The mobility of contaminants in the ground refers to the transport and attenuation of contaminants in the subsurface soils. In this chapter, the transport mechanisms and fate or conversion processes of contaminants constitute the main subjects of interest from the viewpoint of the controls or influence of contaminants on the soil-water system. Figure 9.1 gives a simple example of some of the basic issues relating to contaminant mobility. The left-hand sketch in the figure shows the source of contaminants, that is, discharge of leachate from a landfill. The axis in the contaminant plume identifies the x = 0 location used in the right-hand schematic graph, which portrays contaminant concentration distribution in relation to the distance x from the source at a particular time t. The dispersion distribution of contaminants identifies the situation where the agents involved in the transport of contaminants from the source to a point x are (a) the dilution effect of the soil water and groundwater, and (b) the physical obstruction to the mobility of contaminants, resulting in retarding the flow of contaminants.