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In this example a landfill with both a primary and a secondary leachate collection system is modelled using the Passive Sink special feature. The landfill contains a finite mass of a conservative contaminant species, and is underlain by an aquifer with fixed outflow. A passive sink is used to model the secondary leachate collection system, which is assumed to be composed of a 0.3 m thick granular layer. The Darcy velocity is assumed to be 0.01 m/a downward from the landfill to the secondary leachate collection system, and 0.0 m/a between the secondary leachate collection system and the aquifer (i.e., the water table is assumed to be at the base of the secondary leachate collection system).

 

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 = 7.5 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 (va) is 0.01 m/a, then the Volume of Leachate Collected is:

 

Qc = qo - va = 0.3 - 0.01 = 0.29 m/a

 

The strata beneath the landfill consists of a 1 m clay layer, a 0.3 m granular layer (i.e., secondary leachate collection system), a 2 m aquitard layer, underlain by a 1 m thick 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 4 m/a.

 

The outflow Darcy velocity at the down gradient edge of the landfill (vb) is then by multiplying the landfill length (200 m) by the Darcy velocity below the secondary leachate collection system (0.0 m/a) and adding the inflow, viz:

 

vb = 4 + 200 / 1 * va = 4 m/a

 

When using the Passive Sink Properties special feature the deposit is divided into layers which can have vertical and horizontal flows In the example 3 layers are necessary, the first is from the base of the landfill to the top of the secondary leachate collection system, the second is the secondary leachate collection system, and the third is from the base of the secondary leachate collection system to the aquifer. In the first layer there is a vertical downwards Darcy velocity of 0.01 m/a and no horizontal flow. The second layer has a horizontal flow which is equal to the difference in Darcy velocity between the layers above and below, multiplied by the landfill length and divided by the layer thickness, viz:

 

vs = (va1 - va2) * L /h = (0.01 - 0.0) * 200 / 0.3 = 6.67 m/a

 

In the third layer there is no vertical or horizontal advective flow, there will however still be diffusive flow.

 

Following are the parameters used in this example:

 

 

This example is for a hypothetical landfill and is used to illustrate how to prepare an input file and run an analysis using the Passive Sink 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 Passive Sink option should only by used by someone with the hydro-geotechnical background necessary to appreciate the subtleties associated with the physical situation and the steps necessary for appropriate modeling of this physical situation.