4.2.4. Natural Attenuation

The natural attenuation is a process to reduce the mass, toxicity, mobility, volume, or concentration of contaminants (organic and inorganic) in soil, groundwater and surface water. These processes are grouped as physical, chemical and biological. Physical phenomena are advection, dispersion, dilution, diffusion, volatilization and sorption/desorption. Chemical processes are ion exchange, complexation and abiotic transformation. Biological processes are aerobic and anaerobic biodegradation, plant and animal uptake. Natural attenuation is a cost-effective remediation technology. The advantages and disadvantages of this technique are given in Table 4.4.

In order to use natural attenuation as a cleaning process, sites must meet one or more of the following criteria:

•It must be located in an area with little risk to human health or to the environment.

•The contaminated soil or groundwater must be located an adequate distance from potential receptors.

•There must be evidence that natural attenuation is actually occurring at the site.

•High permeability speeds contaminant spread, low permeability slows the breakdown. Ideally, natural attenuation works best in soils whose permeability ranks somewhere between high and low.

 Table 4.4. Advantages and disadvantages of natural attenuation technique