Bioremediation of a crude oil-polluted soil: Biodegradation, leaching and toxicity assessments

The combined fate and effects of hydrocarbons (HC) on a soil ecosystem affected by bioremediation were studied during 480 days in a field experiment. The HC removal rates, the HC and metabolites mobility and the potential toxicity were assessed. A clayey soil polluted by 18 000 mg HC kg(-1) dry soil, was treated with either static-ventilated biopile or series of five windrows periodically tilled in order to determine the relative influence of nutrients, bulking agents, aeration and soil temperature. HC concentrations were determined by infrared spectrometry, gravimetry, gas chromatography and thermodesorption. Between 70 to 81% of the initial HC were removed through biological processes in fertilized soils, whereas natural attenuation without added nutrients was 56%. When adding fertilizers, residual HC were cyclic compounds poorly biodegraded and strongly trapped on the organo-mineral matter. Leaching of HC and water-soluble metabolites was demonstrated during the first stages of biodegradation. Low levels of the HC were detected in the leachates at day 480. Maximal toxicity was highest immediately after the introduction of oil then decreased as biodegradation proceeded. No toxic effect was recorded on worms survival and on seeds germination at day 480. However growth of plants was reduced in treated soils and a potential residual toxicity was observed on the basis of photosynthesis inhibition and bacterial bioluminescence (Microtox) tests.