Transfer of chlorinated volatile organic compounds from soil and groundwater into indoor air buildings
Résumé
Chlorinated solvents are amongst the most common soil and groundwater contaminants due to their widespread use as dry-cleaning solvents and as degreasing agents. Due to their physicochemical properties, they produce large scale plumes of pollution in the groundwater. In the densely populated regions, these pollution plumes are located under residential and urban development areas and therefore difficultly accessible. Vapours can migrate through building slabs into indoor air. Soil vapor migration into house, with subsequent inhalation, is often the main exposure pathway to humans at sites contaminated with volatile organic compounds (VOCs). Two approaches are commonly used for quantification of indoor concentrations: indoor gas measurement or transfer modeling from the source. Model development is relatively well advanced, but measurements for model calibration and 'validation' hardly exist in the literature. Furthermore, predictions of indoor gas concentrations from different models may vary by several orders of magnitude, depending on the application. The work presented here consists in comparing modelled results and experimental measurements on a pilot site. The pilot site is a factory, where contaminations with chlorinated solvents in groundwater and the vadose zone were observed. Before beginning the campaigns of measurements, some preliminary characterizations were carried out as screening by Cone Penetration Test and BAT sampling. Then an adequate monitoring network was performed (groundwater monitoring and soil-air monitoring wells). Measurements concerned contaminant concentrations in several media (groundwater, soil, soil air, indoor air) and fluxes at soil surface, but also key model parameters (source depth, soil and concrete characteristics and other characteristics of the building like atmospheric / soil pressure difference, ventilation rates...). The analytical modeling was performed with two transport models, one based on the equations of Johnson & Ettinger, the second based on the equations of VOLASOIL adapted for a multilayer soil. The numerical modeling was performed with COMSOL Multiphysics. COMSOL is a finite element analysis for various physics and engineering applications, especially coupled phenomena or multiphysics. Measurements have shown a complex hydro geological context and the presence of multiple contaminations (in several locations and in several dates). Comparisons between measured and modeled concentrations have shown that some building parameters have a great influence on the results and that they must be site specific determined.