You do not need containment until you go above 1100 gallons of TOTAL storage. This includes barrels of oil so even if you only have two 500 gallon tanks but you have three 55 gallon barrels of oil. You would be supposed to have containment structures because of the oil barrels putting you over the 1100 gallon limit. That is even of the oil barrels are stored inside. You would have to have containment structures on all of the fuel/oil storage. This includes the inside barrels.
Think about floor drains in your shop. They do not want an oil barrel leaking on the floor and then running out a floor drain.
The EPA will check this while doing manure management plan checks.
Above 1100 gallons you need to comply with containment and different set back laws. For the average smaller farmer just keep smaller tanks. Say two 350 gallon tanks and then you would not have to have containment structures.
Just had to take a class on this because of the amount of fuel we store. Iowa DNR had the classes. It is new Federal rules taking effect sometime this year. The effective date has been moved several times already.