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This example illustrates the use of the programs to study time-varying rates of advective-dispersive transport from a landfill, using the Variable Properties special feature. The landfill contains a finite mass of a conservative species, and has a leachate collection system. Initially there is an inward hydraulic gradient causing a hydraulic trap. After 20 years the collection of leachate is terminated and the leachate mound begins to build reaching it’s maximum height after another 10 years. The increased leachate mound causes a reversal in the hydraulic gradient, that results in a reversal of the Darcy velocity and the loss of the hydraulic trap.

 

The analysis starts at time zero which corresponds to the completion of the landfill and the development of a peak leachate concentration (co) of 1000 mg/L. It is assumed that the average waste thickness is 6.25 m with a density of 600 kg/m3, and that the contaminant represents 0.2% of the total mass of the waste. Thus the total mass of contaminant per unit area of landfill is:

 

mtc = 0.002 * 600 = 6.25 kg/m2

 

The Reference Height of Leachate (Hr) is then calculated by dividing the total mass of contaminant per unit area (mtc) by the contaminant concentration (co).

 

Hr = (0.002 * 600 * 6.25) / 1 = 7.5 m

 

It is also assumed that the peak concentration in the landfill is reached relatively early in the life of the landfill, and that the analysis starts at this time. Consequently there is no increase in concentration with time and the Rate of Increase in Concentration (cr) with time is zero.

 

The average infiltration through the cover (qo) is assumed to be 0.3 m/a. If the average exfiltration through the base of the landfill is va (which varies with time), then the Volume of Leachate Collected is:

 

Qc = qo - va = 0.3 - va

 

In this example the landfill is situated in a 4 m thick clay, which in underlain by an aquifer. The landfill is assumed to be 200 m long in the direction parallel to the groundwater flow in the aquifer. At the up gradient edge of the landfill the inflow in the aquifer is given by a Darcy velocity of 2 m/a. The outflow Darcy velocity at the down gradient edge of the landfill (vb) is assumed to be 2 m/a from years 0 to 20, then increasing between 20 and 30 years according to the relationship:

 

vb = 2 + 200 * va

 

After 30 years the outflow Darcy velocity (vb) is 6.2 m/a.

 

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 for inward flow (i.e., va < 0), and is 0.4 m for outward flow (i.e., va > 0). The coefficient of hydrodynamic dispersion (D) is then calculated by:

 

D = Dm + α * va/n

 

where n is the porosity, in this example 0.4.

 

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 during operation and failure of 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 with the physical situation and the steps necessary for appropriate modeling of this physical situation.