It encourages tank failure by enhancing corrosive action from the inside. Water can also leak into a tank from ground water if the tank wall is damaged. Underground fuel storage tanks usually fail from rust perforation due to the effects of moisture inside the tank. In the case of heating oil, the combination of water with sulfur in the fuel, bacterial action, and other non compatibility factors. Buried tanks should be tested for the amount of water present in tank bottom, and the water should be pumped out.
Water corrodes the tank which then leads to leaks.
If the tank is to remain in use, ask your fuel supplier about using an additive or other methods to help remove water. A proper abandonment procedure involves pumping out any remaining fuel, cleaning the interior of sludge or residue, removing the tank, and confirming that there has been no leakage. The Department of Commerce maintains the state storage tank database. Both simple pressure-testing and very sophisticated electronic testing are commonly used, mostly on commercial equipment rather than residential tanks. Tank testing methods vary in cost, invasiveness, length of time to complete, and more.