有機物質在不均質土層中傳輸之模擬研究

Modeling the Transport of Organic Compounds in Heterogeneous Soil Matrix

王美雪(Mei-sheue Wang)

指導教授 ： 吳先琪

國立臺灣大學/工學院/環境工程學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.03275

摘要

摘 要
土壤不均質特性讓流體在不飽和土壤中的不同區域有不同的速度，由流體的速度差異區分為流動區域與靜止區域。微量有機物質在土壤中隨空氣或水移動時，於流動區域內之主要傳輸機制為平流與延散，而在靜止區域內之主要傳輸機制則為分子擴散。
本研究將土壤的不均勻地質特性之溶質一階交換速率係數，以一種分佈函數來參數化，進而建立微量有機物質在不均質土壤中之傳輸模式。傳輸模式中除了以分佈的一階交換速率係數描述靜止區域內的物質傳輸擴散機制之外，也包括了流動區域之平流與延散傳輸機制。研究繼而利用實驗室之土壤管柱試驗與現地土壤抽氣試驗，以不同示蹤劑在不同試驗條件下所得到之結果，進行傳輸模式之模擬與驗證。
模式分析結果與試驗結果比較，顯示本研究建立之傳輸模式能模擬有機物質在不均質土壤中的傳輸情況，及其物質傳輸受交換動力限制之狀況。實驗和模擬結果指出當有機物質移動到不流動區域內部的靜止區時，會有類似慢吸脫附之情形，是造成不平衡傳輸或濃度變化曲線有拖尾巴現象的主要因素，同時也是造成土壤抽氣試驗於抽氣停止後土壤中有機物質濃度反彈回升的主要原因。
本研究建立之包含的一階交換速率係數的傳輸模式中，以一套對數常態分佈函數或伽傌分佈函數描述一階交換速率係數的機率密度分佈，便能解決一般模擬有機物質在土壤中傳輸時，缺乏現地地質資料之問題。同時發現傳輸模式中的一階交換速率係數機率密度分佈函數，與土壤系統之系統寬度尺度、土壤含水率、有機物質於土壤固相與流體間之分配係數等場址特性有關。亦發現其中之系統寬度尺度是除了污染物本身之外，最重要的影響因子。此發現將有助於有效地預測有機物質在受污染之土壤及地下水中的分佈和移動，亦可應用於土壤蒸氣抽除（SVE）系統中預估受有機物質污染之土壤區域的整治效率、整治成本及所需的移除時間。

關鍵字

污染物傳輸 ； 吸附動力 ； 一階交換速率係數 ； 土壤異質性 ； 土壤蒸氣抽除(SVE) ； 系統尺度

並列摘要

ABSTRACT
Heterogeneous unsaturated soil layers comprise both mobile and immobile regions as characterized by different flow velocities of fluid. The movement of pollutants is governed by advection and dispersion in mobile regions, and by diffusive mass transfer between mobile and immobile regions.
In this study, the aim is to estimate the distribution of the first-order exchange rate coefficients that reveals the presence of soil-matrix heterogeneity, and to develop an advection-diffusion mass transport model that includes the distributed first-order exchange coefficients in unsaturated soils. The study also simulate the experimental results from laboratory soil columns and a soil-vapor-extraction (SVE) field site by using different organic compounds as tracers and under different test conditions.
Modeling analyses and experimental results indicate that the including a log-normal distributed or a gamma distributed first-order exchange rate coefficient could explain the organic compounds transport in the vadose zone. Experimental results and model simulation could explain that the tailing phenomenon in chemical breakthrough curves, which signifies a non-equilibrium transport, resulting mainly from a rate-limiting exchange into and out of immobile (stagnant) regions. Model simulation could also explain the rebounding phenomenon and fit the data well by only adjusting the volume fraction of the immobile regions, the averaged radius of the aggregates and the standard deviation of the logarithmic first-order exchange rate coefficient.
The problem of the lack of local geological information while modeling chemical transfer in immobile regions can be solved by using a set of first-order exchange rate coefficients in log-normal or gamma distribution, in which the related parameters are functions of the system length scale, the soil moisture content, and the chemical partition coefficient. Among these properties the system length scale is found the most important site-specific property. The findings from this study enable one to better predict the transport of trace pollutants in a heterogeneous vadose zone based on the properties of chemicals and the soil system. The model will be able to predict the organic compounds moving and distribution situation in soil and groundwater. Also the model will be able to assist the designing of the SVE system and operational strategy, and the prediction of the performance of the clean-up actions.

並列關鍵字

contaminant transport ； sorption kinetics ； first-order exchange rate coefficients ； soil heterogeneity ； soil vapor extraction (SVE) ； system length scale

參考文獻

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