The recommendation of  the Washington State Department of Ecology is permanent closure for abandoned underground storage tanks this is also required by many local Fire Departments. Decommissioning a tank is the process of permanently closing a tank. Removal of heating oil and sludge from the tank is also involved in decommissioning abandoned underground oil tanks.
Abandoned underground oil tanks often have been known to contain oil still in them. To help prevent possible soil and groundwater contamination, you should arrange to have the oil removed from your abandoned tank if you do not immediately decommission the tank.