Weathering processes only partially limit the potential for bioremediation of hydrocarbon-contaminated soils

We evaluated the viability of bioremediation of weathered hydrocarbons for a soil affected by diverse and very old crude oil spills. After decades of natural attenuation, the oil in the soil showed complete removal of linear alkanes, monoaromatics and naphthalene. Under such conditions, it is frequently assumed that bioremediation should be ruled out as a practicable remediation alternative since the remaining hydrocarbons are not easily biodegraded. However, we applied a landfarming treatment to the soil in order to determine the hydrocarbon families still susceptible to biodegradation. Five strategies were implemented in plots of 60 m3 each: (i) natural attenuation; (ii) tilling (T) and watering (W); (iii) T,W, and a slow-release fertiliser (SRF); (iv) T,W, SRF and a surfactant; and (v) T,W and an oleophilic fertiliser. To follow the degradation of hydrocarbons, periodic sampling, gas chromatography [GC; quantification of total petroleum hydrocarbons (TPHs)] and gas chromatography-mass spectrometry (GC-MS; qualitative study of different families) were carried out. After 5 months treatment, TPHs were reduced by 50% in plots IV and V and by >65% in plot III. The biodegradation affected mainly isoprenoids and alkyl aromatics; in contrast, steranes and hopanes were unaltered. In conclusion, successful bioremediation of the contaminated soil was achieved, in spite of the absence of readily degradable compounds.