Introduction to Basic Ground-Water Transport
By the earthDr!
Plume Development: from the source-recharge area to downgradient points of the ground-water system
We depicted loading of contaminant mass to the ground water on the previous webpage. It is this mass loading of contamination to the ground water that is a precursor for ground-water plume development. The ground water is flowing downgradient as the mass of contamination discharges to it from above. This first figure is a plan view representation of the source-recharge area plus the ground-water flowpaths depicted by the yellow arrows. The mass loading of contaminants at the source-recharge area is depleted, by this continued downgradient transport of contamination by ground-water flow, extending the area and volume of contaminated ground water within the soil pores or within rock fractures, cracks, crevices, and joints. This downgradient extension of the contaminated ground water within the void spaces of the subsurface matrix (either soil or rock) is called a ground-water plume or more commonly, a plume.

The width of the source-recharge area, measured perpendicular to ground-water flow, defines the minimum width of the plume. Generally, the plume width is much wider than the width of the source recharge area due to dispersion as depicted in the second figure. Dispersion causes mixing of the both the water and the dissolved contamination as the ground-water flows downgradient. Dispersion is a combination of diffusion and mechanical mixing. Mechanical mixing results from the herky-jerky movement of both water and contamination as it forced to move: up, down, left, and right due to the native tortuosity of the microflowpaths in the pores of the soil or in the cracks, joints, fratures, and crevices of the rock.

This tortuous flow, coupled with diffusion, tends to mix the contaminants with cleaner water as the ground water flows downgradient. Ground-water flow is the predominant factor for spreading contamination, but dispersion plays a part in spreading the contamination somewhat farther. Please take note: dispersion influences the spread of contamination in all directions: laterally, vertically, and even longitudinally (in the direction of the ground-water flowpaths). This second figure depicts the concentrations of contaminants dissolved in the ground water. In this figure, the highest concentrations of dissolved constituents are at the source-recharge area, because contaminant mass loading to the water table is ongoing. If the contaminant mass loading to the water table was decreased or terminated, eventually the ground water flow regime would deplete the contaminant levels at the source-recharge area and the plume would migrate downgradient as a slug of contaminated ground water, like a puff of smoke.