There are two types of underground tanks. Heating oil tanks contain oil to heat buildings, and storage tanks contain used oil or other substances.
Different regulations apply to each.
Heating Oil Tanks Heating oil tanks are usually 300-500 gallons. As they age, the chance of leakage increases. The failure rate for 40 year old tanks is 80 percent. If the underground tank contains heating oil and the tank capacity is less than 1,100 gallons, the tank is not regulated by the Department of Ecology. Before removing an underground heating oil tank, check with the local fire marshal.