<< Click to Display Table of Contents >> Navigation:  Appendix A Examples > Example 15: Leachate Collection with Failure > Description

This example is similar to case 14 except the failure of the primary leachate collection system is also modelled using the Variable Properties special feature. Prior to the failure of the primary leachate collection system there is a downward Darcy velocity of 0.01 m/a between the landfill and the secondary leachate collection system. The primary leachate collection system is assumed to fail between 20 and 30 years, causing the leachate mound in the landfill to rise resulting in an increase in the Darcy velocity. After 30 years the collection system has completely failed and the Darcy velocity is now assumed to be 0.1 m/a.

 

As in case 14 the landfill contains a finite mass of a conservative 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 initially 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. As in example 14 the Reference Height of Leachate is 7.5 m, and the Rate of Increase in Concentration 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 (which varies with time), then the Volume of Leachate Collected is:

 

Qc = qo - va = 0.3 - va

 

The strata beneath the landfill, landfill dimensions, and aquifer characteristics are the same as in example 14.

 

 

Passive sink layers are divided the same as in example 14, except that the Darcy velocity in the first layer and the outflow in the second layer will be variable. The Darcy velocity in the first layer will be 0.01 m/a between 0 and 20 years, then will increase linearly between 20 and 30 years to 0.1 m/a, and then will be 0.1 m/a. In the second layer the horizontal outflow 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 = (va2 - va1) * 200/0.3 m/a

 

In the third layer there is no vertical or horizontal advective flow, there will however still be diffusive flow. When using the Variable Properties special feature with the Passive special feature it is possible to specify the Darcy velocities in both features. The Darcy velocity used by POLLUTE will be the result from the multiplication of the two velocities. For most practical applications, it is recommended that the Darcy velocity be entered as 1.0 in one of the features, and then the actual value entered in the other feature. In

this example the Darcy velocity is entered as 1.0 in the Variable Properties special feature, and the actual values are entered in the Passive Sink special feature.

 

Using the Variable Properties special feature the dispersivity can also be specified, in this example it is assumed to be 0.4 since there is outward flow from the landfill.

 

Following are the parameters used in this example:

 

 

When using the Variable Properties special feature the accuracy of the results is dependent on the number of sublayers 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 and 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. These options 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.