Diffusion in Natural Porous Media Contaminant Transport, Sorption/Desorption and Dissolution Kinetics

About this book

This text introduces the general principles of diffusion in the subsurface environment and discusses the implications for the fate and transport of contaminants in soils and groundwater. Emphasis is placed on sorption/desorption and the dissolution kinetics of organic contaminants, both of which are limited by the slow speed of molecular diffusion. The book compiles methods for calculating the diffusion coefficients of organic compounds (in aqueous solution or vapour phase) in natural porous media. The author uses analytical solutions of Fick's 2nd law and some simple numerical models to model diffusive transport under various initial and boundary conditions. A number of these models may be solved using spreadsheets. The book examines sorption/desorption rates of organic compounds in various soils and aquifer materials, and also examines the dissolution kinetics of nonaqueous phase liquids in aquifers, in both the trapped residual phase and in pools. The text concludes with a discussion of the impact of slow diffusion processes on soil and groundwater decontamination and the implications of these processes for groundwater risk assessment.

Contents

1: Introduction.1.1. Persistence of Organic Contaminants in the Subsurface Environment. 1.2. Nonequilibrium Transport - Remediation of Contaminated Sites. 1.3. Diffusion in Sediments and Rocks. 2: Basics of Sorption and Diffusion in Soils and Sediments. 2.1. Sorption of Organic Compounds in Soils and Sediments. 2.2. Mass Transfer by Diffusion. 3: Modeling of Diffusion Processes. 3.1. Intraparticle Diffusion. 3.2. Diffusion into Layers of Low Permeability. 4: Measured Diffusion Coefficients. 4.1. Time-Lag Experiments. 4.2. Diffusion Coefficients from Sorption/Desorption Kinetics. 4.3. Prediction of Diffusion Coefficients. 5: Dissolution Kinetics of Non Aqueous Phase Liquids. 5.1. Aqueous Solubility of Organic Compounds. 5.2. Dissolution Kinetics. 6: Risk Assessment - Remediation. 6.1. Risk Assessment: Release Rates. 6.2. Time Scales of Soil and Groundwater Remediation. 6.3. Transport by Diffusion Compared to Advection. Appendices: I. Analytical Methods: TCE and Phenanthrene. II. Methods of Solids Characterization. III. TCE-Diffusion Data: Time-Lag Method. IV. Tracer Diffusion Data. V. Examples for Long-Term Sorption Kinetics of Phenanthrene.

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