Water contamination has been paid much attention due to its impact on environments. The present study focuses on water contamination within Lampa River in Santiago, Chile, which is recharged exclusively by groundwater. This study shows how the concentrations of both cations and anions are increased and diluted within the river by water and sediment analyses emphasizing the behavior of hazardous elements (Cu, Zn, Ni,Co, Cd, Cr and Pb). Only a single area of high concentrations of cations and SO4(-2) was found at 0.8 km in length along the river out of 60 km examined. The phenomena observed in the high concentration area (HCA) were similar to those of acid mine drainage.

Samples were collected along Lampa River during January-February 2006 and 2007: they were surface (runoff) waters, pore waters in saturated soil, groundwaters (well waters) and sediments from the riverbed.Measurements of pH, Eh, temperature and anion concentrations (SO4(-2), HPO4-, Cl-, NO3-, HCO3 -) were done in the field mainly by colorimetry for the waters and cation concentrations (Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mg,Mn, Na, Ni, Pb, Si, Zn) were determined in the laboratory by inductively coupled plasma atomic emission spectrometry (ICP-AES). For the sediments, X-ray fluorescence spectrometry (XRF), X-ray diffraction analysis (XRD), ICP-AES and observation by scanning electron microscopy equipped with energy dispersive X-ray spectrometer (SEM-EDS) were employed to measure the concentrations of major cations (XRF) and trace cations (ICP-AES), to identify minerals (XRD) and to observe textures (SEM-EDSW). In addition a sequential extraction technique was used to examine how trace hazardous elements were associated with sediments constituents. The geochemical calculations were done by Phreeqc and Geochemist Workbench software.

Analyses of pore and surface waters and sediments have revealed that the cation concentrations in the waters along the river in general are low and normal, and those in the sediments are constant except in the HCA. The HCA was characterized by sudden increase and subsequent decrease in cation concentrations (including those of most of the hazardous cations) and SO4(-2) in the waters, and by corresponding decrease and increase in pH.The cation and SO4(-2) concentration variations are well represented variation of ionic strength (Figure 1),suggesting that ionic strength is a good indicator of contamination. On the other hand, the cation and anion concentrations in the groundwaters are generally low and normal everywhere along the river, which reflects those in the surface and pore waters in other areas than the HCA due to the nature of the river system,groundwater recharging.

The pH decreased from pH =7 in the upstream to pH = 3.1 in the HCA (Fig. 2) resulting from dissolution of Cu/Fe sulfide (e.g., chalcopyrite). The presence of the sulfide and the dissolution texture were confirmed by SEM-EDS (labels A and B, respectively in Fig. 3). The dissolution of Cu/Fe sulfide also caused the increase in Cu, Fe and SO4-2 concentrations in waters in the HCA (Fig. 2), which consequently formed Fe and probability Cu sulfate minerals. The increase in concentrations of Mn, Zn, Ni, Co and Cd (Fig. 2) resulted probably from dissolution of sulfide minerals which further increased SO4-2 concentration. The low pH induced by dissolution of sulfides, then, dissolved feldspars abundant in the Lampa River sediments, which increase the concentrations of Ca, Na, K and Al in the waters (Fig. 2)

The decrease in SO4(-2) concentration in the HCA is concordant with the decrease in cation concentrations (Fig.2) occurred by the short-time consumption of SO4(-2) through precipitation of sulfate minerals such as gypsum (CaSO4:2H2O), rozenite (FeSO4:4H2O) and Al sulfate which was confirmed by XRD and SEM.Thermodynamic calculations suggested that Mg, Mn, Cu and Zn were all possible cations forming sulfate minerals. The decrease in concentrations of cations (excluding Cr and Pb) (Fig. 2) can be explained mainly by sulfate mineral formation. The slower decrease in Ca and SO4-2 (Fig. 2) and the observation of gypsum even in water suspension indicated that gypsum formation is most effective to remove SO4-2, which has not been pointed out for acid mine drainage.

The increase and subsequent decrease in concentrations of Cu, Zn, Ni, Co, Cd, Cr and Pb occurred in the HCA for surface and pore waters. Similar increase and decrease were also observed for the hazardous elements in sediments only in the HCA but not in other areas along the river where their concentrations were generally quite low.

Along with the concentrations variations (Fig. 2c), the sequential extraction (Fig. 4) revealed that Cu, Zn, Ni,Co and Cd behaved similarly. These hazardous elements are released by dissolution of sulfides, resulting in their high concentration in water. The dissolved Cu, Zn, Ni, Co and Cd are then redistributed in sediments in a limited area and remain there but are not transported further downstream. Te sequential extraction results for Crand Pb were different from those of the above hazardous elements in addition to the difference that Cr and Pb concentrations in the waters are not affected significantly by dissolution of sulfides.

Figure 1 - Variations of ionic strength in waters along Lampa River

Figure 2 - Concentration variations of selected cations and SO4(-2) and variations of pH around the HCA for surface waters.

Figure 3 - Backscattered electron image of a sediment in the HCA

Figure 4 - Sequential extraction results for Cu in sediments expressed by concentration accumulation.

本論文は、チリの首都であるサンティアゴ市内を流れる河川の水および堆積物を地球化学的手法で分析し、様々な元素、特にCuやZnなどの有害元素の挙動を解析し、河川水および堆積物中の元素の濃度増減が、堆積物に存在する硫化鉱物の溶解による増加、硫酸塩鉱物の形成による減少に起因していることを明らかにした。本論文は、通常の学術雑誌の論文と同様に(1) Introduction、(2) Sampling and methodology、(3) Results、(4) Discussionより構成されている。

水質汚染の環境へのインパクトを鑑み、その河川水が地下水によってのみ供給されるランパ川を研究対象にすることにより、汚染の発生とその解消の機構がより明確に決定できると本論文は位置づけている。水のサンプリングは、ランパ川の流水経路を考慮し、地下水、堆積物中の間隙水、河川水で行い、様々な陽イオン、陰イオン、さらにpHなどの溶液分析を行った。また、堆積物に対しては、化学分析、鉱物分析、組織分析とともに、連続抽出法を適用し、有害元素がどのような鉱物と関係し存在しているかを調べた。約60 kmにわたり調査したランパ川流域では元素の濃度はほぼ一定であったが、800 mの長さの流域のみで(high concentration area, HCA)河川水および堆積物中の元素の急激な濃度変化が起こっていた。河川水のpHは約7から3に急激に低下し、その後HCA内で徐々に7に上昇する。それに対応してSO42-は約100ppmから3500 ppmに急激に上昇し、その後徐々に約200 ppmに低下する。SO42-以外の陰イオンは、HCA内で大きな濃度変化は認められなかった。陽イオンでは、微量に含まれるCrとPbを除いて、K, Na, Mg, Fe, Mn, Al, Ca, Cu, Zn, Ni, Cd, Coで急激な上昇(バックグランドに対し、最大3桁)とその後の低下が同様に認められた。またSO42-とCaのみはHCAの下流側でもさらに濃度が減少した。河川水では、pH、SO42-、陽イオンの増減はほぼ一致して起こっていた。

一方、堆積物からCuを含む硫化鉱物(chalcopyrite)の存在とその溶解組織が観察された。また、gypsum (CaSO4・2H2O)、rozenite (FeSO4・4H2O)、Al硫酸塩鉱物の存在が確認された。この硫化鉱物の溶解は、溶液中でのCu2+, Fe2+, SO42-の増加とpHの低下を引き起こす。また、硫酸塩鉱物の形成は陽イオンとSO42-の濃度を低下させる。堆積物中のマイナーな元素は含有量が数百からせいぜい数千ppmなので特定の固相、鉱物として特定はできなかったが、河川水の熱力学的分析から、gypsum、rozenite (FeSO4・4H2O)、Al硫酸塩鉱物以外にも、少なくともMg, Mn, Cu, Zn硫酸塩鉱物は沈澱しうることがわかった。また、連続抽出法からCu, Zn, Ni, Cd, Coは硫酸塩鉱物として堆積物中に存在していることが示唆された。硫化鉱物の溶解によるpHの低下は、ケイ酸塩鉱物の溶解を促進することが知られており、堆積物の主要鉱物である長石、角閃石の溶解により、K, Na, Ca, Mg濃度が河川水中で増加したと考えられる。

これに対し、地下水組成はランパ川流域では一定であり、河川水および堆積物中の元素の濃度変化に関係してないことが分かった。従って、HCA内で観察されたpHの急激な低下とその後の増加、SO42-と陽イオンの急激な増加とその後の低下という一連の化学変化は、硫化鉱物の溶解とその後の硫酸塩鉱物の形成で起こったことが強く示唆された。Cu, Zn, Ni, Cd, Co, Cr, Pbという有害元素のうち、Cu, Zn, Ni, Cd, Coの挙動は上記と同様に説明できるが、Cr, PbはHCA内でもその濃度増減がほぼなく、Cr, Pbを含む硫化鉱物が極微量でしか存在しなかったと考えられる。これらの有害元素は、硫化鉱物から溶出してもすぐに堆積物に再分配し、HCAより下流側には運搬されず、汚染はHCA内で留まっていると結論される。汚染の原因は硫化鉱物の溶解であるが、硫化鉱物は堆積物に含まれており、HCAの上流側から運搬された可能性が高い。従って、HCAに見られる汚染は、ランパ川流域の他地域でも発生する可能性がある。

本学位論文は、鉱物-水の相互反応により天然の河川で生じた陽イオンおよび陰イオンの化学濃度変化を測定、分析、そしてその機構解明という一連の解析手法を見いだした点において、今後の関連分野の研究、特に環境中の水質汚染の研究に寄与するところが大であると認められる。この点において、本論文は高く評価され、審査委員全員で、博士(理学)の学位を授与するにふさわしいと判断された。