It would be much safer to require, as "a condition precedent to the Buyer's obligation to close the transaction," that the owner will have removed the tank and will have provided a complete report, prepared by a qualified professional, describing the removal process, and attesting to the fact that there were no signs of leakage. That report should include, as an exhibit, the results of lab tests on soil samples taken from areas below the tank and to the sides of the tank. If there is contamination, you do not want to be in the chain of title prior to complete remediation of the site.