應用化學及生物指標評估不同特性之環境底泥受長期砷汙染之生物有效性及毒性

Abstract

砷為 IARC歸類之第一級致癌物。臺北淡水河流域因其上游地熱谷為溫泉地帶且富含砷母岩，其排放之溫泉水砷濃度高達4 mg/L，因此導致下游淡水河流域為砷污染之高風險地區。砷進入到底泥後，會與底泥中的黏粒、有機質及金屬氧化物等形成鍵結，但當環境條件改變時，可能會再次釋放到表層水中因而對於生物體造成危害。文獻研究指出砷的移動性會受到底泥性質影響，並與生物有效形及毒性相關。現今已有許多化學分析方法被應用於評估重金屬污染底泥之危害風險，然而何者最適合用於評估砷污染底泥風險仍未知。本研究採集淡水河流域內五種性質各異的環境底泥，包括四種無砷汙染底泥及一種高砷污染底泥 (磺港溪底泥)，並應用多種化學指標及生物指標於評估不同基本性質之砷汙染底泥的砷釋出量、生物有效性及毒性。本研究將四種環境底泥 (磺港溪底泥除外) 進行人為砷添加後孵育 90日，以7日齡青鱂魚幼魚為模式生物，進行為期 14 日的流水式底泥生物暴露實驗，並藉由量測系統之表層底泥、表層水和孔隙水之砷濃度，以及利用序列萃取法及氧化鐵濾紙抽出法等化學方法，以評估底泥性質對於砷移動性及生物有效性潛力之關聯性。研究進一步將化學參數與生物指標進行相關性分析，尋找能用於預測底泥生物有效性砷及魚體毒性反應之指標。90日之底泥孵育過程中顯示，低有機質及低金屬氧化物的砂質底泥 (如中正橋底泥及外雙溪底泥) 有較高之砷釋出能力，且14日生物暴露過程中顯示，在生物暴露前之表層底泥中，低有機質及低金屬氧化物的砂質底泥中，弱鍵結的F1 及 F2 較多，因此能釋放到孔隙水中與能被氧化鐵濾紙所吸附的砷較高，但由於底泥釋出至表層水的砷會在14日暴露實驗前期隨著表層水更換而快速且大量流失，因此在實驗中後期能釋放之砷明顯降低；與之相對，高有機質及高金屬氧化物的黏質底泥 (如關渡底泥及大佳碼頭底泥) 能與砷形成穩定的鍵結避免在前期大量的流失，並於生物暴露試驗中後期逐漸被釋出，造成較高之魚體砷累積量。故相較之下，暴露試驗前期採樣之化學參數 (如孔隙水、序列萃取分析及氧化鐵濾紙抽出量) 會高估砂質底泥之砷釋出能力，而在暴露試驗後期採樣之化學參數與生物有效性砷則有較好的相關性，且以氧化鐵濾紙抽出法與生物有效性砷有最高的相關性 (R2 = 0.86)。此外，暴露於含砷底泥之魚體的 GST 及 SOD 酵素活性顯著高於空白組，其 ROS、CAT酵素活性與魚體體長及體重生長也顯著被抑制，且魚體體重抑制率為較敏感之生物毒性指標，能夠反應不同性質底泥之有效性砷差異，並與魚體砷累積量有最高的相關性 (R2 = 0.84)。高砷含量之磺港溪底泥因其生物有效性砷種類以及底泥性質與其他四種環境底泥不同，因此不適用於實驗室建立之生物暴露模型。綜上所述，高有機質及高金屬氧化物的黏質底泥之砷生物有效性較高。實驗室建立之生物暴露系統在試驗後期採集之化學參數較能預估含砷底泥對魚體造成的影響；所有化學指標中以氧化鐵濾紙抽出法與魚體砷累積量相關性最佳。本研究確立氧化鐵濾紙抽出法應用於評估不同性質之砷汙染底泥之生物有效性及毒性之可行性。

Arsenic (As) is the 20th most common element in the earth's crust, and certain places, such as Tamsui River, have been reported to have high risk of As contamination due to its geological characteristics. The highest As concentration reported in the upstream of Tamsui River was 4 mg/L, resulting in extremely high As pollution risk of the lower reaches. After As enters aquatic environments, it can be settled down through formation of chemical or physical bond with sediment particles (e.g., clay, organic matters or metal oxide). As bound to the sediment particles can be released back to the aquatic environment if the sediment is distributed or environmental conditions are altered. The remobilization and toxicity of As are strongly affected by sediment properties. Many chemical assessments have been developed to evaluate the ecological risk of heavy metal contaminated sediments, but the most suitable methods to assess the risk of As-contaminated sediment remain unknown. Therefore, this study sampled five environmental sediments in the Tamsui River, including four uncontaminated environmental sediments and one geologically As-contaminated sediment then multiple chemical and biological indicators were used to evaluate As mobility, bioavailability and toxicity of As-contaminated sediments. The uncontaminated sediment was artificially spiked and aged for 90 days. 7-day-old larvae of medaka fish (Oryzias latipes) was used as model organism in a 14-day automatic water-renewal toxicity test to evaluate the bioavailability and toxicity of As-polluted sediments. Correlation analysis of chemical analysis (Dissolved As in overlying water and pore water, sequential extraction method, iron oxide impregnated filter paper (FeOF) extraction method, etc.) and biomarkers (As body burden, growth toxicity and oxidative stress indicators) were performed to investigate the relationship between chemical predictors, bioavailability and toxicity. During 90-day aging, sediment with lower clay content, organic matters, CEC and metal oxides (e.g., ZZ and WS) showed higher As releasing capacity due to its lower binding affinity to As. The result of 14-day exposure demonstrated that sediment with lower clay content, organic matters, CEC and metal oxides had higher pore water As concentration, higher FeOF extracted amount and higher As content of F1 and F2, which represented the most bioavailable parts in sediment. However, the weakly-bound As will be released more easily into overlying water, flowing out rapidly and massively along with the renewal of overlying water. This process resulted in the lower-releasing capacity of As at the later stage of the experiment. In contrast, the sediment with higher clay content, organic matters, CEC and metal oxides (e.g., GD and DJ) can form stable bond with As, preventing As in sediment from flowing out along with overlying water renewal. These strongly-bound As would gradually be released into overlying water at the later stage of the experiment, eventually causing higher As bioaccumulation. As a result, the chemical parameter sampled at the early stage would overestimate the risk of As-contaminated sediments. Among all chemical risk assessment methods, bioavailability of As was most strongly correlated to FeOF extraction method (R2 = 0.86). As exposure significantly increased the activity of GST and SOD but reduced fish growth and the activity of ROS and CAT. Among these toxicity indicators, body weight inhibition rate was the most sensitive biomarker and was most strongly correlated to As body burden (R2 = 0.84). However, because of the significant difference of sediment properties and bioavailable As species between the geologically-contaminated sediment (HG) and the laboratory-spiked sediments, the exposure-dose-response model was not suitable for HG. Overall, our study indicated that the sediment with higher clay content, organic matters, CEC and metal oxides had higher As bioavailability. Moreover, the chemical parameter sampled at the late stage of experiment had higher correlation to As bioavailability than that sampled at early stage of experiment. Finally, the result showed that FeOF extraction method and overlying water have potential to assess As bioavailability.