連續萃取法對曾文溪流域含水層水－岩反應之研究

Study of the water-rock interaction in Tsengwenshi groundwater system using sequential extraction procedure

鄧瑞元(Jui-yuan Teng)

指導教授 ： 呂學諭

國立中正大學/理學院/地球與環境科學系應用地球物理與環境科學研究所/碩士(2012年)

摘要

地下水一直以來為台灣重要的用水來源，但這些地下水中 Fe、Mn 以及許多重金屬離子含量常有過高的趨勢，除人為污染的來源外，重金屬元素多源自於天然地下水與圍岩之間的水－岩反應。然而過去的研究中鮮少有較大範圍及深度之探討。本研究將以曾文溪流域含水層做為研究區域，並藉由天然示蹤劑的概念來探討土壤釋放重金屬汙染的情形。
本研究利用連續萃取方法針對沉積物樣本進行多階段的萃取，判斷與重金屬富集的相關相態，並加以推論可能的化學過程，利用感應耦合電漿質譜儀(ICP-MS)與離子層析儀(IC)進行土壤與地下水中的元素分析，了解曾文溪流域的水－岩反應過程。
地下水質分析結果顯示，此區地下水以末次冰期為界限，分為淺層含水層及深層含水層，淺層含水層因潟湖環境具有較高的鹽度，但 As 與重金屬含量均低，深層含水層則因陸相沉積環境具有較低的鹽度，但 As 與重金屬含量均高。藉由 BCR 溶出試驗之分析結果，主要元素、Mn與Sr在可交換相中含量較高，可能代表在pH值大於8且略微氧化的環境；Fe、As與其他重金屬元素以鐵錳氧化物形態存在；在有機物相中富集的元素僅有Th與Li。
分層結果顯示，As在深層含水層中的平均含量約為淺層的1.7倍，但其他重金屬元素則約略相等或反序，此與地下水質的分布有明顯差異。由稀土族元素的分析結果顯示，地下水含As量高者，幾乎沒有Ce負異常的現象。Ce負異常指示水體曾發生過強烈的氧化反應，顯示地下水體的 As 含量與氧化作用的發生有直接的關係。

關鍵字

水岩反應 ； 連續萃取法 ； 重金屬

並列摘要

The heavy metals in groundwater seriously risk the human wealth, agriculture and the aquaculture, especially, if the water is the major source of daily use. Generally, in spite of anthropogenic source, the heavy metals in groundwater are released during water-rock interaction. However, there are many mineral phases being capable of releasing heavy metals. It would need a sequential extraction procedure to identify the source mineral phase in the aquifer. In addition, the geochemical reactions after the release of heavy metals are also important to modify the concentrations. In this study, the rare earth elements are used to be a natural tracer for this purpose. The study area, Tsengwenshi watershed in southern Taiwan, is an alluvial fan with all kinds of land uses and is notorious of arsenic contamination. The groundwaters sampled in this study show that arsenic is enriched in deep aquifer (depth>150m), which is composed of sediments deposited in the last glacial period (18 ka). Based on this conceptual model, the results of BCR sequential extraction procedure are categorized into shallow aquifer (depth<150m) and deep aquifer; and, the averages of heavy metals in two groups can be subsequently obtained to take account of extensive water-rock interaction in the groundwater system. The results show that arsenic and other heavy metals are mostly binding with Fe-Mn oxides. To compare the ratios between deep and shallow aquifers for all heavy metals, the pattern of groundwaters does not show the similar type with those of extracted phases from soils. It is believed that the released heavy metals were strongly modified by the geochemical reactions during the transportation in the groundwater system. In addition, the analysis results of the rare earth elements demonstrates that almost all groundwaters with high arsenic do not have Ce negative anomaly; and, on the contrary, those with low arsenic are generally characterized by strong negative anomaly. Generally, the Ce negative anomaly is a prominent indicator of oxidation reaction even though the groundwataers are very negative in redox state in the present. However, the relationship between Ce negative anomaly and arsenic is deserved to be evaluated in more detail.

並列關鍵字

water-rock interaction ； sequential extraction procedure ； heavy metal

參考文獻

李春樹(2003)，銅鋅鉛在污染土壤中之化學型態及其萃取性研究，博士論文，國立成功大學環境工程學系，共141頁

Bacon, J.R., Hewitt, I.J., Cooper, P. (2005) Reproducibility of the BCR sequential extraction procedure in a long-term study of the association of heavy metals with soil components in an upland catchment in Scotland. Science of the TotalEnvironment 337:191– 205.

Cohen, D. and Ward, C.R., 1991, SEDNORM- A program to calculate a normative mineralogy for sedimentary rocks based on chemical analyses. Computers and Geosciences, 17, 1235-1253.

Davranche, M., Grybos, M., Gruau, G., Pédrot, M., Dia, A. and Marsac, R. (2011) Rare earth element patterns: A tool for identifying trace metal sources during wetland soil reduction. Chemical Geology 284, p.127–137.

Doelsch, E., Moussard, G. and Saint Macary, H. (2008) Fractionation of tropical soilborne heavy metals—comparison of two sequential extraction procedures. Geoderma, v. 143(1-2), p. 168–179.