Question

In a groundwater plume undergoing advection and dispersion, what happens to the peak concentration of the contaminant as the travel distance from the source increases?

Accepted Answer

As a contaminant plume travels away from its source, its peak concentration decreases. This reduction in peak concentration is driven primarily by the process of hydrodynamic dispersion. Advection refers to the movement of the contaminant at the same average linear velocity as the surrounding groundwater. Dispersion is the process by which a contaminant spreads out in all directions—both longitudinally along the path of flow and transversely perpendicular to it—because water molecules take different paths around individual soil grains and travel at varying velocities within the pore spaces of the aquifer. Because the mass of the contaminant is spread over an increasingly larger volume of groundwater as it travels, the concentration at the center of the plume, known as the peak concentration, must diminish to conserve mass. For example, if a specific volume of dye is injected into a flowing stream, it starts as a concentrated cloud; as it moves downstream, it spreads into a longer and wider shape, causing the color to appear fainter at the very center. In a groundwater system, this same principle applies, leading to a predictable decrease in peak concentration as the travel distance from the source increases.

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In a groundwater plume undergoing advection and dispersion, what happens to the peak concentration of the contaminant as the travel distance from the source increases?