For purposes of the SPCC requirements, "secondary containment for bulk storage facilities must be constructed to at least provide for the capacity of the largest single tank with sufficient freeboard for precipitation.  EPA believes that the proper standard of "sufficient freeboard" to contain precipitation is that amount necessary to contain a 25-year, 24-hour storm event."  While EPA believes that a 25-year, 24-hour storm event is appropriate for most facilities and protective of the environment, it did not make it rule standard because of the difficulty and expense for some facilities of securing recent information concerning such storm events at this time.  There are several different types of secondary containment measures that could be used at a facility.  EPA does not dictate which method must be used, only that it must meet at least the requirements discussed above.