It has been shown that having earthworms present in petroleum polluted soil will decrease the levels of these chemicals over time.

There are a number of papers that discuss the effect that earthworms have on bioremediation of soil, both in combination with other presences like compost, and alone.

Here is an abstract of one article, called "Effects of Lumbricus terrestris, Allolobophora chlorotica and Eisenia fetida on microbial community dynamics in oil-contaminated soil" From the journal Soil Biology & Biochemistry. 2005, vol. 37, no11, pp. 2065-2076 [12 page(s) (article)] (1 p.1/4)

"Oil spills are one of the most common types of soil pollution. Bioremediation has become an attractive alternative to physicochemical methods of remediation, where feasible. Earthworms have been shown to stimulate the degradation of petroleum hydrocarbons in soil, and it was hypothesized that the role of earthworms in remediation lies in the enhancement of an oil degrading microbial community. The aim of this study was to characterize microbial activity and community dynamics in oil-contaminated soil incubated with or without earthworms. Three earthworm species (Eisenia fetida, Allolobophora chlorotica and Lumbricus terrestris) were incubated in crude oil polluted soil (ca. 10,000 mg/kg total petroleum hydrocarbons (TPH)) and a reference soil for 28 d. Control treatments with manual mixing and/or cattle dung amendment were also included. In the oil-contaminated soil, respiration and concentration of microbial biomass was significantly enhanced by earthworm amendment, and TPH concentrations decreased significantly. These effects were less evident in treatments with A. chlorotica, possibly due to a difference in behavior, since individuals of this endogeic species were found in a state of inactivity (aestivation). Microbial community dynamics were described by phospholipid fatty acid (PLFA) analyses. After 28 d, similar shifts in the soil PLFA composition were observed in the oil-contaminated soil irrespective of worm species. Fungal:bacterial ratios were increased in the presence of worms, but also by addition of dung as a food source, indicating a non-specific effect of metabolizable substrates. In contrast, the fatty acids 17:1ω8 (=Δ9-heptadecenoic acid) and 20:4ω6c (arachidonic acid) were specifically stimulated by the presence of earthworms in the oil-contaminated soil. The results showed that earthworms can contribute positively to bioremediation of oil-contaminated soil, but that the effect may be species-dependent."

There are also a number of books available on this subject that should be available to you at your local public or university library. For example:

Remediation of Petroleum Contaminated Soils: Biological, Physical, and Chemical Processes
By Eve Riser-Roberts

Please let us know if you have any other questions on this subject, and thanks.