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The Horizontal Migration option is used to quickly enter a model for the horizontal migration of a contaminant from a waste mass to the site boundary. The model may contain a primary composite liner and an aquitard. In this option the primary composite liner can be composed of a geomembrane and a primary liner. If the geomembrane is present the leakage through the geomembrane is calculated using equations by Rowe et. al., 2004.
In this model the attenuation layer is modeled as a passive sink, where there is downward flow due to infiltration from precipitation and horizontal flow due to a difference in heads between the waste and the attenuation layer. The infiltration will have the effect of removing contaminant by acting as a passive sink with inflow. To avoid this effect set the infilration into the attenuation layer to zero. Unless yor really understand what you are doing, it is recomemended that you set the infiltration to zero.
At the site boundary, the attenuation layer is assumed to continue indefinitely and is modelled as an Infinite Thickness boundary.
If a geomembrane is present the horizontal flow is calculated using the leakage through the geomebrane calculated using the equations by Rowe et. al., 2004. And if the geomembrane is not present the horizontal flow is calculated using the average gradient (using the head in the waste and the head in the attenuation layer) and the harmonic mean hydraulic conductivity between the head measurements.
