Validated approaches to restoring the health of ecosystems affected by soil pollution

Abstract

The diagnosis of soil pollution in relation to the most commonly used remediation methods is an innovative way of tackling this problem. To this end, the theoretical framework of Ecosystems Health has been instrumental, since this area is presently emerging as a new language for general discourse on pollution. For example, the use of the term stressor for a pollutant implies the study of behavior responses in living organisms (at different levels of organization, from the cell to ecosystem) both in terms of impacts (toxicity) and tolerance towards the pollutant (their adaptation). Two very common scenarios in the central Iberian Peninsula have allowed us to approach the understanding of this area of study: (i) old abandoned mines and, (ii) old solid waste (urban and industrial) landfills. The soils of these systems are mainly polluted with heavy metals (Zn, Cu, Pb, Cd), they overlie different lithological substrates (granites, limestones and especially arkoses), and are highly representative of soils of the Mediterranean region (mainly regosols, luvisols and cambisols). The landfills of the mines or waste tips have slopes exceeding 40%, such that any leachates produced transfer pollutants to adjacent ecosystems at lower altitudes (stream bed pastures, marshes and surface water courses). Moreover, since the topsoil layer is usually sandy with low proportions of clay and organic matter, deep leachates also transport pollutants to ground water systems. We focused on the autotrophic component of the affected ecosystems for several reasons: toxicity tests could be used to examine the physiological and behavioral responses of organisms (mortality, injury, metabolic changes) as well as population (population density, risk of extinction) or community (structure, diversity, biomass, nutrient flow changes) variables. The build-up of a heavy metal in the above-ground part of a plant (phytoaccumulation) consumed by herbivores is also detrimental for health, due to transfer to the trophic network. Root systems may play a role in phytostabilizing heavy metals and in preventing them from passing to deeper soil layers. Finally, given the erosion problems of fine materials in landfills and waste tip slopes, vegetation helps avoid the movement of topsoil layer pollutants to other ecosystems. The methodological approaches validated by results obtained over the last twenty years can be summarized as: studies of polluted sites based on phytoecological sampling, analysis of soil chemical and physical properties, georeferencing of the heavy metals they contain for further sampling in areas showing the highest levels, collecting and chemically analyzing plants at these sites, and the use of soils with their seed banks to perform experiments in microcosms in controlled conditions. The idea is to use a combination of field and laboratory methods that simulate real scenarios in which soil pollution occurs.