What is the spreading of contaminants in a liquid, gas, or solid phase due to impingement of the contaminant by that phase material?

• A. Advection or convection
• B. Dispersion
• C. Attenuation
• D. Water solubility

Answer: (B)

Dispersion describes the spreading of contaminants within a liquid, gas, or solid phase as it interacts with that phase and is carried by it. This spreading comes from two main processes: molecular diffusion, the random motion of molecules that causes the contaminant to mix with the surrounding phase, and mechanical dispersion, which arises from velocity differences and mixing as the contaminant travels through the medium (such as in porous media or turbulent flow). Together, these mechanisms broaden the contaminant plume around its average path, even as the bulk flow moves the center of mass forward. This is distinct from advection or convection, which describes the bulk transport with the flow; attenuation, which is the loss or reduction of concentration through processes like decay or sorption; and water solubility, which is about how much contaminant can dissolve in water rather than how it spreads within a phase.