沿海地區砷在鹽化地下水之生地化循環: 硫同位素之研析

Biogeochemical cycling of arsenic in coastal salinized aquifers: evidence from sulfur isotope study

高雨瑄(Yu-Hsuan Kao)

指導教授 ： 劉振宇

國立臺灣大學/生物資源暨農學院/生物環境系統工程學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.03254

摘要

台灣沿海地區地下水常發生砷污染及鹽化問題，包括濁水溪沖積扇及蘭陽平原。前人研究指出此兩區地下水中，砷主要釋出機制為含砷之無晶型氧化鐵的還原溶解作用，然而鮮少研究探討鹽化地下水中硫的氧化還原循環對於砷移動性之影響。本研究以地下水中硫酸鹽的硫與氧同位素 (δ34S[SO4] 及δ18O[SO4])及沈積物中硫化物的硫同位素(δ34S[FeS2])之分析，以評估硫酸根還原作用與硫化物再氧化作用對於砷釋出機制之影響。依據δ34S[SO4] 、δ18O[SO4]、與砷濃度，可將濁水溪沖積扇的53 個地下水樣本區分為三類，分別為高砷且鹽化(Type A)、低砷且非鹽化(Type B)、與高砷且非鹽化(Type C)。在沿海地區Type A地下水因受到海水入滲的影響，當大量硫酸根進入還原性的地下水環境，伴隨著硫酸還原菌的作用，造成δ34S[SO4] 與δ18O[SO4] 的富集。以Type A 地下水含有高18O富集因子(ε18O [SO4-H2O]) 及高34S富集因子(ε34S [FeS2-SO4]) 所示，故元素硫的歧化作用(disproportionation)與含砷氫氧化鐵的還原溶解，可能為地下水中砷濃度偏高的原因。反之，Type B與Type C地下水僅呈現δ18O[SO4] 富集，此結果顯示有其他較重的氧同位素來源影響地下水中δ18O[SO4]值。以硫化物(FeS2)氧化之硫酸根18O質量平衡式估算結果顯示，Type C地下水之δ18O[SO4]含有大氣中溶氧的訊息，說明有人為超抽地下水之影響，可能導致額外的氧氣進入含水層，改變地下水氧化還原條件，並促使含砷的硫化物行再氧化作用，使得吸附於硫化物表面上的砷釋出於地下水中。同樣地，可將蘭陽平原31 個地下水樣本依濁水溪之分類方法區分成三類。Type A與 Type C地下水均含有高的18O及34S富集因子，亦說明還原環境中元素硫的歧化作用伴隨還原溶解含砷之氫氧化鐵，可能為Type A與Type C地下水中砷釋出之機制。位於扇頂補注區中的Type B地下水，其δ34S[SO4] 與δ18O[SO4] 值可能受到不同來源的硫酸鹽(例如蘭陽溪水、泉水)及黃鐵礦氧化所影響。蘭陽平原之鹽化地下水主要是受到養殖魚池水之滲入及古海水殘留所導致，與大範圍受海水入滲之濁水溪沖積扇不相似。因此，由極高之34S富集因子(ε34S [FeS2-SO4]) 說明本區存在拘限性古海洋沈積物之地層，產生顯著的硫酸鹽還原菌之還原作用，形成之硫化物可能與地下水中的砷形成共沈澱(co-precipitation)，降低的地下水中的砷濃度。此外，應用地化模式模擬於不同情境條件(如非鹽化地下水的流入、養殖魚池之有機質的入滲、抽水造成溶氧的溶入及海水入侵等)下，砷在鹽化含水層的移動，其模擬結果與硫同位素之生地化循環雷同。由本研究結果可知含砷之氫氧化鐵的還原性溶解，同時伴隨硫酸鹽的還原作用、硫化物之再氧化作用、有機碳的降解反應，均可為地層中砷含量釋出的主要途徑，而海水入侵、大量超抽地下水與古海水殘留也可能影響此兩區砷的循環之重要因子。

關鍵字

砷 ； 地下水 ； 硫同位素 ； 人為影響

並列摘要

As-contamination of groundwater, accompanied by critical salinilization, occurs in the coastal area of the Choushui river alluvial fan and Lanyang Plain, Taiwan. Statistical analyses and geochemical calculations indicate that a possible source of aqueous arsenic is the reductive dissolution of As-bearing iron oxyhydroxides. There are few reports of the influence of sulfate-sulfide redox cycling on arsenic mobility in brackish groundwater. This study evaluated the contribution of sulfate reduction and sulfide re-oxidation on As enrichment using δ34S[SO4] and δ18O[SO4] sulfur isotopic analyses of groundwater. In the Choushui river alluvial fan, fifty-three groundwater samples were divided into groups of high-As content and salinized (Type A), low-As and non-salinized (Type B), and high-As and non-salinized (Type C) groundwater, based on hydro-geochemical analysis. High18O enrichment factor (ε[SO4-H2O]) and 34S enrichment factor (ε34S[FeS2-SO4]) indicated that the disproportionation and dissimilatory sulfate reduction were both involved in the Type A groundwater. Sulfur disproportionation is an important process during the reductive dissolution of As-containing iron oxyhydroxides. In contrast to this, Type B and Type C groundwater samples showed high δ18O[SO4] and low δ34S[SO4] values under mildly reducing conditions. Based on 18O mass balance calculations, the oxygen sources of sulfate are from infiltrated atmospheric O2, caused by additional recharge of dissolved oxygen and sulfide re-oxidation. The anthropogenic influence of extensive pumping also promotes atmospheric oxygen entry into aquifers, altering redox conditions, and increasing the rate of As release into groundwater. For the Lanyang Plain, thirty-one groundwater samples are divided into three groups, which are similar with those of the Choushui river alluvial fan. High 18O enrichment factors and large range of sulfur isotope fractionations in Type A and Type C groundwater of Lanyang plain closely relate with the microbial disproportionation reactions and bacterial sulfate reduction. Samples of Type B groundwater are mostly located in mountainous recharge area, causing the mixing of different sources of sulfate. Based on the infiltration of marine brackish water from fish pond and affect of isolated brackish groundwater were responsible for groundwater salinization in Lanyang Plain. Hence, the highest δ34S[SO4] and δ18O[SO4] values caused by significant sulfate reduction of paleo-seawater in Type A groundwater of Lanyang Plain. Moreover, the results of geochemical simulations on the influence of various anthropogenic factors including mixing with non-saline natural water, organic-rich infiltration water, oxygenic rain water and sea water on As mobility in salinized groundwater agreed with the proposed biogeochemical cycle based on the sulfur isotopic analysis. The major As release mechanism by the reductive dissolution of As-Fe oxyhydroxide, accompanying with sulfate reduction, sulfide re-oxidation, carbon degradation and oxygen consumption, the extensive pumping, seawater intrusion and paleo-seawater residues govern As enrichment in saline groundwater of Choushui alluvial fan and Lanyang plain. Two conceptually models that summarized As cyclic redox reactions in the Choushui river alluvial fan and in the Lanyang plain were herein proposed to illustrate the complex interations of As in the saline and non-saline groundwater aquifers.

並列關鍵字

Arsenic ； Groundwater ； Sulfur isotope ； Anthropogenic influence

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