Subsoil Heterogeneities Controlling Contaminant Mass and Microbial Diversity in Porewater in Mega-site Contexts

The aim of this study is to gain a better understanding of the conceptual model of contaminant transport and fate in cases of NAPL pollution in mega-site contexts in order to improve the design of remedial strategies. The area where the study was carried out is an unconfined aquifer of alluvial fans in the Tarragona Petrochemical Complex (Spain). Two boreholes were drilled and continuous cores were recovered in order to carry out a geological and hydrogeological testing at centimeter scale. A comprehensive sampling of subsoil materials was also undertaken. The biogeochemical heterogeneity occurring in the mega-site context is controlled by the existing conjunction of lithological, hydrochemical and microbiological heterogeneities. Biodegradation processes of the lightest compounds take place not only at the level of the dissolved fraction but also at the level of the fraction retained in the fine, less conductive materials. The quantification of this pollutant mass must be taken into account when designing remedial strategies in order to forecast back-diffusion. Sampling the low-conductivity levels contributes to the definition of the conceptual model of transport and fate from the spatial and temporal viewpoints.