嘉南平原高砷地下水腐植物質的特性

Abstract

嘉南平原烏腳病盛行地區的地下水，含有高濃度的砷及腐植物質。早期的研究顯示，居民長期飲用當地地下水可能導致烏腳病及癌症。水體中的腐植物質具有多官能基的特性，對金屬有很強的螯合能力，形成能產生螢光的「有機金屬錯合物」，對砷在沈積物及水體間的遷移扮演著重要的角色。本研究採取嘉南地區7個觀測站的13個分層地下水樣進行研究。以分子篩配合500及5000分子量的濾膜及DAX-8樹脂分離水中之有機物，利用水樣的光學特性、碳與的氮的比值以及碳同位素，了解腐植物質的組成及來源，並探討水中砷及有機物質含量間的關係。 研究結果顯示在常溫下，水中腐植物質螢光強度有隨樣本保存時間而衰減的現象，且衰減的程度為陽光照射組＞日光燈照射組＞黑暗組。可能是腐植物質受光照而發生光化學反應導致。故欲分析腐植物質濃度的水樣必須以遮光和冷凍的方式保存，並儘快分析。本研究所採水樣的腐植物質約占溶解性有機碳(DOC)的80％，平均發光能力約為8.7 QSU/mg/L。由總鹼度、溶解無機碳的碳十四年代與穩定碳同位素，以及水樣的螢光強度和DOC濃度相比較，可以得知嘉南地區地下水中DOC主要由沈積物的有機碳轉化而來，造成越老地下水的螢光強度也越高。 利用螢光激發－發射矩陣光譜的最高峰位置，及紫外光－可見光光譜於波長 254 nm和 436 nm的吸收值來看，可以發現這些水樣中的有機物主要以陸源性的腐植物質為主，含有較高比例的芳香環成分及較強的金屬結合能力。以紫外光－可見光光譜測量腐植物質特性的方法，較不受腐植物質不均質性及水樣離子強度、pH、有機物濃度等因素的干擾，可以達到方便、快速判斷水中腐植物質的來源及組成的目的。 本研究大部分水樣的砷與分子量大於500 Da.的腐植物質結合。以DAX-8樹脂分離腐植物質的實驗發現水樣以鹽酸酸化至pH 2時，砷會從腐植物質釋出，顯示嘉南地區地下水中的砷主要與腐植物質的官能基結合，並形成「有機金屬錯合物」。砷濃度的高水樣，通常腐植物質的濃度亦偏高；但腐植物質濃度高的水樣，其砷濃度卻不一定高。因此，腐植物質可能為嘉南平原地下水中砷富集的重要控制因素之一。

It has been well known that there are high contents of arsenic and humic substances in deep groundwater of the Chianan plain. Drinking the groundwater of this area may causes cancers and blackfoot disease. Humic substances have varieties of functional groups and high metal-chelating ability, which forms organo-metallic complexes and emits fluorescence when irradiated by ultra-violet light. Humic substances may play an important role in mobilization of arsenic in sediments and groundwater. In this study, 13 groundwater samples from 7 monitoring well sites were collected to isolate humic substances by DAX-8 resin or Molecular/Por® stirred cell to explore the relationship between humic substances and arsenic in groundwater. Fluorescence characteristics and UV/visible light absorption of groundwater were also used to investigate the composition and origin of dissolved organic matters (DOMs). The results obtained show that the decay rate of fluorescence intensity decreases in the order of sunlight exposure, lamplight exposure and light-shield. Accordingly, it is suggested that samples must be frozen and store in the dark to prevent decomposition of humic substances. In average, about 80% of total dissolved organic carbon in these samples is humic substances, and their fluorescence efficiency is about 8.7 QSU per mg/L of humic substances. By combining the results of total alkalinity, C-14 age and δ13C of dissolved inorganic carbon, dissolved organic carbon and fluorescence intensity, the author suggestes that the dissolved inoeganic carbon and humic substances were derived from sedimentary organic matters. The peaks of excitation-emission matrices (EEMs) and absorption rate of UV-visible light at 250 and 436 nm suggest that that the dissolved humic substances of the Chianan plain are mainly of terrestrial origin with high contents of aromatic ring, which leads to higher metal-chelating ability. Because of the variable compositions of humic substances and background quality of groundwater samples, UV-visible spectrum analysis is an easier method to investigate the origins and compositions of humic substances than EEMs method. It has been found that dissolved arsenic of the Chianan plain is combined mainly with humic substances with molecular weights greater than 500 Dalton. Arsenic is released from humic substances when groundwater is acidified to pH 2 with HCl during humic substance isolation processes. In addition, the results show that arsenic is mainly chelated with humic substances, forming organo-metallic complex. Samples with high concentrations of arsenic often have high humic substances contents, but not vice versa. Obviously, humic substances content is one of the important factors controlling arsenic concentrations of groundwater in the Chianan plain.