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Über dieses Buch

Diffusion in Natural Porous Media: Contaminant Transport, Sorption/Desorption and Dissolution Kinetics 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.
Diffusion in Natural Porous Media: Contaminant Transport, Sorption/Desorption and Dissolution Kinetics compiles methods for calculating the diffusion coefficients of organic compounds (in aqueous solution or vapor 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.
Diffusion in Natural Porous Media: Contaminant Transport, Sorption/Desorption and Dissolution Kinetics 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.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
This chapter contains a brief introduction concerning the fate and transport of organic contaminants in soils and groundwater. Physical and chemical properties of some common groundwater contaminants are provided, and non-equilibrium effects in advective solute transport in groundwater due to slow diffusion are discussed. In addition, a general overview is given of processes in the subsurface in which molecular diffusion is important (sorption/desorption kinetics, matrix diffusion, radioactive waste disposal, liners for waste disposal sites, hydrocarbon migration in reservoirs, gas transport in soils, etc.). These processes are discussed in detail in the following chapters.
Peter Grathwohl

2. Basics of Sorption and Diffusion in Soils and Sediments

Abstract
The objective of this chapter is to briefly introduce the basic concepts of organic pollutant sorption in soils and sediments and then to discuss diffusive mass transfer phenomena. Methods for the calculation of diffusion coefficients in vapor and aqueous phase are introduced and the definition of diffusion coefficients in porous media for steady state and transient conditions is given. Surface diffusion and diffusion in natural organic matter (polymers) are briefly reviewed at the end of Section 2.
Peter Grathwohl

3. Modelling of Diffusion Processes

Abstract
This chapter provides methods for the calculation of diffusion rates for various boundary conditions in porous particles and low permeability sediment layers. Emphasis is placed on the modeling of sorption/desorption kinetics (including heterogeneous aquifer materials) and diffusion across low permeability layers (e.g. mineral liners).
Peter Grathwohl

4. Measured Diffusion Coefficients

Abstract
Diffusion coefficients in natural porous media can be determined from field data and in laboratory experiments (e.g. see review by Shackelford, 1991). Since laboratory experiments allow well-controlled initial and boundary conditions, they are usually much more suitable for accurate measurement of diffusion parameters. In this chapter the time-lag method for the separate measurement of effective diffusion coefficients and capacity factors is introduced. Examples of measured diffusion parameters are given for porous rocks, clays, and polymer materials (Section 4.1). The parameters obtained with the time-lag method are then compared with results from sorption/desorption experiments (under transient conditions) using aquifer materials (sand and gravel) and soil samples (Section 4.2). Details on the analysis of samples and characterization of solids are given in tables. Finally, empirical correlations are summarized for the estimation of diffusion coefficients in natural porous media (Section 4.3).
Peter Grathwohl

5. Dissolution of Nonaqueous Phase Liquids

Abstract
Organic contaminants may occur in the subsurface as nonaqueous phase liquids (NAPL), which form pools or are trapped in the porous media as blobs or ganglia. The following chapter addresses the mass transfer processes which control the release of organic compounds from NAPLs and multicomponent NAPL mixtures such as coal tar or creosote. The mass transfer into the aqueous phase depends on the compound’s water solubility and molecular diffusivity. Since the water solubility is a key parameter in the dissolution kinetics of NAPL this issue is discussed briefly in Section 5.1. The dissolution kinetics of organic contaminants from residual phase liquids (“blobs”) trapped in granular porous media is discussed based on Fick’s 1st law (film diffusion, Section 5.2). Fick’s 2nd law is needed in order to calculate the pollutant mass transfer rates from pools of liquid contaminants floating on the groundwater table (light nonaqueous phase liquids: LNAPL) or ponding on low permeability layers in the aquifer (dense nonaqueous phase liquids: DNAPL).
Peter Grathwohl

6. Risk Assessment — Remediation

Abstract
The purpose of this chapter is to provide examples of diffusion limited contaminant transport for some cases of practical interest. The length of the mass transfer zones is briefly introduced which can be used for risk assessment, e.g. to distinguish maximum contaminant concentrations (equilibrium, e.g. dissolution from residual phase) from maximum contaminant flux (non-equilibrium, e.g. due to slow desorption) into groundwater. Transport of volatile compounds across the capillary fringe (both directions) is treated analogously to pool dissolution (semiinfinite diffusion). Typical dissolution time scales are discussed, together with desorption processes desorption processes, limiting aquifer and soil remediation. Finally a comparison of advective and diffusive solute transport is provided.
Peter Grathwohl

Backmatter

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