4.4 Landfill Leachate Plume Example
The relative importance of the transport processes discussed in the previous subsection changes with time, hence the type of convection regime (free, forced or mixed) changes with time as well. As dense plumes disperse and diffuse, density contrasts become smeared out, which diminishes the driving force for density-driven flow and increases the relative importance of forced convection. This can be illustrated using the case of a dense leachate plume infiltrating from a landfill into an aquifer with a background flow field. Figure 12 shows the concentration patterns that may develop, as calculated using a numerical model. Groundwater flow is from left to right, with inflow of pristine groundwater from the upgradient section of the aquifer to the left and as recharge from infiltration at the surface. A 100 m long landfill is located between 50 and 150 m. Chloride is often present in high concentrations in landfill leachate due to the presence of soluble salts in household waste. Infiltration through the landfill dissolves these salts which increase the density of the recharge water in that zone. Color-coded concentration is used here to visualize spreading of the plume. Three different snapshots show the concentration pattern after 20 years of continuous leaching for leachate chloride concentrations of 250, 2500 and 10000 mg L–1, which will be referred to as the low-, medium- and high-density cases, respectively. These values are based on reported concentrations for Danish landfills (Christensen et al., 2001).
Figure 12 – Three different snapshots of numerical simulations showing the concentration distribution of a landfill leachate plume with chloride concentrations for the low-, medium- and high-density cases of: a) 250 mg L−1; b) 2500 mg L−1; and, c) 10000 mg L−1 after 20 years of continuous leaching. The animated version of this illustration is provided here.
The difference between the three snapshots in Figure 12 illustrates again that density differences can have an enormous impact on the flow dynamics and hence the spreading of contaminants. For the low-density case (Figure 12a), the density contrast between the landfill leachate and the pristine groundwater is too low to cause any noticeable deviation from the plume spreading that might be expected under the prevailing flow conditions. As the plume emanates from the landfill, it gets pushed slightly deeper into the aquifer by the uncontaminated rainwater that recharges the aquifer downstream of the landfill. The peak concentrations decrease with distance downstream due to spreading caused by hydrodynamic dispersion.
For the medium-density case (Figure 12b), the plume thickness increases significantly, and lobes of contaminated groundwater have formed. This is the result of the downward flow caused by the negative buoyancy of the denser leachate plume. At a distance of almost 400 m, the plume even reaches the bottom of the aquifer, and in some places, the concentrations increase with depth. This is entirely counter-intuitive for a system in which the contaminant source is at the top of the aquifer, yet this situation is sometimes observed under field conditions near landfills in Denmark (Christensen et al., 2001).
While the plume of the medium-density case has some resemblance to the low-density case just downstream of the landfill, the situation is entirely different for the high-density case (Figure 12c). There, the density contrast between the leachate and the groundwater is so high that convective fingering starts immediately below the landfill and while the lobes are entrained with the regional groundwater flow, the direction of spreading is strongly vertical. Interestingly, the peak concentrations are dissipated much more efficiently in the high-density case compared to the other two cases. This is because the formation of fingers leads to a larger contact area between the contaminated and pristine groundwater, which enhances mixing by dispersion and diffusion. While the positive effect is that the peak concentrations of the contaminants are attenuated, the adverse effect is that a much larger volume of groundwater becomes contaminated.
It is further interesting to note that the dissipation of mass strongly reduces the density contrast and hence the propensity for variable-density flow downstream of the landfill. The animated version of Figure 12 (video) shows that once the plumes have reached the bottom of the aquifer and the concentrations have decreased, the migration is essentially horizontal as dictated by the regional flow field. Thus, the high-density case is free convection dominated under the landfill, but transitions to a forced convection mode once the driving force for density-driven flow is diminished. The low-density case is dominated by forced convection throughout. The medium case is an example of a mixed convection regime.