BTW, check your deed for any strange "mineral rights" clauses-- a house I bought years ago for The Front Porch ( one of my stores ) had a specific clause which meant that any oil, gas, or valuable minerals were not mine, but reverted to the former owner. Didn't mean a damn thing in practice, since no such deposits existed, but the seller was covering all his bases.
I was thinking along the same lines that perhaps a neighbor has been changing oil in his yard and dumping the old oil which has eventually seeped into other properties.