On different occasions works have been carried out in the subsoil, treating old contaminations. In these cases (as in the treatment in the subsoil of the Renfe in Madrid), once extracted the hydrocarbon by mechanical tools and pumping, the inoculum is infiltrated to the subsoil through piezometers in order to access to the pollution plume.

When working in the subsoil with aerobic microorganisms, it is advisable to enrich the soil by adding H2O2 at low dilution, 3 or 4 days after inoculation. Obtaining O2 enrichment can also be achieved through the insertion of bags containing calcium carbonate Ca Co3.