The seller is liable for damages from the dumping prior to the sale of the property.
Explanation:
It has been clearly mentioned in the CERCL Act of 1980, that the owner is directly responsible for any damage caused to the property either because of his activities or his negligence regarding the maintenance of the property. In this case, the seller seems not to be the first owner of the property. But now, because the previous owner is no more legally bound to compensate for the damage caused to the land (as the land does not belong to him anymore), only the current owner is liable for all the damages irrespective of when they were caused.
It can be concluded from the case given, that it is primarily important to seek and secure all the information about a property before purchasing it.