台灣留學生出席國際會議補助

2010年4月14日星期三

Evaluation of Tortuosity Models for Predicting Solute Diffusion in Unsaturated Soils

 

論文發表人;周心儀 (加州大學河濱分校環境科學研究所土壤與水資源科學組 博士班)

 

https://www.acsmeetings.org/

 

http://a-c-s.confex.com/crops/2009am/s1/papers/index.cgi?invited=&sessionid=6031&username=52240&password=166079

 

對於溶質在土壤中的傳輸,擴散(Diffusion)是一種重要的機制。溶質在土壤中的擴散率會比在純液相中的擴散率低;因其擴散是經由土壤孔隙中不連續分布的液體。在非飽和的土壤中,溶質擴散率會隨著土壤含水量降低而減少;因其造成溶質擴散傳輸路徑的增長和液體媒介斷面積的減少。Fick's擴散傳輸方程式便是描述溶質在非飽和土壤中的擴散率,而其中曲折係數 (tortuosity factor)便是用以量化溶質在土壤液相中之擴散率隨液體分布變化的情形。

直接測量溶質於土壤中的有效擴散係數(Effective diffusion coefficients)來求得曲折係數是耗時耗工的方法,所以曲折係數通常會藉由曲折係數模型來預估。這些模型可分為四類:(1)含水量為基礎,(2)含水量、孔隙度及總體密度為基礎,(3)含水量、總體密度和土壤水分特性曲線為基礎,及(4)概念化模型。本研究的目的為評估上列模型的預測度及應用性。

本研究使用離子交換試膜(ion exchange membrane) 方法測量溶質於砂土、黏土和砂質黏壤土之三種不同質地土壤中,不同含水量之下的有效擴散係數,進而求得曲折係數,並用以評估上列之曲折係數模型。

結果顯示第一類和第二類模型的可應用性相對地較低,因提高其預測度需要針對不同的土壤質地和不同的總體密度輸入不同的參數。而第三類模型有較高的預測度和可應用性,因為這類模型在輸入溶質於三種實驗土壤中的飽和有效擴散係數和其土壤水分特性曲線於對數座標上的斜率後,便可以提供較高準確度的預測。第四類模型對於曲折係數的預測度很低,但其比前三類模型更能夠描述曲折係數隨含水量變化的趨勢,可用以解釋溶質於非飽和土壤中的擴散動力機制。

Diffusion is an important mechanism of solute transport in porous media. It is often described by Fick's Law. In unsaturated porous media, the rate of solute diffusion decreases with decreasing soil water content due to the reduced cross-sectional area of the liquid phase and the more tortuous diffusive pathway. This reduction is accounted by the 'tortuosity' factor, the fraction of diffusion coefficients of solute in soil and liquid phase. Direct measurement of effective diffusion coefficient to obtain tortuosity is time and labor consuming, thus many models have been proposed to predict the tortuosity for unsaturated porous media. These models can be categorized into four groups: (1) water content based, (2) water content-porosity/bulk density based, (3) water content-bulk density-soil water characteristic based, and (4) conceptual models. In this study, effective diffusion coefficients of packed soil cores of a sand, a sandy clay loam, and a clay were measured to evaluate the aforementioned tortuosity models. Soil cores were equilibrating with 0.01M KBr solution to a range of desired water contents obtained by applying 1.53 x 105 cm-H2O to 10 cm-H2O matric potential. The effective diffusion coefficients of bromide were measured by ion exchange membrane method. Results show that the water content based and water content-porosity/bulk density based models have very limited prediction capability, as they require individual empirical constants to model soils in different textures and bulk densities. Water content-bulk density-soil water characteristic based models give better prediction if the measured effective diffusion coefficient at saturation and the soil water characteristic data are available. The conceptual models have limited prediction capability, but they serve to explain the dynamics of the diffusion process and can predict the trend of the effective diffusion coefficient change as water content changes.