土壤中砷、汞生物可及性、移動性、及健康風險評估

Abstract

因土壤與地下水污染，台灣環保署已經公告超過2,900個整治場址及控制場址，表示台灣居民正暴露於一個高風險的環境下。土壤直接攝入為重金屬經污染土壤進入人體的途徑之一 ，其中又以六歲以下孩童容易將手放入口部行為使孩童的暴露風險高於成人，因此有必要進行污染土壤直接攝入的健康風險評估。然而，以往的健康風險評估皆基於土壤中污染物的總量做為計算，明顯造成風險結果過於高估；已經有研究指出當誤食重金屬污染土壤時，並非污染土壤中的所有重金屬皆會被人體吸收，需要考慮重金屬之生物有效性(bioavailability)，然而體內萃取試驗(in-vivo test)費時且成本高，進而發展出體外萃取試驗(in-vitro test)，以生物可及性 (bioaccessibility)評估取代。 為了得到更為合理的健康風險評估結果，本研究以台灣之砷、汞污染土壤場址為例，包括台北關渡平原天然砷污染場址、台南安順、高雄前鎮汞污染場址以及屏東赤山巖砷、汞污染場址，以生理萃取試驗法(physiologically based extraction test, PBET)及(simplified bioaccessibility extraction test, SBET)兩種體外萃取試驗進行生物可及性評估 ，再以序列萃取法(sequential extraction procedure, SEP)探討重金屬的化學型態與生物可及性濃度的關係，進而提供建立土壤中重金屬對於孩童的健康風險評估中暴露劑量的參考。實驗結果顯示As在模擬胃腸道中之生物可及性與非專一性吸附、專一性吸附有顯著正相關(p<0.05); Hg 之生物可及性則與水可溶解態、可交換態亦有顯著正相關(p<0.01) 。土壤有機質、粘粒與As、Hg之生物可及性濃度有顯著負相關(p<0.05)。 最後，將所得之生物可及性作為本土砷、汞污染場址健康風險評估中暴露劑量之參數，顯示健康風險值確實降低 ，此結果可提供政府管理單位在未來對污染場址管理與決策做為更適當的參考依據。

Taiwan Environmental Protection Administration has declared more than 2,900 control and remediation sites due to soil and groundwater contamination. People who live on or close to these contaminated sites may have high exposure risk. Moreover, higher-frequency hand-to-mouthing behaviors could lead children under six years old to greater health risk than adults due to direct ingestion. Hence, health risk assessment is necessary for children living in the neighborhood of the contaminated sites. However, the assessment based on total content of contaminants may overestimate the risk; consequently, it is important to determine bioavailable metals in soils. Although in-vivo approaches reflect quasi-physiological conditions, these methods are expensive and time-consuming. Therefore, in-vitro methods are developed and recognized as fast screening tools in assessing bioaccessibility of metals in the soils of contaminated sites. In this study, eight asenic (As)-contaminated and mercury (Hg)-contaminated soils were collected from four different sites in Taiwan (i.e., Guandu, Cianjhen, Anshun and Chishan-Yen sites). One of the objectives is to provide more proper results for exposure assessment via acquisition of As and Hg bioaccessibility by using two in-vitro assays, namely physiologically based extraction test (PBET) and simplified bioaccessibility extraction test (SBET). Moreover, sequential extraction procedure (SEP) was employed to understand the solubilities and motilities of As and Hg in the soils. The experimental results showed that positive significant (P<0.05) relationships were found between As proportions in fraction 1, fraction 2 of SEP and As bioaccessibility; the positive significant (P<0.01) relationships was also established between Hg proportions in fraction 1, fraction 2 of SEP and Hg bioaccessibility. The soil properties (e.g., organic carbon and clay content) had significant (P<0.05) correlation with As and Hg bioaccessibility. As expected, the risk results after adjustment by bioaccessibility were lower than that obtained based on total metal contents. Results obtained in this research provide suggestions to the decision makers on setting new strategies in the risk management of metal-contaminated soil sites.