Lab-scale biopile assessment for the evaluation of diesel fuel soil remediation

A soil of north-eastern Italy was interested by a strong diesel fuel spill. A wetting system was used to periodically release a microbial inoculant, containing a dried consortium of bacterial strains proven effective by previous microcosm evaluation. Investigations indicated that oil degrading bacterial strains were shown not to enhance significantly the remediation when the polluting agent was present in the soil for many years (Basaglia et al., 2003). To make a distinction between the role of the remediation activity of oil degrading bacterial inoculants and that of the aeration and nutrients application, four lab-scale biopiles were set up and monitored. This assessment was also necessary to dispose of suitable controls and make the right comparison between inoculated and not inoculated soil (Aichberger et al., 2005). Two different soil types were used in two different sets of experiments: set 1 - two lab-scale biopiles loaded with an aged polluted soil coming from a 30 years contaminated site: this soil was supposed to contain a well established oil tolerant and oil degrading microflora. set 2 - two lab-scale biopiles loaded with an unpolluted soil, artificially contaminated with diesel fuel: this soil was supposed not to contain a well established oil tolerant and oil degrading microflora. In both the above experiments the control was represented by a non inoculated column. Soil microbial community composition, as well as that of the commercial inoculant used, was mainly determined by denaturing gradient gel electrophoresis (DGGE) (Muyzer et al., 1998). A number of microorganisms capable of living in the presence of diesel as the only carbon source were also isolated and identified by 16s rDNA analysis. Among the bacterial species isolated and identified in the active biopiles Sphingomonas sp. was almost constantly detected. DGGE analysis performed on set 1 showed that the profile related to the inoculant did not cluster with any of the profiles obtained from the two biopiles, at any time. Since gas-chromatographic analysis indicated that both the biopiles were fully active in the remediation process, the biodegradation occurred was to be ascribed to the autochthonous microflora that experienced a strong and continuous selective pressure during the long period (more than 30 years) of continuous pollution. After so long time the microbial community evolved in a hydrocarbon resistent and actively degrading population. The results obtained from the experiment set 2, instead, indicated that the community profile of the inoculated biopile clustered with that obtained from the inoculant. This indicates that the inoculated bacteria did colonize the soil. While GC analysis demonstrated that they effectively metabolized the hydrocarbons, very reduced degradation activity was detected in the not inoculated control. The above results clearly suggest that while bioventing technology always stimulates the natural in situ biodegradation, bioaugmentation could significantly contribute to the remediation process only if rapidly applied after the contamination occurs.