As I recall, the secondary containment for a storage tank does not have to be impermiable forever...
I belive that the US federal government defines a 72 hour period for containing the liquid.
Suggest that you consider re-posting in the "Storage Tank Engineering" forum....
Concrete is appropriate since the fluid is flammable. Concrete is considered to be sufficiently impervious such that it generally does not require permeablility testing. Sufficient impervious means that the effective secondary containment must be able to contain the liquid until the liquid is cleaned up. USEPA does not specify permeability or retention time performance criteria for these provisions. Instead, USEPA gives the owner/operator and the certifying PE flexibility in determining how best to design the containment system to prevent a discharge. This determination is based on a good engineering practice evaluation of the facility configuration, product properties, and other site-specific conditions. Ultimately, the determination of imperviousness should be verified by the certifying PE.
Concrete will be a recomendation. I'll double check on the USEPA and particularly the MAEPA(which always seems more stringent than the Feds) on retention time.
