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. 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.
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. 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.