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Transport of contaminants in the groundwater system is affected by different processes. They include advection, dispersion, diffusion, adsorption and decay. These processes can work together or separate in groundwater flow.
Advection is the movement of the contaminant with the groundwater flow. The rate of flow is determined by Darcy's law, where the hydraulic conductivity of the soil is divided by its porosity to find the average linear velocity:
As the groundwater moves it will carry the pollutant with it in the direction and distance it is traveling. Adsorption is the ability of a compound to "attach" itself to the soil. Its determined highly on the properties of the soil and the compound. Differences in solubilty and reaction with organic materials help make up a wide range of adsorption strengths. Decay does not affect how fast or how far the pollutant will travel. Biological or chemical processes will reduce the amount of compound traveling thorough the system so that the effects of the advection will stay the same but with a lesser amount present. The main topic of this page is to discuss the effects of diffusion and dispersion on contaminant migration.
Dispersion is categorized in two ways, mechanical dispersion and hydrodynamic dispersion, hydrodynamic being mechanical plus diffusion. These occur at both a microscopic and macroscopic level.