They also found 2.5 fuel line leaks per 1000 customers. We do not have similar data for buried gas tanks in residential application. Westinghouse Hanford to participate in an interesting non-residential study. Tank Failure Causes Underground fuel storage tanks usually fail from rust perforation due to several effects of water inside the tank including, in the case of heating oil, combination of water with sulphur in the fuel, bacterial action, and other factors. External rust, unless very heavy, isn't highly correlated with internal rust.
Leaks can occur due to tank damage or at piping connections.
A new tank involves significant expense. Tank Testing Specialty companies and some oil companies have equipment to test buried tanks for leaks. Both simple pressure-testing and very sophisticated electronic testing are commonly used, mostly on commercial equipment rather than residential tanks. Testing for water in the tank is simple and can be done by any service person. Tank testing methods vary in risk to the tank, cost, invasiveness, length of time to complete, and more.