台灣西南沿海地區地下水砷之生物地化特徵及循環

Abstract

本研究進行詳細生物地質化學分析以了解影響台灣西南沿海地區含水層中砷分布特徵及循環。此區地下水主要為鈉-鈣-碳酸氫根型態，碳酸氫根為主要陰離子，地下水中砷濃度最高可達562.7 μg/L，且以三價砷為主。遭受鹽化的水樣集中於淺層含水層並含有高砷濃度，然而統計分析或實驗結果皆無法證明鹽化與高砷濃度之相關性。利用因子分析可將本區地下水水質分為四個主要因子，包含鹽化、鐵還原溶解、砷還原及化學潛勢。黏土礦物為本區地層中砷最主要的儲源，而連續萃取結果指出於扇尾地區固相砷平均分布於不同操作相別，扇央地區則集中於鐵氧化合物中，鐵氧化合物及含砷硫化物為此區最主要含砷化合物。本研究使用的鐵還原菌能同時還原固相砷及鐵化合物，並進一步證實鐵化合物之生物可利用性對於地層中砷釋出的重要性。利用孵育實驗證實，添加有機碳可改變砷與鐵的還原序列，而含砷鐵化合物的螯合溶解為另一種現地環境中造成砷釋出的可能機制。由脫附實驗結果得知，土樣中砷的價數及不同添加陰離子，皆可影響砷的脫附行為，於添加相同濃度時，磷酸根較碳酸氫根造成較多砷脫附至溶液中，但於現地環境中，因碳酸氫根濃度遠高於磷酸根濃度，故碳酸氫根對於砷脫附影響將遠大於磷酸根，此結果也可用於解釋水質分析中砷與鐵濃度之不一致性。總而言之，微生物驅動的鐵化合物溶解為造成此區地下水中高砷濃度之主要原因。利用區別分析所得之結果，可藉由簡單現地量測項目(pH和Eh)可初步評估此區地下水中砷濃度是否超過飲用水標準(10 μg/L)。

Comprehensive biogeochemical studies have been carried out to elucidate the controlling factors on the partition and the cycling of arsenic (As) within coastal aquifers in the southwestern part of Taiwan. Most groundwater samples were characterized as Na-Ca-HCO3 with HCO3- as the dominant anion. Total arsenic (As) concentration, predominantly as As3+, ranged from <1.0-562.7 μg/L. Saline water type, which was mostly concentrated in the uppermost aquifer in the study area, generally retained highly dissolved As concentrations, albeit no correlation was observed among the analyzed parameters and leaching experimental data. Factor analysis proposed a four-factor model, comprising salination, reductive dissolution of Fe/Mn oxyhydroxides, As reduction and chemical potential factor, and explained 89.94% of total variance in groundwater. Clay minerals were evidenced as the main pools for sedimentary As. Sequential extraction data indicated the partitions of As in the distal-fan were distributed evenly, independently of the weakly adsorbed As phase, but high As fractions were concentrated in amorphous and less crystalline Fe hydroxides phase in the mid-fan. Iron oxyhydroxides were regarded as the dominant sinks sorbed As as well as As-bearing sulfides in the well screen level. Stable cultures of iron-reducing bacteria (IRB) were capable of reducing both Fe and As within sediments, and further shed light on the influence of bioavailable Fe minerals. While suspending in a non-sterile, in-situ groundwater, addition of acetate as carbon source would affect the reactive pathways of Fe and As, resulting in the decoupled processes of these two redox couples. At this circumstance, solubilizing Fe(III) accompanied a significant increase of As(III) concentration implicated another release pathway for converting sedimentary As into groundwater during the onset of reducing environment. Desorption behaviors of As were relevant to its valence in the sediments and the co-existence of anions. Although bicarbonate addition resulted in less As desorption than that of phosphate on a molar basis, the contribution of bicarbonate to the total release of As was still greater than phosphate due to the much higher concentration of bicarbonate in groundwater. This result also in part explained the poor correlation between Fe and As concentrations in groundwater. As a consequence, the dissolution of As-related Fe oxyhydroxides mediated by microbial activities was the prerequisite for elevated concentration of As in the subsurface. Two-parameter (pH and Eh) model derived from discriminant analysis can be used for preliminary assessment to determine whether the As concentration exceeds 10 μg/L with simple field measurements in this area.