Underground fuel storage tanks usually fail from rust perforation from the inside of the tank, due to several effects of water inside the tank including, in the case of heating oil, combination of water with sulphur in the fuel.
So if a test shows that there is a lot of water in a buried oil tank one would be more pessimistic about its remaining life.
Water in home heating oil joins with sulphur in this case to become acidic and corrosive. It causes tank failure by rust penetration from the inside. Also, there may be a bacteria living in tanks, existing at the water/oil interface, digesting organics and excreting acids.
The corrosiveness of this activity is often most significant at the water-oil interface in the tank, which explains why some tank leaks will develop not at the very bottom of the tank (but look there too) but instead, a few inches up, along the side of the tank.
The height of this corrosion line along the sides of the inside of the oil storage tank depends on the amount of water in the tank and thus the location of the water/oil interface line on the side of the tank.
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Billy Willard, Author
I have been involved in the environmental consulting field since 1995. I have been involved in removing and the remediation of hundreds of oil tanks.