが重金属の保持能力特性が大きく異なることを明らかにしている。そして、表層及び下層のコンポジット土壌を用いた酸性条件での重金属吸着実験を実施して、表層土壌の吸着ポテンシャルの高さとイオン交換部位や炭酸塩への吸着部位が深く関与している可能性を示唆する知見を得ている。

第5章では、競合的な吸着平衡関係を調べるために、表層及び下層の土壌について、それぞれ6種類の重金属(Zn, Cd, Ni, Cu, Pb, Cr)を単独添加および二種類の重金属を添加する計21ケースの吸着実験を4段階の濃度条件(0.1, 0.2, 0.3, 0.5mM)で実施した結果を報告している。Cr, Pb, Cuは相対的に高い吸着ポテンシャルを有していること、同時にZn, Cd, Niは移動性が高いことを示している。そして、BCR逐次抽出法による存在形態別の重金属量を測定した結果から、前者の重金属類の吸着はイオン交換部位や炭酸塩への吸着部位だけでなく、有機物や鉄・マンガン酸化物の吸着部位も大きく貢献していることに対して、後者の重金属類の吸着は、イオン交換部位や炭酸塩への吸着部位のみが主たる支配していることを実験的に明らかにしている。

また、Langmuir平衡式とFreundlich平衡式の適用可能性とその係数値を求めて、競合的な吸着関係を定量的に整理している。そして、この競合関係は表層土壌と若干性格を異にしているが、下層土壌でより顕著であることを明らかにしている。

第6章では、雨水浸透施設内の堆積物を採取して、Zn, Cd, Pbが土壌に比較すると高濃度に保持されていることを確認した上で、堆積物中重金属の溶出ポテンシャルを調査した結果を示している。イオン交換部位や炭酸塩への吸着重金属のように移動性の高い画分量が、溶出ポテンシャルを支配することを明らかにした。そして、雨水浸透施設に流入する道路排水中の有機物による錯体形成、酸性雨によるpH低下、堆積物の蓄積による嫌気・還元状態化を想定して、各影響要因が溶出に及ぼす影響を調べた結果、有機物による錯体形成やpH低下により、イオン交換部位や炭酸塩への吸着部位だけでなく有機物の吸着部位の重金属にも影響を与える可能性があることを指摘している。したがって、移動性が高いZn, Cd, Niだけでなく、Pb, Cuも堆積物からの溶出もありえることを示している。一方、Crはその存在形態は変化するものの、溶出しにくいことを明らかにした。

7章では、高速道路の清掃時に回収される道路塵埃を用いて模擬道路排水を作成して、関東ローム土壌を充填したカラムに連続および間欠通水することにより、土壌による重金属の吸着や蓄積を調べた結果を示している。溶存態の重金属としての浸出はないものの、コロイド状態でのカラム内移動の可能性が考えられること、間欠通水は連続通水に比較すると土壌中の有機物分解を進行させ、有機物に吸着する重金属量が低下する可能性などを示している。また、道路排水中のCaの表層部への蓄積により、Cu, Cr, Pbがより移動性の高い吸着部位に変換するため、pH低下により溶出しやすい可能性も指摘している。

第8章では、上記の研究成果から導かれる結論と今後の課題や展望が述べられている。

以上の成果では、都市域の雨水浸透施設の周辺土壌を対象に、重金属の存在状態や環境条件変化に対する挙動に関して行った多くの実験や競合的な吸着特性に関する考察を行っている。これらの知見は、雨水浸透施設内に保持された重金属による地下水汚染の可能性を検討する際に役立つだけでなく、都市ノンポイント汚染対策や雨水浸透施設の設計や維持管理の在り方に有用なデータや知見を提供しており、都市環境工学の学術の進展に大きく寄与するものである。

よって,本論文は博士(工学)の学位請求論文として合格と認められる。