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.
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.