What is the normal migration or movement of a contaminant through a medium?

• A. Advection or convection
• B. Dispersion
• C. Attenuation
• D. Transformation and degradation

Answer: (A)

The movement of a contaminant through a medium is governed by advection, which is the bulk transport carried by the flowing fluid. In groundwater or air, the contaminant simply rides along with the moving water or air, following the velocity of the medium. Convection is often used interchangeably with advection in many contexts, describing this same bulk transport.

Dispersion describes how the contaminant plume spreads out as it moves—caused by velocity variations at different scales and by molecular diffusion—so the plume broadens but its forward movement is still driven mainly by advection. Attenuation refers to the decrease in concentration over time due to dilution, sorption, or other loss processes, not the transport mechanism itself. Transformation and degradation are chemical or biological changes that alter the contaminant's form, not the physical movement through the medium.

So the normal migration or movement of a contaminant through a medium is advection, the bulk flow carrying the contaminant along with the fluid.