In the present era of high population growth, rapid urbanization and climate change, uncertainty of natural phenomenon and land use pattern are rapidly increasing. Particularly rainfall frequency and its amount became too unpredictable to regulate stormwater runoff or wet weather flow. Further, urbanization is a global phenomenon that includes various point and diffuse non-point pollution sources, which can impart various toxic contaminants into the environment. Particularly, diffuse non-point pollution sources are difficult to identify, categorize, control and prevent due to their dynamic nature. Road dust is one of such non-point pollution source of heavy metal which is toxic, prevalent and persistent in the urban environment. On the other hand, urban areas require artificial infiltration facilities (AIF) to enhance groundwater recharge and to regulate storm water runoff and wet weather flow. However, AIF systems such as soakaways, provide a sink which traps the road dust coming with urban runoff. Prolonged accumulation of road dust in soakaway sediment is likely to cause groundwater contamination.

Although the information of total metal content in the solid phase of contaminated soil/sediment is useful to estimate its overall contamination potential, the magnitude and intensity of contaminant mobilization to groundwater does not necessarily depend directly on total metal content. Precise assessment of heavy metal fate and its toxicity must take into account of the partitioning of heavy metal. It is a known fact that trace metals are distributed among several different soil components with varying degree of strength and ways. This strength and ways of association with different component enable trace metal to behave as highly labile species, easy to get exchanged with ambient environment, through to increasingly non-labile (fixed) forms. However, under certain condition, these fixed forms of metal also can turn into the labile pool. In response to this accepted fact, researchers from worldwide conducted various research to find the most accurate technique to quantify the labile pool of heavy metal.

One of the most widely used technique for speciation is selective sequential dissolution (SSD) that relies on the solubility of individual solid-phase components by selective reagents where each reagent in SSD targets one major solid phase. However, in no case a reagent can remove all of a targeted solid-phase without attacking other components. In addition, metal redistribution and re-adsorption may occur during a given step. While, isotope dilution technique (IDT) is highly accurate with a 0.2% of possible error of estimation of isotopic ratio against 5% of error in estimation of elemental concentration. IDT is one of the most accurate methods available to quantify exchangeable metal concentration (E-value) which is based on the assumption that stable isotopic tracers added to the soil solution will exchange with the potentially mobile forms of the elements present in the solid phase. There are limited studies available in literature that had used both techniques for the mobile metal pool assessment.

Moreover, there is no scale present to measure the heavy metal retention properties of soil and sediments. Henceforth, the present research aims at precise understanding of the heavy metal sorption properties of soil/sediment using an integrated analytical approach for column leaching experiment. In addition, the compliance between SSD technique proposed by the Community Bureau of Reference (BCR) and isotopic dilution techniques for the determination of mobile pool of heavy metal contained in soakaway sediment, road dust, and soil samples were also examined. Study also has an objective to observe the influence of soil properties on the metal sorption capacity and its variation with ageing.

More specifically, the objective of this study includes (i) Understanding the metal partitioning and their relative affinity for different fractions of soil and sediments with the comparison among different metals. ii) Tracing the source of metal contamination using isotopic fingerprinting. iii) Probing the reactivity, fate and mobility of heavy metal using the sorption coefficient (Kd) and isotopically exchangeable metal concentration (E) in soakaway sediments of AIF. iv) Understanding the factors that affect heavy metal retention by soil and sediments. v) Tracing the ageing effect on the metal sorption, and vi) Development of heavy metal retention index (HMRI).

Four sediments samples were collected in July and November 2007, from different infiltration facilities (constructed in the early 1980's) at Nerima ward, Tokyo. All four soakaway sediments were different on the basis of their locations viz besides the park, near residence with and without receiving some portion of domestic water use, and near parking area. Two soil samples representing the surface (depth<1.0m) soil and bottom soil (depth >1.0m) were also collected besides. Apart from this highway road-dust samples were also collected. The samples were processed and preserved in refrigerator till analysis. Initially, samples were characterised by determining the parameters like pH, moisture content, organic content (i.e. ignition loss) and cation exchange capacity (CEC) using standard methods. For speciation analysis, the 'BCR (three steps) sequential extraction method' was applied in order to sequentially extract metals in the order of decreasing mobility. The three fractions represent the exchangeable and carbonate bound lead, the Fe/Mn oxide bound lead and organic bound lead, respectively. The remaining amount, termed as 'residual fraction' which is supposed to be associated with silicate fractions. The total lead content and their isotopic ratios were measured separately with ICP-MS.

Isotopic data were used in this study to first distinguish the partitioning of anthropogenic and natural lead in different fractions, obtained by BCR sequential extraction, and then to anticipate their mixing process in the soakaway sediment of AIF. In general, total contamination level of a particular metal in different samples followed similar order of road dust > soakaway sediment >> soil, except that of Pb which showed higher value for soakaway sediment. Trend showed that heavy metal contamination threat to groundwater from road dust and soakaway sediment is much higher than that of soil. The results obtained through BCR fractionation showed that irrespective of total metal burden, the exchangeable fractions of all metals were found maximum in soakaway sediment except Cd. This is due to changing oxidation-reduction condition due to wet and dry periods in soakaway sediment. In general, residual fractions shares highest percentage of total metal among different fractions. This observation is valid for each sample and metal with only exception in case of Cu in Soakaway sediment, which is found to be associated with oxidisable fractions more.

Investigation of the difference in Pb content, partitioning and its isotope signature among four sediment samples considering their sampling locations showed that the lowest 206Pb/207Pb ratios were mostly observed in exchangeable fractions of soil and sediment samples, while residual fractions mostly showed the highest 206Pb/207Pb and 208Pb/207Pb ratios than those of other fractions. In general, both ratios were higher in the soil than those of sediments. Among soil samples, residual fraction of bottom soil exhibited higher ratios than surface soil indicating higher contribution of natural lead with depth. The plot of 206Pb/207Pb versus 208Pb/207Pb showed two well demarcated cluster formations by soil and sediments samples that describe the partitioning between anthropogenic and natural lead; and some points falling in between soil and sediment samples pertinently illustrated the mixing processes between these two different pools of lead.

It was found that only exchangeable fraction measured through BCR is not a good criterion for the estimation of mobile metal pools. It is also pointed out that metal extracted by chemical reagents not only underestimates the exchangeable portion but also overestimates the potential mobile pool of heavy metal as evident from the comparison of percentage of E-value and potential mobile pool. Although there is a difference in quantification of mobile pool between the two techniques, mobility potential ranking of heavy metal can be evaluated by BCR. Nevertheless, sequential extraction shares a common goal with IDT to determine the reactive pool of metal. Moreover, combination of isotope dilution analysis with selective dissolution method can give more insight into mobile pool of heavy metals. The information obtained in the study is a vital contribution for obtaining international standardized agreement between both techniques.

Column leaching experiments were conducted using surface soil, underlying soil and soakaway sediment to first understand the potential mobility of heavy metals and then to distinguish the difference of mobility characteristics of these contaminant among different samples. Artificial road runoff (ARR), prepared in the laboratory using highway road dust and pure water (L/S=25), was used as leaching solution to mimic actual condition of urban runoff received by artificial infiltration facilities (AIF). Continuous leaching was carried out by feeding ARR with flow rate of 0.12 ml/minute to simulate 10mm/hr of infiltration. Leachates were collected at the regular interval of 24hrs (daily basis) and analysed for pH, DOC (dissolved organic carbon), major ions and heavy metals. Experiments were carried out in batches for 30 days under continuous flow conditions as well as for 20 days under intermittent flow condition. An extra batch of columns operated under continuous flow condition was installed to compare with the intermittent flow. This is because the amount of eluent passed to column under intermittent flow condition for 20 days is equivalent to the amount that was passed in continuous flow mode for 10days. At the end of experiment each column samples were divided into two i.e. upper half and lower half and then analyzed for several parameters to evaluate the change during experiment. The E-value increases with time which was the highest for Pb and least for Cu, some exception for specific sample did exist. The most important thing observe was higher isotopically exchangeability for Cu and Zn after 20 days of intermittent leaching than that of after 30 days.

The strong retention of Pb to the solid system may be one of the reasons of not having much difference between two flow conditions. The important change in chemical partitioning before and after leaching was seen as the increase of exchangeable fraction and decrease of percentage contribution of residual fraction with the time of leaching. The easy transport of mobile pool associated with solid system but due to higher dynamic attachment of free and labile metal complexes of feed water on solid system likely is the reason behind change in metal speciation. A general finding was the change in reducible fraction which was the most significant for Pb while oxidisable fraction change exhibited highest change for Cu. The fraction of Zn bound to organic matter changed insignificantly except the initial stages of experiments of the column packed with soakaway sediment. Pb showed the highest retention than that of Cu and Zn. Further, the concentration of stable complexes in the collected leachates seems to be quite stable throughout the experiments especially in the case of soil samples. Therefore, the increase and decrease of metal retention is due to the variation in the retention of free and labile complexes of metals. It can thus be concluded that it is labile fraction which is scanned by the solid materials for retention. Henceforth, high labile concentration in ARR is a subject of more concern from the pollution point of view through infiltration facilities.

The hierarchical levels of normalized factors established for heavy metal retention index (HMRI) was designed to estimate heavy metal retention capacity. Three different index are proposed based on six physicochemical parameters (classical HMRI), three special parameters for each Cu (specific HMRI) and nine combined parameters (integrated HMRI). All three HMRI is validated with actual results obtained for column leaching experiment. Integrated HMRI not only decreases the extra sensitivity of specific parameters but also decreases the roughness of physicochemical parameters.

Results obtained in this study effectively predict the risk associated with the release of retained heavy metal and nitrate with changing environmental conditions in AIF. The information obtained in the study can be utilised in better management of non-point pollutants accumulated in infiltration facilities in urban area.

気候変動に伴う集中豪雨の頻発とともに少雨による渇水が想定されるなか、都市域の洪水や浸水対策だけでなく、都市域におけるストック型自己水源としての地下水の管理は重要性を増してきている。その観点から、屋根排水だけでなく、道路排水も浸透させることは、地下水涵養を通じた水資源確保、河川流量の確保や湧水復活などにも有効であると考えられる。しかしながら、道路排水の浸透を推進するためには地下水汚染のリスクも考慮する必要がある。特に、都市ノンポイント汚染対策の観点と地下水涵養の両者を組み合わせて、雨水浸透施設の維持管理のあるべき姿や将来像を明確するために、雨水浸透施設における重金属の動態や挙動、さらには施設下層に位置する土壌による重金属の保持能を正しく、定量的に評価することが期待されている。

本研究は,「Comparative assessment of potential mobility of heavy metals in soakaway sediment of infiltration facilities and soil using sequential extraction and isotopic dilution techniques :逐次抽出法と同位体希釈法による浸透ます堆積物及び土壌中の重金属移動性の比較評価」と題して,8つの章から構成されている.

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,まず、土壌中重金属の存在形態分析のための逐次抽出法とその適用事例、同位体希釈法による交換可能な重金属量定量法とその適用事例、雨水浸透施設における重金属の除去などについて既存の研究をとりまとめている。また、鉛の安定同位体比による起源解析の研究事例の整理も行っている。

第3章では,研究対象となる浸透施設堆積物、土壌、道路塵埃の採取方法の説明に加えて、それらの化学成分や陽イオン交換容量など、重金属の保持能に関わる特性を整理している.また,土壌中の重金属の存在形態を調べるために広く用いられている逐次抽出法、交換性の重金属量を測定するための同位体希釈法の具体的な手順および、重金属の分析方法についても記載されている.

第4章では,浸透施設堆積物、土壌、道路塵埃の重金属全含有量を調べるだけでなく、重金属の移動性を理解するために、逐次抽出法による形態別存在量を求めている.そして、道路塵埃の重金属量が高いこと、それぞれに存在形態割合が異なることなどを明らかにしている。特に、鉛に関して3つの安定同位体の存在比が、鉛鉱石や自然土壌などと石油や塗料などと異なることを利用して、人為的な汚染が道路塵埃や浸透施設堆積物などにどのように影響しているかを検討している。その際、206Pb/207Pb と208Pb/207Pbの比を同時に用いることで、わかりやすく人為汚染の影響を識別できることを提示している。

第5章では,土壌などに保持されている交換態重金属量を測定するために、同位体希釈法を堆積物、土壌などに適用して、各重金属の吸着係数の大小を調べている。この測定では、試料と蒸留水を混合した溶液に対して、試料とは大きく安定同位体存在比が異なる重金属スパイク溶液を添加して、72時間で同位体比が安定することを確認した上で、その安定した比率を用いて交換態の重金属量を求めている。そして、同時に実施した逐次抽出法による酸で溶出する画分、還元状態で溶出する画分、過酸化水素水による酸化分解で溶出する画分と交換態重金属量とを比較することで、両者の方法での重金属の移動性評価の違いや関係を議論している。この成果は、雨水浸透施設における堆積物やその下層に位置する土壌での重金属の移動性を評価する上で非常に貴重なものである.

第6章では,道路塵埃と蒸留水と混合することで模擬的な道路排水を作成して、道路排水を受け入れる雨水浸透施設での重金属の吸着過程を調べるカラム実験を行っている。その際、二種類の土壌と浸透施設堆積物を対象にして、連続的に流入させる系と間欠的に流入させる系の二つについて実験を行い、溶出液の水質測定と実験後の土壌や堆積物の重金属存在形態の変化を調べて、鉛に比較して亜鉛の保持能が低いこと、土壌などによる除去や溶出、除去された重金属の保持形態が溶出液のpHや有機物濃度と関連している可能性を示唆している。また、有機物が少ない土壌では、間欠流入による有機物分解や硝化反応の促進が鉛の保持能を低下させる結果も示されている。さらに様々な実験条件での調査が必要であるものの、これらの知見は雨水浸透施設における重金属保持能力を検討するために有意義な実験的成果である.

第7章では,重金属保持能に関わる土壌の物理特性や環境因子を整理して、それらに重み付けを行い、総合的に重金属保持能の新たな指標を開発することを試みている。重金属ごとに土壌や堆積物の交換可能量や吸着係数の情報を加味することで、重金属を一律に扱う従来型の保持能力指標を、重金属種ごとに評価するものへ発展させている。そして、6章で実施した土壌カラム実験での重金属保持能をここで提案した指標で評価可能かの検討を行い、その適用性と限界について考察を加えている.

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,土壌や雨水浸透施設堆積物による重金属保持能の定量評価への提案や展望が述べられている.

以上の成果は,都市ノンポイント汚染対策として、雨水浸透施設を高度に活用する観点から、間欠的に道路流出水が導入される施設内堆積物や下層に位置する土壌における重金属の保持や溶出などの動態を詳細に研究したものである。特に、逐次抽出法と同位体希釈法の両手法を適用することで、重金属に移動性を検討した点を含め、これらの知見は,地下水汚染防止の観点から浸透施設における重金属動態や土壌の重金属保持能を評価し、その指標を開発する上で非常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.