台灣西南沿海地區地層環境中砷之來源與釋出機制

Abstract

台灣西南沿海地區之嘉南平原早在1960年代便曾出現大規模烏腳病疫情，經研究證實其與深層地下水中之無機砷有關。此外，自1992至2005年，於緊鄰嘉南平原之濁水溪沖積扇南翼淺層地下水中，亦檢測發現含有高砷濃度，目前當地居民雖已不再直接飲用含砷地下水，但仍大量抽取地下水用以灌溉、養殖、公共、民生給水等多項用水標的，此舉可能造成砷透過食物鏈累積，進而對人體造成健康危害，故本研究之目的為探討此區高砷地下水之來源與其釋出機制。本研究針對濁水溪沖積扇南端之13口地質鑽探井，分析地層岩心之砷含量分布，並比對現有之水文地質資料，建立地層中砷存在與地質年代及地層地化環境之相關性，探討地層環境中砷可能之來源。進而針對嘉南平原之33個地下水觀測站水質資料，利用因子分析法歸納影響水質結構之主要因子，並結合地下水氧化還原狀態之判定及地球化學模式之計算，與濁水溪沖積扇之結果比較，進而推測此兩區域之地下水砷可能釋出機制之異同。最後利用高砷含量之地下水培養與砷釋出相關之鐵還原菌，將其添加至合成之含砷氫氧化鐵之批次實驗中，隨時間量測物種砷與二價鐵之濃度變化，藉以歸納砷於地層環境中之釋出過程與物種變化，及鐵還原菌於此循環過程中所扮演之角色。 由濁水溪沖積扇南翼之地質岩心分析結果顯示其總砷含量較一般土壤高，且海相層之地層岩心總砷濃度大多較陸相層高，並呈現由西向東遞減之趨勢。再者，不論出現高砷含量之深淺，其岩心之地質年代大多約介於7,000-9,000年前間之上部海相層，且主要為泥質地層，而由阻水層岩心與含水層岩心總砷含量分別與地下水總砷含量進行迴歸分析之結果中，更可發現地下水中之砷含量不僅與岩心總砷含量有關，地下水中高砷含量之來源更可能為阻水層岩心之砷含量，由此推測距今最近一次之全新世冰河時期，其海進海退造成大量的砷累積於潟湖相之沉積層中。濁水溪沖積扇與嘉南平原地下水之主要特性皆為海水導致之鹽化與高砷污染，但此兩區地下水氧化還原狀態之空間分部則出現相反之情形，主要為水文地質條件之差異所致。此兩區大量抽取地下水不僅可能造成地下水鹽化，亦可能導致地下水氧化還原狀態改變，進而促使地層中含砷之鐵氧化物的還原與溶解，導致砷釋出於地下水中。此外由批次實驗之結果可發現，固相之氫氧化鐵與液相之五價砷皆受鐵還原菌影響而呈現還原反應，且於含砷氫氧化鐵之實驗組別中得知，大分子有機碳易與五價砷競爭氫氧化鐵表面之吸附位置，而導致五價砷脫附至水溶液中，而小分子有機碳則較利於鐵還原菌作用於氫氧化鐵之還原溶解，進而減少五價砷之吸附表面而使五價砷釋放至水溶液中。當氫氧化鐵之還原溶解達平衡時，水溶液中已釋出之五價砷進而成為鐵還原菌之電子接受者，故存在於地層環境中之鐵還原菌不僅影響砷釋出之地下水中，更可能改變水中物種砷之價態。本研究提出台灣西南沿海高砷地化環境之概念模式，有助於吾人了解砷在地下水系統中之釋出與遷移，及在整體地質環境中之宿命及循環過程。

High arsenic (As) contents in groundwater were found in two neighboring catchments of southwestern Taiwan- the southern Choushui river alluvial fan and the Chianan plain. To investigate the sources of As in groundwater, a total of 655 geological core samples from 13 drilling wells situated at the southern Choushui river alluvial fan of Taiwan were collected and analyzed. High As contents were found primarily in aquitards, to a maximum of 590 mg/kg. The contents were correlated with the locations of the marine sequences. Additionally, strong correlations among the As concentrations of core samples, the clay, and the geological age of the Holocene transgression were identified. Most of the As in groundwater originated from the aquitard of the marine sequence. The high As content in marine formations with high clay contents may be attributable to the bioaccumulation of As in the sea organisms, which accrued and were deposited in the formations. A preliminary geogenic model of the origin of the high As concentration in the shallow sedimentary basin of the Choushui river alluvial fan of Taiwan is proposed. Furthermore, the groundwater quality, the redox potential and the As distribution of the Chianan plain were characterized using factor analysis, redox zoning and a geochemical program, PHREEQC. The results were compared with those of the southern Choushui river alluvial fan to allude the possible release mechanisms of As in groundwater of the southwestern Taiwan. Factors 1 and 2 determined by the factor analysis of the groundwater in the Chianan plain - the salinization factor and the As pollutant factor - are similar to those in the southern Choushui river alluvial fan. However, the spatial distribution of reductive tendency in the Chianan plain is different from that in the Choushui river alluvial fan, yielding spatially distinct hydrogeochemical environments in these two neighboring areas. The reduction potential in the Chianan plain is stronger than that in the Choushui river alluvial fan. The difference of the reduction potentials between these two vicinal areas affects the distribution of As concentrations in groundwater. The reductive dissolution of As-rich iron oxyhydroxide is postulated to be the major mechanism of the release of As to the groundwater in the Chianan plain and the Choushui river alluvial fan of Taiwan. The release mechanism is generally driven by reducing bacteria. Experiments approach was carried out by coupled synthetic As-contained amorphous iron oxide (HFO) and inoculation of iron-reducing bacteria (IRB) to evaluate the contribution of IRB on the mobilization and transformation of As. The results of respective HFO and As5+ reducing experiments show that both of the reduction reactions are promoted by IRB. However, the rate and extent of As5+ reduction are similar with different treatments of organic substrates. In As-contained HFO reducing experiment, both the IRB and the competition of organic carbon play important roles on As desorption and mobilization. Sequentially, aqueous As5+ becomes an electron acceptor after that the solid phase of ferric iron has been reduced, but the conversion of As5+ to As3+ were affected by different treatments of organic substrates. Hence, Fe-related reducing bacteria may be capable of reducing aqueous As5+ after the reduction of iron minerals. The working hypothesis model of As biogeochemical cycling proposed by this study sets up the framework for investigation of the fate and transport of As in the groundwater of the southwestern Taiwan.