Effect of leachate mounding and sand lenses on contaminant migration from a landfill cell

Abstract

The effect of leachate mounding between drains at the edge of a landfill cell upon the flow field and contaminant migration to an underlying aquifer is examined. It is demonstrated that the impact is controlled by the downward component of the velocity and that once contaminant reaches an aquifer very little will return to the collection drains even if there is upward flow from the aquifer to these drains. It is shown that the simplified procedure of using the average velocity beneath a landfill in simple contaminant transport analyses is not valid when there is a significant component of the flow which is upward to the drains. However, it is also shown that simplified analyses can be used to get reasonable predictions of impact provided that the representative advective velocity is based on the average downward velocity to the underlying aquifer.The effect of the presence of up to 15% sand lenses in the aquitard is also examined. It is found that the sand lenses have relatively little effect on the impact in an underlying aquifer for the range of cases considered. The calculated potential impact for the non-uniform flow field corresponding to leachate mounding and the presence of sand lenses was obtained using a finite element analysis. The results were compared with calculated impacts based on relatively simple finite layer analyses and it is shown that using reasonable idealizations, quite good predictions could be made using the simplified models for non-uniform flow fields both with and without the presence of sand lenses.