In the state of NJ, the banks require that we keep taking offers until they have accepted an offer and all contingencies of that offer have been met.
I am only basing my information on NJ and suggest you speak to your agent or the listig agent (whoever submitted your offer to the bank on your behalf) but I believe that you were making an offer based on certain criteria and disclosures by the bank,,,those criteria have changed. I believe if you ask your agent to resubmit your office acknowledging that you are aware of the oil spill, you will again be on the table.
Your question interested me because I had this happen to one of my listings (except I did not fill the oil tank -the owner did) and we became aware of the situation during the buyer's inspections. It took almost a year to get the property cleaned up along with health officals certifications as to ground water quality that needed to be done every 3 months for year. This might not be true in your state but it will still get complicated...perhaps you should speak to an insurance specialist and your mortgage reps to make sure that don't have special requirements too with regards to this issue...