Infoscience

Thesis

Méthode de détection et de caractérisation de pollutions du sous-sol par les sites contaminés à l'aide des traces inorganiques

Contaminated sites form one of the major causes of environmental pollutions, in particular those of the soil and of the underground. One of the main problems presented by these sites is their location. Since water is the main contamination support in soil and underground, it is necessary to get good indicators to be able to detect pollution sources in areas where suspected sites location remains unknown. This is also necessary for the monitoring of contaminated sites in time and space. The aim of this project is to enhance the use of inorganic trace elements by offering specialists a new investigation tool adapted to a survey strategy of contaminated sites in various geological conditions like those found in Switzerland. This study has shown, through numerous site descriptions (41 sites) and the statistical approach of collected samples (222 samples), that the use of inorganic trace elements is helpful for the survey of contaminated sites. The most interesting result shows specific indicators for hazardous waste. Principal results Some inorganic trace elements represent almost exclusively hazardous wastes, since they are missing in the domestic and in the inert wastes. We also showed that these trace elements are typical indicators allowing to detect the presence of hazardous waste mixed with domestic wastes. These elements are : Se, As, W, Cs, Sc, Bi, Ga, Hg, Ag. Domestic and inert wastes do not show any characteristical trace elements. In general one can find the following elements in more or less important amounts in domestic wastes : Cu, U, La, B, Al, Ni, Co and Zn, Li in inert wastes. We also noticed that main inorganic trace elements in landfill leachates are : B, Ba and Br, of which the concentration is often close to or greater than mg/l. Tritium is also often associated to landfills. The comparison between uninfluenced groundwater collected during this study and polluted groundwater has allowed to value some inorganic trace elements. These elements differ significantly from the natural background when polluted sites are encountered. We distinguished specific indicators that are not present in uninfluenced groundwater : Cs, Sb, Sn, Ag, Bi, La, Se and tritium. We also put forward the presence of typical tracers showing anomalous concentrations in case of pollution by landfills, such as : Ti, Co, V, Ni, Cu, Ba, Br and B. Guide values could be established for these elements for their use in detecting contaminated sites. The same process was adopted for a limited amount of samples in surface waters. The significant trace elements are the same as for groundwater, except Sn and La. Hg, Cd and Pb which, on the other hand, could be detected at significant concentrations in cases of pollutions by contaminated sites.

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