Changes in toxicity during in situ bioremediation of weathered drill wastes contaminated with petroleum hydrocarbons

Bioremediation of weathered drill wastes severely contaminated with total petroleum hydrocarbons (TPH) (90,000–170,000 mg kg−1) and BTEX (51.2–95.5 mg kg−1) to soil standards was achieved over a 3-year period in three phases: initial remediation, basic bioremediation and inoculation with a biopreparation. Fourteen non-pathogenic indigenous bacteria species belonging mainly to the Actinomycetales were identified and shown to be able to degrade 63–75% of nC9–nC20, 36–51% of nC21–nC36, 36% of BTEX and 20% of PAHs (polycyclic aromatic hydrocarbons). Addition of five non-pathogenic fungi species to the bacterial consortium allowed degradation of 69–89% of nC9–nC20, 47–80% of nC21–nC36, 76% of BTEX, and 68% of PAHs. Microtox, Ostacodtoxkit, Phytotoxkit and Ames tests indicated that changes in toxicity were not connected with the decrease in TPH contents, possibly due to the formation of toxic indirect metabolites during bioremediation. No toxicity was found in the soil after bioremediation.