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Break through curve

Video: Break through curve (2:00 min)
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A chemical substance, which is applied onto a homogeneous soil column moves through the column with the moving water. This mechanism of displacement is referred to as convection. The travel velocity of the dissolved substance depends on the pore water velocity if the substance does not interact with the solid surfaces. Such a substance is called conservative, i.e. non-reactive. If the dissolved substance is partially retained at or in the solids, its mobility is then affected by the chemical as well as physical properties of the dissolved substance and on those of the solid surfaces in contact with the moving water. The local pore water velocities depend on the soil structure.

In the course of the displacement process the water and the dissolved substance is locally mixec because the local velocities of the water moving through the pore space varies considerably. This mixing mechanisms is called hydrodynamic dispersion. The mixing due to molecular diffusion contributes to the mixing as well, but matters only if the pore water velocities are small. If the substance is applied onto the column surface as a single pulse of mass this pulse is smeared as it travels through the column, i.e. it is getting wider and wider and less concentrated. The mixing is also affected if the pulse is not homogeneously applied onto the column surface.

Plotting the concentration of the dissolved substance leaving the bottom of the column as a function of time is referred to as break through curve. Since the flow is constant the time coordinate can also be expressed as the cumulated amount of water leaving the column (outflow). This amount can in turn be expressed as number of pore volumes. One pore volume is the amount of water in the column, which actually participates in the displacement process. Hence, the water in inaccessible niches (immobile water) should not be included in the pore volume.

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