The factors which govern the migration of a contaminant can be considered in terms of transport processes and attenuation processes. The transport processes can be mathematically represented by equations based on flow laws. The two basic transport processes are advection and dispersion. Advection is the movement of the contaminant with the flowing water.
Dispersion is the apparent mixing and spreading of the contaminant within the flow system.
The advection and dispersion transport processes can be illustrated by considering a steady flow of water in a long pipe filled with sand. The mass flows along the pipe with a constant velocity v. This transport process is called advection. The contaminant mass occupies an increasingly longer length of the pipe, thereby decreasing in concentration with time. The spreading out of the contaminant is called dispersion. Figure 1.2 illustrates the transport process when a continuous source of contaminant mass is injected into the pipe.