**4. Understanding the complex nature of landfill soil caps with the view of restoring the impacts of pollution**

The mountains of waste and rubble we have created are new landscape features that most often emerge in areas around cities. These scenarios can be viewed as laboratories for research into the environmental impacts of landfills that were capped without prior treatment of the deposited waste. Even considering that the restoration of degraded ecosystems is a systemic topic, the functionality of this epistemological approach arises from the fact that ecosystems are dynamic systems that evolve and co-evolve with human activity.

**Figure 7.** Principle components analysis of the soil chemical variables showing points appearing on the new coordi‐ nate axes. A) PCA of the whole set of variables, N = 29, B) PCA excluding As, Hg, hydrocarbons and PCBs increasing the number of cases to N = 52, C) representing the 52 points on the new axes created by the PCA in Figure B), D) expan‐ sion of plot C) from –2.5 to 1 abscissa and -2 to 2.5 ordinate. L, landfill proper; T, rubble tips; Pl, platforms; S, slopes; FS, foot of slopes; D, discharge zone.

The complexity of the problem faced arises from questions related to the secondary ecological succession (from the capping soil's seed bank), which interacts with the primary succession that is possible in this new ecosystem in the landscape. Besides restoring its impacts, efforts need to also focus on revegetating the landfill system itself.

Hence, these landfills may be considered a new type of ecosystem in which primary and secondary successions coincide. They are thus of great interest for ecological science since they provide a real scenario for investigating the measures we should install to restore a degraded and polluted ecosystem and help us identify the plant species related to their varied forms of pollution. This will enable researchers to select the most appropriate plant species for revege‐ tation efforts rather than simply establishing a green cover once a landfill has been capped.

**4. Understanding the complex nature of landfill soil caps with the view of**

The mountains of waste and rubble we have created are new landscape features that most often emerge in areas around cities. These scenarios can be viewed as laboratories for research into the environmental impacts of landfills that were capped without prior treatment of the deposited waste. Even considering that the restoration of degraded ecosystems is a systemic topic, the functionality of this epistemological approach arises from the fact that ecosystems

**Figure 7.** Principle components analysis of the soil chemical variables showing points appearing on the new coordi‐ nate axes. A) PCA of the whole set of variables, N = 29, B) PCA excluding As, Hg, hydrocarbons and PCBs increasing the number of cases to N = 52, C) representing the 52 points on the new axes created by the PCA in Figure B), D) expan‐ sion of plot C) from –2.5 to 1 abscissa and -2 to 2.5 ordinate. L, landfill proper; T, rubble tips; Pl, platforms; S, slopes; FS,

The complexity of the problem faced arises from questions related to the secondary ecological succession (from the capping soil's seed bank), which interacts with the primary succession that is possible in this new ecosystem in the landscape. Besides restoring its impacts, efforts

need to also focus on revegetating the landfill system itself.

are dynamic systems that evolve and co-evolve with human activity.

**restoring the impacts of pollution**

218 Environmental Risk Assessment of Soil Contamination

foot of slopes; D, discharge zone.

**Figure 8.** Spatial distribution of Zn, Cd, total hydrocarbons and electrical conductivity

The ecological theory that is most applicable to the restoration of the environmental impacts of capped landfills addresses the stress and ecological strategies of herbaceous species.

The classification of plant life cycle strategies described by Grime combines the stress intensity with the perturbation intensity [7, 28]. Thus, "competing species" are more appropriate for landfills with a low intensity of perturbation and stress, "ruderal" species adapt better to conditions of low stress and intense perturbation, and "stress-tolerants" are ideal for settings of intense stress and scarce perturbation. When both these factors are excessive, this approach is ineffective.

It should not be forgotten, however, that different types of ecosystem respond differently to a given perturbation, and vice-versa, that a given ecosystem can respond in many different ways to different perturbations. We also need to be aware of the vast environmental variability and randomness that exists along with other associated forms of uncertainty [29].
