ABSTRACT
Waters and soils have an almost infinite healing capacity, including negative feedback mechanisms responsible for reinstating an optimal equilibrium in element cycling and ensuring an acceptable habitat for the biota. In addition to negative feedback processes, high diversity and flexibility of the biota is required to ensure a dynamic equilibrium and adaptation to actual conditions. The biota is constantly short of nutrients, which leads to maximal exploitation of the available resources, thus even contaminants may serve as nutrient supply for aquatic and soil microorganisms. This way, contaminants become part of the biogeochemical element cycling and, depending on their persistency, may disappear or accumulate in the environment. Water and soil processes have considerable impact on the atmosphere as well, as hydrogeochemical element cycling may result in gas emissions/release with a long residence time in the atmosphere. Biological CO2 sequestration and high soil organic matter content could mitigate the release of greenhouse carbon dioxide into the air.
If the natural buffering and balancing system of the environment does not function properly, many of the unregulated processes lose their capacity of self-control. In spite of being aware of this, we hardly learn from it, and our environmental management is still not based on it. One of the positive lessons learned is to use natural attenuation in environmental remediation. Unfortunately, this “attenuation” in the environment is often identified/interpreted as a concentration decrease. However, from the point of view of environmental load, the total contaminant amount and, from the perspective of the ecological and human receptors, the specific land use risk posed should be considered. Those natural processes are acceptable in environmental remediation, which can diminish adverse impacts of contaminants and irreversibly reduce the environmental risk posed to the ecosystems and humans via the environment.
A proper attenuation process reduces the contaminant quantity in parallel with its concentration. Decreasing the concentration without removing the contaminant may affect increasingly large areas. In the short term this may not be visible locally, but a generic increase in the contaminants’ background concentration poses a high risk to element cycles and ecosystem stability at watershed or even global scale. This phenomenon has been experienced in the last century, and today nobody knows what the result of these changes will be, such as the increases in the background values and the additional environmental loads, as surplus to the already existing levels. We can only hope that the Earth finds a new equilibrium that is still beneficial to the human species.
Natural attenuation and engineered (enhanced) natural attenuation used in environmental remediation are considered real technologies/processes occurring in quasi-reactors (see Chapter 2) operating with or without engineering interventions. Depending on the scale of intervention, management activity is denominated differently such as monitored natural 96attenuation, engineered or enhanced natural attenuation, soft remediation, close-to-nature bio- or ecotechnologies, in situ bioremediation, phytoremediation, artificial wetlands, living machines, etc. Many of these technologies include almost no intervention, while others can be highly controlled and regulated bio- or ecotechnologies.
This chapter discusses the process itself and the monitoring of natural attenuation as well its application as a remedial option and its management as a nature-based technology.
