Dielectric Properties Of Polluted Soils As A Potential Tool To Detect Low Level Organic Pollution In Unsaturated Soils

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The detection of organic pollution in soils is usually made by chemical analysis. Some geophysical methods are available to detect massive and transient pollution. But the standards used in Europe to define a non polluted soil are very strict : some organic compounds (as Polycyclic Aromatic Hydrocarbons) have to be less than 10 ppm. Hence, a geophysical tool allowing to prospect large areas with low rate organic pollution has to be developed. In this aim, our work is the first step of the evaluation of a geophysical method based on the dielectric characteristics of soils, our final goal being in situ measurementso f those parameters. Several geophysical methods are already used to monitor organic pollution of groundwater. Both electric (Olhoeft G.R., 1985) and dielectric properties of these pollutants can be used. Conventional DC resistivity measurementsa nd EM methodsc an provide good results when polluted groundwater is linked with high salinity, while GPR and radiowave EM can be used to detect hydrocarbons by measuring soil dielectric properties. The problem of detecting organic compounds is different for saturated and unsaturated media because the dielectric constant E, of water (- 80, depending on temperature and frequency) is higher than that of dry soils (2 organic liquids (E,, reduces the water saturation component and, consequently, the global dielectric constant of the soil in comparison with non-polluted soil or rock. GPR method can detect contrasting dielectric permittivities due to such phenomena. This method is efficient for high levels of pollution ( > 30% by weight) in a saturated zone and for transient flow because it is easier to compare several GPR sections versus time than to interpret one section without references. Our bibliographic study indicated the lack of geophysical methods to detect low level organic pollution, but some papers appear to be optimistic (Yong R.N. and Hoppe ES., 1989, Vanhala H. and al, 1992) concerning the detection of pollutants in soils measuring the dielectric behavior in polluted soils. Consequently, we made some laboratory measurementso f the dielectric properties of both polluted and non polluted soils.