台灣西南海岸平原地下水砷之遷移與富集

Abstract

台灣西南海岸平原自西元1920年始，即有居民罹患烏腳病(blackfoot disease)。前人研究顯示此病係因飲用富含砷與腐植物質的地下水所致，然而砷與腐植物質的來源與富集機制之了解尚有不足。 本研究除分析28站84個地下水樣品及孔隙水83個樣品之各項水質參數(氧化還原電位、電導度、酸鹼值、鈉、鉀、鎂、鈣、硫酸根、氯離子、鐵、錳與砷，並引用穩定碳同位素與碳十四年代等資料)外，分析地表24個及鑽井岩芯534個沈積物樣品中可溶出的砷與鐵等元素(連續階段萃取法與鹽酸氫氨-磷酸萃取法)以及42個樣品有機碳含量，以了解砷與腐植物質的來源與富集機制。 鑽井岩芯孔隙水之分析結果顯示，因本研究尚難避免樣品在採樣過程中的氧化還原電位改變且取得的孔隙水樣不足，故僅能了解水樣的主要離子濃度，無法了解孔隙水砷與腐植物質的正確濃度。鑽井岩芯與地表沈積物所含可酸溶釋出之砷含量平均約5mg/kg，有機碳含量約0.3%；砷與有機碳含量與沈積環境無直接關連。唯細顆粒沈積物含量比例越高，砷與有機碳含量越高。水中砷與腐植物質濃度與相對應之層位的沈積物砷與有機碳含量無直接相關。沈積物砷與有機碳含量應非直接控制水中砷與腐植物質濃度的因素。 高濃度的砷(>0.1mg/L)與腐植物質(>200QSU)地下水大多分布於本區地下水下游之深層含水層，這些地下水之停滯時間極長。地下水中的砷主要來自鐵氧化物經還原作用而釋出。隨還原程度增高，硫酸還原產生的硫化氫，與鐵氧化物還原而釋出的砷與鐵，形成硫化砷與硫化鐵沈澱，而降低砷濃度；此一推論可由大文(II)之水井的無晶型硫化鐵沈澱物的砷含量高以及PHREEQE程式演算結果得到佐證。沈積物中的有機物，經微生物礦化作用於現地生成腐植物質，與硫化氫競爭水中的砷，使水中近90% 的砷與分子量500 Dalton以上的水溶性腐植物質錯(螯)合而保留於水中，導致本區域地下水砷與腐植物質濃度偏高。待硫化氫與被持續還原釋出的鐵形成硫化鐵沈澱而耗盡後，水中的鐵與砷濃度又逐漸提高，並與腐植物質錯(螯)合。

The Chianan coastal plain in southeastern Taiwan has long been well known for the prevalence of black-foot disease since 1920s. Previous literatures showed that this disease resulted from drinking groundwater rich in arsenic and dissolved humic substances. However, the occurrence and enrichment mechanism of arsenic and their controlling factors have not yet been studied in detail. Well water samples from a total of 84 newly established monitoring wells and 83 pore-water samples from 4 wells were analyzed in this study to investigate major chemical compositions (such as Eh, pH, EC, Na+, K+, Mg2+, Ca2+, SO42-, Cl-, ΣFe, ΣMn and ΣAs, δ13CDIC and 14CDIC-age). Extractable arsenic contents of 534 sediment samples from fully cored boreholes and 24 sediment samples from ground surface were analyzed. Total organic carbon contents of 42 sediment samples were also analyzed. The technique and equipments we used for pore-water sampling were still difficult to avoid strong oxidization during sampling to understand the distribution of arsenic and humic substances. The average acid extractable arsenic content and total organic carbon (TOC) content in both core and surface sediment samples are about 5 mg/kg and 0.3 %, respectively. Besides, arsenic and TOC contents increase with the proportions of the clay-sized fraction. However, there is no significant correlation between both the arsenic and TOC contents with depositional environment. In addition, no significant correlation was found between concentrations of arsenic and humic substances in groundwater and arsenic and TOC contents of the corresponding core sediments. This demonstrates that both arsenic and TOC contents of the sediments are not direct controlling factors for the concentrations of arsenic and humic substances in groundwater. Almost all groundwater from the deep aquifers in the down-gradient (western) side have long residence time and are relatively high in arsenic contents (> 0.1 mg/L) and humic substances (>200 QSU). Under strong reduction condition, H2S (or HS-) was generated and precipitated with either arsenic or iron as AsS2(s) or FeS(s). The black precipitates collected from Da-Wen(II) well water is high in arsenic content supporting mineral phases inferred by the PHREEQE program. In-situ generated humic substance competed with H2S (or HS-) for the arsenic. About 90% of arsenic in groundwater is combined with humic substances which are greater than 500 Dalton in molecular weight. It is not uncommon that the groundwater in the Chianan plain are both high in arsenic and humic substances. The relatively high contents of arsenic and iron in groundwater resulted from the lack of H2S (or HS-) under mild reduction condition and the content of chelated arsenic increases with that of the dissolved humic substances.