探討土壤中鎘、鉛之健康風險評估−以臺灣桃園農地土壤為例

Abstract

截至目前為止，臺灣環保署列管土壤污染場址中，以重金屬污染佔比最高。長期引灌含有重金屬的廢水，會導致土壤中重金屬的累積，並經由誤食土壤、吸入揚塵、皮膚接觸或透過食物鏈等暴露途徑影響人體健康，因此土壤重金屬污染問題不容小覷。本研究以桃園蘆竹鄉福興段與德林段的污染農地為例，藉由土壤性質分析、序列萃取法以及體外萃取試驗，探討農地土壤性質、土壤中鎘、鉛化學型態及生物可及性之間的關係。此外，考量農地休耕現況，目標受體(孩童與成人)暴露到鎘、鉛主要經由誤食土壤、皮膚接觸、吸入揚塵以及日常食物攝入，進行健康風險評估。 由土壤性質結果顯示，福興段土壤pH約為4.89、有機質含量3.44%、黏粒含量13.9%，為砂質壤土或壤土；德林段土壤pH約為6.32、有機質含量1.56%、黏粒含量40.2%，為黏質壤土或壤土。此外，在福興段的土壤中，重金屬鎘主要以酸可溶解態存在，其次是殘餘態；德林段中則以酸可溶解態與鐵錳氧化態為主。在兩段土壤中，重金屬鉛則是以鐵錳氧化態與有機質氧化態為主。在不考慮參數分布的情況下，健康風險評估結果顯示，誤食土壤貢獻一定的風險比例(鎘: 13.4−50.2%; 鉛: 61.9−95.2%)，因此土壤重金屬的生物可及性有必要被考量，以提高評估的準確度，使模擬結果更符合實際狀況。此外，經蒙地卡羅模擬可知，鎘的機率分布主要受到飲食習慣的影響，而鉛則是與土壤的暴露有較明顯的關係。整體而言，當生物可及性被考量時，經由誤食土壤暴露到重金屬鎘、鉛的風險比例會降低。因此建議在未來的健康風險評估中，應加入生物可及性之概念，提高評估的真實性，而本研究之結果可供為政府與相關單位在未來對污染場址管理與決策的參考依據。

Based on the statistical data of Taiwan Environmental Protection Administration, soil contaminated by heavy metal was severe in a number of control and remediation sites. With long-term irrigation by water containing heavy metals, these metals would accumulate in soil and then cause health injury through the oral soil, dermal contact, inhalation, and other exposure pathways. Therefore, the exposure to metal-contaminated soils is highly needed to be noticed. In this study, soil samples were collected from contaminated farmland in Fuxing and Delin sections of Taoyuan City in Taiwan. Soil property analysis, sequential extraction procedure (SEP), and two in-vitro assays, namely physiologically based extraction test (PBET) and simplified bioaccessibility extraction test (SBET), were used to discuss the relationship between soil properties, metal speciation in soil, and the bioaccessibility of Cd and Pb. Moreover, the target subjects, namely the children and the adults, were evaluated assuming mainly exposing to Cd and Pb through the oral soil, dermal contact, inhalation, and daily food ingestion. The adverse health effects of exposure to soils containing Cd and Pb were then evaluated based on risk assessment approaches. The results of soil property analysis showed that soil texture of Fuxing section was sandy loam and loam (pH = 4.89, organic content = 3.44%, clay = 13.9%), and that of Delin section was clay loam and loam (pH = 6.32, organic content = 1.56%, clay = 40.2%). In Fuxing section, the main speciation of Cd was residual (55.6%) and acid-exchangeable (44.4%); in Delin section, the major forms of Cd was acid-exchangeable (48.8%), followed by Fe/Mn oxides (35.5%). Both in Fuxing and Delin sections, the major form of Pb was Fe/Mn oxides, followed by residual ones. Without considering the distribution of parameters, the health risk assessment results showed that the risk ratio of oral soil exposure occupied a significant proportion (Cd: 13.4−50.2%; Pb: 61.9−95.2%); so the bioaccessibility of heavy metals in soil should be considered to increase the accuracy of assessment results and make the simulation more realistic. Additionally, according to the Monte Carlo simulation, the probability density distribution of Cd was principally affected by the dietary habits, and Pb was mainly from soil exposure. In general, when the bioaccessibility of heavy metals was applied to the risk assessment, the risk ratio of oral soil would decrease. Consequently, the results of this study could be a reference for government and associated organizers to reasonably manage the contaminated sites based on health risk assessment approaches.