嘉南地區地下水砷濃度之研究

Arsenic Concentrations of Groundwater in the Chianan Plain, Taiwan

歐東坤(Tung-Kun Ou)

指導教授 ： 劉聰桂

國立臺灣大學/理學院/地質科學研究所/碩士(2005年)

https://doi.org/10.6342/NTU.2005.01828

摘要

1970年代的研究即顯示台灣嘉南平原，尤其是沿海地區，地下水砷濃度偏高。當時調查分析之水樣採自民間鑽鑿的水井，可能為不同含水層之混合水。本研究分析地下水觀測網計畫所建置的28個觀測站總共84個分層觀測井水樣，包括在野外測量溶氧 (DO)、氧化還原電位 (Eh)、pH、電導度 (EC)；室內分析總鹼度、陰離子 (Cl-、SO42-)、陽離子 (Na+、K+、Mg2+、Ca2+、總鐵、總錳、總砷濃度) 與螢光強度等項目，以了解本區地下水砷濃度分布之全貌及其與沈積環境的關係，進而探討砷與其他地球化學參數間的關係，瞭解本區砷釋放的機制。
本研究結果顯示，高砷濃度的地下水處於極還原的缺氧環境 (溶氧皆小於1 mg/l)，氧化還原電位皆小於 -110 mV；大部分呈微鹼性。第二、三與四含水層水的砷濃度普遍明顯比第一含水層高，且在八掌溪、急水溪、曾文溪及鹽水溪下游一帶較高。與以往取用民井水樣分析所得的砷濃度分布型態大體上一致。本研究也顯示，在河口灣相地層內的地下水，砷濃度普遍偏高。
本研究利用分子篩過濾法分離地下水中的腐植物質。選用五個不同分子量的篩膜，分離出六個不同分子量範圍的腐植物質。結果顯示腐植物質的分子量主要大部分介於500∼10000 Da.之間，約佔總螢光強度的50∼90％。腐植物質濃度較高的水樣，腐植物質的分子量以1000∼5000 Da.居多。砷主要與分子量介於500∼10000 Da.間的腐植物質結合，約佔總砷濃度的40∼90％；而當水樣的砷濃度較高時，砷亦比較集中在1000∼5000 Da.的腐植物質內。顯示砷在地下水中應是與腐植物質結合。
本區地下水高濃度砷的出現，最重要機制研判是：原本沈積於海陸交界 (尤其是河口灣) 的鐵錳氧化物，因地層轉變為還原狀態而將吸附的砷釋放至水中。海陸相交界的河口灣沈積物，一方面有機物及黏土含量較高，一方面河水中鐵、錳離子因入海時酸鹼度、鹽度改變而易產生沈澱。鐵錳氧化物、黏土礦物及有機物具有強吸附力，能將水中的砷一起沈澱下來。在地層深埋之後，有機物在細菌長時間的新陳代謝下，使得地下水環境慢慢轉變為還原環境，鐵錳的氧化物與氫氧化物，以及有機物與黏土礦物，在還原環境下將砷釋放至水中，使得水中砷濃度增高。當還原程度高時，硫酸根還原而生成的硫離子和水中的鐵離子形成黑色硫化鐵沈澱，使水中的鐵離子濃度大為降低。但釋出至水中的砷因為又與腐植物質結合而大部份仍溶解在水中，造成水中砷濃度高而鐵濃度低的現象。另外，高HCO3-濃度和高pH值也可以造成鐵氧化物表面所吸附的砷釋出，此論點亦可由砷濃度與HCO3-濃度及pH值有正相關性而得到支持。

關鍵字

嘉南地區 ； 地下水 ； 砷

並列摘要

The Chianan plain in SW Taiwan was well known for its high concentrations of arsenic and humic substances in groundwater, which were considered to be responsible for the endemic Blackfoot disease. Eighty-four groundwater samples were collected from aquifers. Dissolved oxygen (DO), redox potential (Eh), pH and electric conductivity (EC) were measured on-site; major anions (Cl- and SO42-), cations (Na+, K+, Mg2+, Ca2+, ΣFe, ΣMn and ΣAs), and fluorescence intensities were measured in the laboratory. The objectives of this study are the following: (1) to determine geochemical conditions of the aquifers; (2) to obtain the distributions of arsenic concentrations and relationship between arsenic concentration and depositional environments; (3) to understand the mechanism of arsenic release to groundwater.
Groundwaters of high arsenic concentration are characterized by low DO (< 1 mg/l), low ORP (< -110 mV) and pH＞7. Arsenic concentrations of the 2nd, 3rd and 4th aquifers, especially in the downstream areas of the Pa-Cheng Stream, Chi-Shui Stream, Tseng-Wen Stream and Yen-Shui Stream, are obviously higher than the 1st aquifer. The arsenic distribution pattern of the 2nd, 3rd and 4th aquifers are similar to that obtained from the private wells. Water samples collected from the estuarine strata were generally high in arsenic contents.
This study used molecular membrane ultrafiltration technique to separate the humic substances into six different molecular weight intervals. Most of the humic substances (ca. 50-90％) have molecular weights of 500-10000 daltons. The distribution pattern of water arsenic distribution in each aquifer was simlar to that of humic substances. The positive correlation between humic substance and arsenic concentrations suggest that arsenic was combined with dissolved humic substances.
The main mechanism responsible for high arsenic concentration of groundwater in the Chianan area was inferred to be the reductive the desorption of arsenic under reducing environment from Fe and Mn oxides previously deposited in the estuary. Estuarine, the border between marine facies and continental facies sediments contain the high content of organic matter and clay minerals, and precipitation of Fe and Mn ions result from alternation of ph and salinity when the river flow into the sea. The very high adsorption capacity of Fe oxides, Mn oxides, clay minerals and organic matters result precipitation of aqueous arsenic. Because after burial of sediment microbial metabolism of organic matter produces a reducing condition. In reducing condition, arsenic was released into solution by Fe- and Mn-oxyhydroxides, clay minerals and organic matters and lead to increase concentration of arsenic. During SO42- reduction, the consequent S2- reacts with Fe2+ to produce FeS and ultimately to pyrite (FeS2), decreasing iron concentraction in groundwater. Because the greater part of arsenic which combines with humic substance dissolve into groundwater, negative correlation exists between arsenic and iron. High HCO3- concentration and high pH mobilize arsenic from surface of iron oxides. The very strong relationship between arsenic and HCO3- concentration and pH support the hypothesis.

並列關鍵字

Chianan ； groundwater ； arsenic

參考文獻

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