Polycyclic aromatic hydrocarbons (PAHs) can be found under natural conditions but they can be produced by pyrolysis processes. They can be found in former industrial sites subsoil, especially on Manufactured Gas Plant sites (MGP sites).Gaz de France has inherited the patrimony of former French gas companies on nationalisation in 1946; consequently, Gaz De France is still the owner of 467 of manufactured gas plants. Even if no impact on human health has been detected and even it the risks seem to be virtual, Gaz de France has to prevent any environmental consequence due to the possible presence of residues in the subsoil of the sites: a protocol has been signed with the French Ministry of Environment. Following the investigations on the site, a risk assessment, which takes into account the future use of the site, is achieved. The definition of remediation and monitoring of the site is based on risk assessment; the pollution is treated on the basis of specifications defined in accordance with public authorities. Biotreatments are potentially the most well adapted techniques to remediate big volumes of low or medium contaminated soils.