底泥重金屬化學指標與生物毒性之比較

Abstract

河川中重金屬經由化學沈澱、吸附離子交換及化學錯合等反應沈積於河川底泥中，導致河川底泥貯存大量重金屬，底泥中之重金屬可能因環境因子的改變而經由脫附、溶出等反應釋出至水體，再度污染河川水質。因此，評估底泥中重金屬對底棲及水體中生物的危害性便將是一重要議題。 對於河川重金屬污染之評估，已由過去僅以水質重金屬含量分析，到針對河川底泥重金屬總量之分析，但由於底泥中重金屬再釋出之潛能取決於重金屬與底泥間的鍵結型態，僅移動性高的重金屬能對環境中的生物造成危害，因此本研究討論底泥重金屬之結合型態，由過去底泥重金屬總量分析，再進一步探討底泥移動相重金屬之含量。並以Microtox®生物毒性試驗評估底泥重金屬對螢光菌抑制的情形作為本研究中之生物指標。 另外，再以過去相關文獻中建立之各底泥化學毒性指標如酸揮發性硫化物與同步萃取之重金屬（Simultaneously Extractable Metals /Acid Volatile Sulfide, SEM/AVS）、酸抑制力與酸產生力（Acid Consuming Capacity-Acid Producing Potential, ACC-APP）、利用底泥標準PEL(Probable Effect Level)計算出之毒性單位（Toxic Unit, T.U.）與以分佈係數（Partition Coefficient, KD）換算之底泥標準等指標評估底泥的污染程度並與螢光菌生物毒性結果作一相關性之分析。 本研究結果顯示，鎳、鋅、鉻與鉛之移動相含量與毒性之相關性相較於總量與毒性之相關性皆有較顯著之相關性。因此，以移動相含量評估底泥生物毒性，將會較總量評估得到更有力之資訊。而銅之碳酸鹽相含量卻呈現著促進螢光的趨勢。以主成分分析（Principal Component Analysis, PCA）與因素分析（Factor Analysis, FA）分析後將各重金屬移動相依各金屬對於螢光菌抑制的程度予以分類，第一類為鋅、鉻與鎳，第二類則為鉛，而第三類為銅。 以化學性之移動相進而定義生物性之生物可利用相，則因為重金屬之不同而有差異，鋅與鉛較適合以可交換相、碳酸鹽相與鐵錳氧化相來定義生物可利用相，鎳與鉻則適合以可交換相與碳酸鹽相來代表生物可利用相。 在其餘化學毒性指標中，T.U.值之評估包含了底泥標準建立時所依據的龐大數據，評估時僅需以重金屬總量就足夠，最為方便。ACC-APP指標之評估後發現共五個採樣點之底泥呈現污染性，因此可將ACC-APP指標視為一般性的評估方式。以SEM/AVS指標評估底泥後，僅一個採樣點為呈現污染性，該採樣點同時為毒性最顯著之底泥，因此，若SEM/AVS評估為污染性底泥，則可判斷該底泥造成螢光菌毒性污染之可能性極高。因此，若以化學毒性指標評估底泥毒性，可先以ACC-APP指標做初步篩檢底泥之毒性，再以SEM/AVS指標作最後的評估。

Most of heavy metals discharged into the river are found to deposit in bottom sediments via the complex physical, chemical and biological processes. However, the accumulated heavy metals may be remobilized from sediments by natural and man-made process, which can pose a risk to the aquatic ecosystem. Mobility and toxicity of metals associated with sediments are generally affected by metal speciation and sediment compositions. Due to the various speciation of heavy metals in sediments, it is not reliable to assess the potential toxicity of heavy metals on aquatic environment with the total concentration of heavy metals in sediments. In this study, the contamination and potential toxicity of heavy metals in sediments were evaluated by chemical and biological assessments. The results show that the evaluation of mobile phase metal was better than total metal in the comparison with bio-toxicity (Microtox®) test. However, exchangeable, carbonates and Fe-Mn oxides phases of zinc and lead were able to be considered as the bioavailable phase of zinc and lead. And, exchangeable and carbonates phases of nickel and chromium could be considered as the bioavailable phase of nickel and chromium. Besides, other chemical assessments showed that evaluation of Toxic Unit was the most convenient method. On the other hand, it was found that difference between ACC (Acid Consuming Capacity) and APP (Acid Producing Potential), defined as ACC-APP, would be useful for long-term toxicity assessment. But the SEM/AVS could be considered as an important parameter for acute toxicity assessment.