Contribution of earthworms to PCB bioremediation

Twenty cm deep columns containing Aroclor 1242 contaminated soil were bioaugmented with the PCB-degrading micro-organisms, Ralstonia eutrophus H850 and Rhodococcus sp. strain ACS, each of which were grown on sorbitan trioleate, and induced for PCB degradation by salicylic acid and carvone, respectively. Treatments consisted of soils with and without earthworms. Earthworms were utilized to enhance the dispersal of the bioaugmented PCB-degrading micro-organisms, while simultaneously improving soil aeration, increasing soil carbon and nitrogen content, and modifying the soil microbial community. Bioaugmented soils containing the earthworm Pheretima hawayana achieved 55% removal of total soil PCB as compared to only 39% in identically treated soils without earthworms. Earthworm-treated soils achieved upwards of 65% PCB degradation at subsurface depths, as compared to 44% in soils without earthworms and prior reports of only 10% degradation in soils treated without manual mixing of the inoculum into the soil (McDermott et al., 1989. Two strategies for PCB soil remediation: biodegradation and surfactant extraction. Environmental Progress 8, 46-51). A methane diffusion study demonstrated that soils containing earthworms attained greater gas diffusion rates. Breakthrough of the methane tracer through the 20-cm column was detected after only 10 min in soils with earthworms, while 340 min was required before breakthrough in soils without earthworms. Using a gas diffusion model, the experimental diffusion coefficients were calculated to be 4.45 x 10(-3) and 5.0 x 10(-4) cm2 s(-1), respectively. The higher diffusion rate of oxygen into the soil profile provided greater concentrations of the necessary terminal electron acceptor for aerobic PCB degradation. Methane depletion was observed only in soils with earthworms and was attributed to microbial communities unique to the earthworm treated soils. The potential contribution of these communities toward PCB degradation is discussed.