There a lots of ways to heat a house. The most common heat sources in Portland are natural gas and electricity. Not long ago though, oil was a very popular heat source and it is still in use in a lot of older houses and is a viable heat source. The heater is fueled by oil stored in a tank. The tank is usually in the basement or buried outside. It is the buried tanks that we are most concerned about.
The Oregon Department of Environmental Quality manages the Heating Oil Tank program. If soil is contaminated around an oil tank, the homeowner has 30 days to remedy the problem. When a tank is properly decommissioned, a certificate is issued and kept on file with the DEQ. There is a FAQ page about what sellers should know about selling a house with oil heating.
If a home inspector finds evidence that the house was once heated by oil but cannot find where the tank was actually located they may recommend that a “tank locate” is performed. From the DEQ:
The current owner of the property is responsible for any necessary cleanup even if the leak happened before he or she bought the property. If you buy property knowing that a tank is present and no work has been done to determine if it has leaked, you also become responsible for any leaks discovered in the future. You then would have to contact an attorney for advice if you wanted to recover any costs from the prior owner.
If a tank is found and it is no longer in use, it must be decommissioned.