<< Click to Display Table of Contents >> Navigation:  Appendix A Examples > Example 4: Finite Mass Source > Description

In this example the input data file from Case 3 will be edited to include a source with a finite mass of waste and a leachate collection system. The hydrogeology is comprised of a 4 m thick layer with a finite mass source at the top, and an underlying aquifer at the base with fixed outflow as discussed in Case 3. All of the parameters are the same as in Case 3, except the vertical Darcy velocity will be 0.03 m/a, the horizontal inflow velocity will be 4 m/a and there will be a finite mass top boundary condition. The finite mass top boundary condition requires the input of the Reference Height of Leachate (Hr), Rate of Increase in Concentration (Cr), and the Volume of Leachate Collected (Qc).

 

It is assumed in this example that the waste has an average thickness of 6.25 m and a density of 600 kg/m3, and that chloride represents 0.2% of the total mass of the waste. Thus, the total mass of chloride per unit area of the landfill (mtc) is calculated by multiplying the proportion of chloride by the density of the waste and the thickness of the waste.

 

i.e. mtc = 0.002 * 600 * 6.25 kg/m2

 

A peak concentration (co) for chloride of 1000 mg/L (i..e., 1 kg/m3) is assumed. The Reference Height of Leachate is then:

 

Hr = mtc / co = 0.002 * 600 * 6.25 / 1 = 7.5 m

 

If the peak concentration is reached relatively early in the life of the landfill and the analysis starts at this time, then there will be no increase in concentration with time. The Rate of Increase in Concentration (Cr) would then be zero.

 

The Volume of Leachate (Qc) collected is equal to the difference between the infiltration through the cover (qo = 0.3 m/a here) and the exfiltration through the base (va = 0.03 m/a here), and is given by:

 

Qr = qo - va = 0.3 - 0.3 = 0.27 m/a

 

In this example the inflow in the aquifer at the up gradient edge of the landfill will be 4 m/a and the outflow at the down gradient edge (vb) is then:

 

vb = (vb(in)*h*W + va*L*W)/(h*W) = va(in) + va*L/h = 4 + 0.03*200/3 = 6 m/a

 

The following parameters are assumed for the example:

 

 

The landfill length is measured in the direction parallel to groundwater flow. And the landfill width is the direction perpendicular to groundwater flow, since this is not a 3D analysis this parameter has no effect on the results.