<< Click to Display Table of Contents >> Navigation:  Appendix A Examples > Example 11: Time-varying Source Concentration > Description

In this example there is a time-varying source concentration history and diffusive transport of a conservative species (i.e., no sorption) from a landfill. Time zero corresponds to the excavation of a landfill cell, the cell then filled quickly with water to a depth of 6 m. No waste was added to the cell for 7 years. The landfill is situated in a clay that contains chloride in its pore water at a concentration of 120 mg/L. During the 7 years that the cell contained water the chloride began to diffused out of the clay pore water and into the cell water. Between the years 7 and 10, waste was added to the cell and the source concentration of chloride increased linearly with time reaching a peak value at year 10 of 2100 mg/L. The source concentration of chloride then remained relatively constant between the years 10 and 13. During the years 13 to 15 the source concentration decreased linearly with time to a value of 1180 mg/L at year 15. The source concentration then remained relatively constant again from years 15 to 19. This example will calculate the predicted chloride distribution with depth at year 19.

 

There is no leachate collection system in the landfill, and the water level in the waste corresponds to the natural water level. The hydraulic gradient is zero, and hence the Darcy velocity is zero. And the clay is sufficiently thick that it can be assumed to be infinite for the time period under consideration.

 

When using the Variable Properties special feature it is possible to independently specify the diffusion coefficient (Dm) and the dispersivity. In this example the dispersivity is assumed to be zero since there is no flow. Clearly if there is no flow then the value of the dispersivity is not relevant since the coefficient of hydrodynamic dispersion (D) is then calculated by:

 

D = Dm + α * va /n

 

The Reference Height of Leachate for this example is the same as the depth of water in the cell (i.e., 6 m). In this example the source concentration is assigned specific values at various times by setting the value of the Reference Height of Leachate very large. Setting the Reference Height of Leachate very large will ensure that the source concentration remains constant during that time interval.

 

Following are the parameters used in this example:

 

 

When using the Variable Properties special feature the accuracy of the solution is dependent on the number of sub-layers used.

 

This example is for a hypothetical landfill and is used to illustrate how to prepare an input file and run an analysis using the Variable Properties option. The example is not a prescription for modeling contaminant migration from a landfill. Each landfill has its own unique characteristics and no general prescription can be made. The Variable Properties option should only by used by someone with the hydrogeologic and engineering background necessary to appreciate the subtleties associated withthe physical situation and the steps necessary for appropriate modeling of this physical situation.