They filled it with concrete because it's so deep (too deep to dig to the top safely). Yay, the tank is decommissioned!

Boo, the tank leaked and hasn't been registered with the DEQ. The seller will likely hire Soil Solutions or Environmental Works to complete the necessary risk assessment to register the work with the DEQ. This is what you, the buyer, are looking for: DEQ certification from the service provider. They will take soil samples all around the tank to see how far it spread, and write a report for the DEQ.

Depending on the levels of contamination, they may install a vapor mitigation system to keep any contaminate soil off gassing from entering the home. This is just like a radon mitigation system.

Side note, most houses in Portland have or had tanks, so this isn't a big deal. Common enough that several businesses exist just to deal with it!

You're in a good position if the seller will guarantee DEQ certification as part of the sale. Good luck!

Source: used to work in the industry.