(1) Secondary containment shall contain at least one hundred twenty-five percent of the volume of the largest storage container within the area plus the displacement of all other tanks, appurtenances, and other items within the containment area: Provided, That permanent storage facilities that have tanks of one hundred thousand gallons or greater capacity may use the following method to meet the capacity requirement: Secondary containment shall contain at least one hundred ten percent of the volume of the largest storage container within the area plus the displacement of all other tanks, appurtenances, and other items within the area plus sufficient volume to contain the precipitation from a twenty-five year, twenty-four hour storm event.

(2) If the secondary containment is located indoors or under a roof to prevent accumulation of rainfall, the area shall contain at least one hundred ten percent of the volume of the largest storage container plus the displacement of all other tanks, appurtenances and other items within the containment area.

(3) Secondary containment in operation prior to March 1, 1994, having a capacity of at least one hundred ten percent of the volume of the largest storage container within the area plus the displacement of all other tanks, appurtenances, and other items within the containment area shall be considered to be in compliance with this section: Provided, That upon alteration to the secondary containment or increase of storage container volume the secondary [containment] shall be brought into full compliance with the specific capacity requirement of this section.