“Tank,” “underground storage tank,” “underground tank system,” and “tank system” have the same meaning as defined in Chapter 6.7 (commencing with Section 25280), except as follows:

1. (a) These terms mean only those tanks that contain only petroleum or, consistent with the federal act, a mixture of petroleum with de minimis quantities of other regulated substances.

2. (b) These terms include all of the following components that are connected either directly or indirectly to the tank:

   1. (1) Spill containment structures that are substantially or totally beneath the surface of the ground.
   2. (2) Those portions of vent lines, vapor recovery lines, and fill pipes that are beneath the surface of the ground.