非飽和層土壤中殘留之三氯乙烯污染物濃度對地下水影響探討

一般而言，地下水含氯揮發性有機化合物（Chlorinated Volatile Organic Compounds, CVOCs）之污染整治難度相當高，例如，三氯乙烯、四氯乙烯等，由於其歸類於所謂「重質非水相液體（Dense Non-Aqueous Phase Liquids, DNAPLs）」，比重大於水（> 1），在水中的溶解度（Solubility）低，一旦從地表洩漏滲入到土壤裡，在重力的作用下會不斷地向下沈移進入含水層，直至低透水性或不透水層為止。在下沈移動的路徑過程中，DNAPLs常以四種相態，包括「揮發性氣態（Gaseous Vapor）」、「水相溶解態（Dissolved Phase）」、「殘留態（Residual）」，以及「自由相（Free-phase）」等分佈於土壤及地下水環境中，如圖I所示。其中仍殘存於非飽和土壤層（Vadose Zone）內之濃度測值（mg/kg）通常並不高，且大多相較於現行國內土壤污染管制標準明顯為低，因此，經常可看到一些國內CVOCs污染調查案例裡，土壤之濃度測值往往因依據土壤污染管制標準的概念而被忽略其對地下水的影響衝擊程度及潛在於場址內的污染意涵。另一方面，目前針對非飽和層之「土壤氣體（Soil Gas）」調查與評估方法裡仍似尚不足夠，一般在國內常見的「薄膜介面探測（MIP）」或土壤採樣篩測所使用的PID/FID等攜帶型偵測器，在污染物之定性與定量的即時分析上仍顯不足，所謂「好的開始是成功的一半」，重視土壤層內殘留CVOCs污染量之分析調查即是所謂好的開始，對於建構一個有效的場址污染概念模型（Conceptual model）應具有其重要性。本研究之目的係透過國內一處土壤及地下水受三氯乙烯污染之實場案例具體探討上述非飽和土壤層內的殘留污染量之影響問題，本案例採用「排土法及現場土壤篩分滾動的氣相抽除法」進行熱區土壤層內三氯乙烯的清除，而三氯乙烯的處理標準是否能採用現行土壤管制標準，同時回填土是否會再造成地下水之污染測值超過現行管制標準等問題，期以透過本研究建立一實場評估方法之外，也提供未來其他場址進行土壤分析調查之評估參考，以利於研擬更有效的整治策略；本實驗方法包括先建立現場PID（光離子偵測器）與實驗室GC（氣相層析儀）分析結果之間的相關性，進而再測試與分析評估回填土接觸地下水後從土壤釋出進入地下水之平衡濃度結果，以決定現場回填土之品管要求。

關鍵字

地下水含氯揮發性有機化合物；土壤氣體；光離子偵測器；氣相層析儀；場址污染概念模型

並列摘要

In general, the pollution treatment of groundwater-containing Chloro-Volatile Organic Compounds (CVOCs) is quite difficult, for example, trichloroethylene, tetrachloroethylene, etc., because it is classified as a so-called "heavy non-aqueous liquid. (Dense Non-Aqueous Phase Liquids, DNAPLs), the specific gravity is greater than water (> 1), and the solubility in water is low (Solubility). Once it leaks into the soil from the surface, it will continuously sink downward under the action of gravity. Enter the aquifer until a low water permeability or impervious layer. During the path of sinking movement, DNAPLs often have four phases, including "Gaseous Vapor", "Dissolved Phase", "Residual", and "Free Phase". (Free-phase) is distributed in soil and groundwater environments. The concentration values (mg/kg) remaining in the unsaturated soil layer (Vadose Zone) are usually not high, and most of them are significantly lower than the current domestic soil pollution control standards. Therefore, some domestic In the CVOCs pollution investigation case, the soil concentration measurement is often ignored due to the concept of soil pollution control standards, and its impact on groundwater impact and potential pollution within the site. On the other hand, the current “Soil Gas” survey and assessment method for unsaturated layers still does not seem to be sufficient. It is generally used in domestic “membrane interface detection (MIP)” or soil sampling screening. Portable detectors such as PID/FID are still insufficient in the analysis of the qualitative and quantitative analysis of pollutants. The so-called "good start is half the success", and the analysis of the pollution of residual CVOCs in the soil layer is called A good start should be important for constructing an effective site concept of a conceptual model.