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In this example a phase change in the secondary leachate collection system is modelled using the Phase Change special feature. The landfill has a secondary leachate collection system and liner which overlies a 1 meter thick aquifer. A phase change occurs in the secondary leachate collection system at the interface between the unsaturated and saturated zones, assumed to be .2 and .1 meters thick respectively.
The landfill contains a constant concentration of DCM, which experiences biological decay in the landfill, primary and secondary liners, and the aquifer. A half-life of 10 years in the landfill and 40 years everywhere else is assumed. No biological decay is assumed to occur in the secondary leachate collection system.
The diffusion coefficient of the DCM in the unsaturated zone of the secondary leachate collection system is assumed to be 300 m2/a, and in the saturated zone to be 100 m2/a (to represent a high degree of mixing in the saturated zone). The phase change parameter for the DCM in the unsaturated zone is Henry’s Constant which is assumed to be 0.1 for DCM in this example.
Two layers are used to model the unsaturated and saturated zones of the .3 meter thick secondary leachate collection system. The first layer represents the unsaturated zone and is .2 meters thick. And the second layer represents the saturated zone and is .1 meter thick.
A Darcy velocity of 0.003 m/a is assumed through the primary liner, and 0 m/a through the secondary liner. Thus, for a 500 meter long landfill the outflow rate in the saturated portion of the secondary leachate collection system would be:
Outflow Rate = (500 * 0.003) / 0.1 = 15 m/a
This example is for a hypothetical landfill and is used to illustrate how to prepare an input file and run an analysis using the Phase Change option. The example is not a prescription for modeling contaminant migration during operation of a landfill. Each landfill has its own unique characteristics and no general prescription can be made. The Phase Change option should only by used by someone with the hydrogeologic background necessary to appreciate the subtleties associated with the physical situation and the steps necessary for appropriate modeling of this physical situation.