淡水河流域及其河口近岸海域沉積物汞之時空變化

Abstract

本研究分析2009年淡水河流域及其河口近岸海域之表層沉積物中總汞、鋁、銅、鋅、鉛、鎘、有機碳、總氮等含量及粒徑分布，以了解總汞在研究區域內空間分布及時間變化、污染情形與通量變化。本研究結果顯示表層沉積物總汞濃度平均值為61 ± 42 ng g-1 (範圍介於6 ~ 284 ng g-1)，而總汞濃度最高值分別位於主河道鄰近獅子頭抽水站及近岸海域之隸屬八里污水處理廠放流管附近區域，其主要受到上述設施排放大量含汞之廢污水所致。另一方面，表層沉積物顆粒大小及有機碳分別與總汞具有良好的正向線性關係，表示顆粒大小及有機碳皆為影響總汞空間分布變化的主要控制因子。在近岸海域其總汞濃度之時間變化的部分，根據單因子變異數分析(One-way ANOVA)的結果顯示2009年近岸海域內表層沉積物總汞並無明顯的顯著差異，即無明顯的季節變化性存在。由本研究區域內表層沉積物總汞的EF (0.18 ~ 6.47)及Igeo (-3.32 ~ 2.24)之結果顯示，淡水河流域之主河道靠近獅子頭抽水站之河段及河口近岸海域的放流管附近區域皆有明顯的汞富集現象存在。另外，主成份分析(PCA)的結果除了指出顆粒大小及有機碳為影響表層沉積物總汞及其他重金屬之空間分布的主要二個因子之外，同時顯示本研究區域內重金屬總汞、銅、鋅、鉛、鎘的污染情形亦相當嚴重。最後，藉由本研究結果以估算近岸海域之海洋放流管附近區域的正常年年度顆粒態及總汞通量。就總汞而言，通量計算所概括之區域內主要的總汞輸入源為海洋放流管( ~428 kg yr-1，占總輸入量之88%)及淡水河(56 kg yr-1，占總輸入量之12 %)；另一方面，主要的輸出源為此區域內總汞經沉降並累積於表層沉積物的部分為5 kg yr-1，而經計算而得到之移出此區域送入外海的總汞通量約為479 kg yr-1，其中顆粒態汞約238 kg yr-1 (占總汞49.7 %)。其顯示自淡水河及海洋排放管所輸入的總汞大約有98.9% (顆粒態汞：97.9 %)在海洋放流管附近區域被移除。儘管有高達 98 %以上的顆粒態或總汞被移除，但遺留下來的部分仍造成近岸海域受到相當程度的汞污染，顯示八里污水處理廠之海洋放流管的放流水對淡水河流域及其河口近岸海域之影響不言而喻。The spatial and temporal distributions of Hg in surficial sediments in the DanShuei River and adjacent coastal area (i.e., ocean outfall area) off the northern Taiwan were investigated in 2009. In addition, heavy metals (Al, Cu, Cd, Pb, Zn) and geochemical parameters (TOC, TN, grain size) were measured to examine the controlling mechanisms, to assess the extent of Hg contamination and further estimate the one-box Hg budget around the ocean outfall area. Results showed that total Hg in all sediments ranged from 6 to 284 ng g-1 and averaged 61 ± 42 ng g-1 (n=93). The high Hg levels were observed within the River near Shizitou wastewater pumping station, and ocean outfall of the Bali wastewater treatment plant (Bali WTP), respectively. The Hg well correlated with mud fraction (< 63μm) and TOC in sediments, suggesting they both significantly affect the spatial distribution of Hg. According to one-way ANOVA analysis, there were no significant differences in seasonal changes of Hg around the coastal area. The assessments of enrichment factor (0.18 ~ 6.47) and geoaccumulation index (-3.32 ~ 2.24) of Hg both showed that contamination of Hg in the middle of the DanShuei estuarine and coastal areas near the ocean outfall are severe. That is due to the Hg inputs into the estuarine and near-shore environments of the DanShuei River as a result of human activities (e.g., wastewater discharges). Furthermore, through the analysis of PCA, the results not only further demonstrate both fine fractions and TOC regulating the spatial distribution of Hg but also assure the DanShuei River and adjacent coastal area receiving trace metal pollution (e.g., Cu, Cd, Hg, Zn). Finally, based on the Hg data obtained in this study, the one-box annual particulate and total Hg budgets around the ocean outfall area were constructed. The principal inputs of total Hg( ~484 kg yr-1) to the ocean outfall area are from the DanShuei River ( ~56 kg yr-1; 12 % of total inputs) and the Bali WTP ( ~428 kg yr-1; 88 %), respectively. On the other hand, the major sink of total Hg into surficial sediments is ~5 kg yr-1. Consequently, about 99 % ( ~479 kg yr-1) of the total Hg inputs was removed away from the ocean outfall area.