PRINCIPLES OF ENVIRONMENTAL TOXICOLOGY AND CONTAMINATION (PART II)

Table of Contents

1. Index
2. UNIT 5 - Transport and fate of contaminants in the environment
   1. Introduction
   2. Transport processes
   3. Environmental persistence and transformation processes
   4. Bioaccumulation
   5. Environmental fate models
   6. Brief conclusion
3. UNIT 6 - Principles of toxicokinetics and toxicodynamics
4. UNIT 7 - Biotransformation
5. UNIT 8 - Toxicity Testing
6. Glossary of key terms associated with environmental toxicology

Transport processes

Following the release of a contaminant into an environmental compartment, transport processes will determine its spatial and temporal distribution in the environment. The transport medium (or fluid) is usually either air or water, while the contaminant may be in different phases (gaseous, dissolved, condensed, or particulate). The physical transport can be categorized as:

Advection (often called nondiffusive transport) - is the passive movement of a chemical in bulk transport media either within the same medium (intraphase or homogeneous transport) or between different media (interphase or heterogeneous transport). Examples of homogeneous advection include transport of a chemical in air on a windy day or a chemical dissolved in water moving in a flowing stream, in a discharge effluent (point source), or in surface runoff (nonpoint source). Examples of heterogeneous advection include the deposition of a contaminant sorbed to a suspended particle that settles to bottom sediments, atmospheric deposition to soil or water, and even ingestion of contaminated particles or food by an organism (i.e., bioaccumulation). Advection takes place independently from the presence of a chemical; the chemical is simply going along for the ride. Advection is not influenced by diffusion and can transport a chemical either in the same or opposite direction as diffusion.

Diffusion - is the transport of a chemical by random motion due to a state of disequilibrium. For example, diffusion causes the movement of a chemical within a phase (e.g.: water) from a location of relatively high concentration to a place of lower concentration until the chemical is homogeneously distributed throughout the phase. Likewise, diffusive transport will drive a chemical between media (e.g.: water and air) until their equilibrium concentrations are reached and thus, the chemical potentials or fugacities are equal in each phase.

Note that: 

Advective processes in water or air usually transport chemicals at a faster rate than either molecular or turbulent diffusion.