Controlled Natural Attenuation For Organic Pollutants: Measuring Tools And Models

Natural attenuation (NA) refers to all the mechanisms that lead, without human intervention, to a reduction in the mass, toxicity, mobility, volume, flow or concentration of pollutants in soil or groundwater. Like any other management measure, natural attenuation involves reducing the concentration at the target.

Different scenarios and the types of targets involved are considered. For the technique to be applicable on a site, it must be controlled, which is why the term "controlled natural attenuation" is often used.

There are two main processes:

• non-destructive mechanisms, including dilution, dispersion, adsorption and volatilization;

  the transformation of pollutants and destructive mechanisms adapted only to certain organic molecules (metallic pollutants are never destroyed).

On the majority of sites, natural attenuation is only envisaged where there is significant destruction of pollutants.

Due to their presence on numerous sites (Basol database of polluted sites and soils), the objective is to focus on the biodegradation of BTEX (benzene, toluene, ethybenzene, xylenes), followed by that of chlorinated solvents (perchloroethylene, trichloroethylene and their derivatives). The extent of this biodegradation is quantified using models at two sites, demonstrating different efficiencies for different substances. This approach can be completed by setting up microcosms and monitoring pollutant isotope levels.

Because of the slow rate of flow and the quantities of pollutants in source areas, AN is essentially a process that extends over several years, and the timescales that can be envisaged can be defined on the basis of real-life examples.