I have worked in the oil patch in Sask. Canada cleaning up oil spills and reclamation(turning a old well sites back to natural landscape. Hydrolic oil is one of the worst things. All the oilspills in the area get cleaned up and put into a holding bin. All sorts of bad stuff but hydrolic oil spills like yours would have to put in another pile because it is more harmful to the environment and is disposed of differently. I would dig it up. You will be able to tell how deep it went
