What describes the retardation, degradation, or adsorption of a contaminant?

• A. Advection or convection
• B. Dispersion
• C. Attenuation
• D. Vapor pressure

Answer: (C)

Attenuation describes how a contaminant’s presence decreases as it moves through a medium due to physical, chemical, and biological processes. Retardation is the slowing of movement caused by the contaminant sticking to soil or aquifer solids (sorption), so it travels more slowly than the water itself. Degradation refers to chemical or biological reactions that transform the contaminant into other compounds, reducing its concentration. Adsorption is the process of contaminant molecules binding to solid surfaces, removing them from the flowing water. When these processes occur together, they lessen both how far and how quickly the contaminant travels, which is the essence of attenuation.

Advection or convection is about the bulk movement of water carrying the contaminant, not the reduction in concentration. Dispersion describes the spreading and dilution of the plume due to velocity variations, but it doesn’t inherently account for removal or transformation of the contaminant. Vapor pressure concerns how readily a substance partitions into the vapor phase, not its attenuation in groundwater.