All outdoor storage facilities for liquid fuels, chemicals, industrial wastes, and potentially harmful materials shall be located on impervious pavement or other impervious surface approved by the Director of Public Works or their designee, and shall be completely enclosed by an impervious dike which shall be high enough to contain the total volume of liquid kept within the storage area, plus the rain falling into this storage area during a 100-year, twenty-four-hour storm, so that such liquid shall not be able to seep into the ground surrounding the paved storage area. Storage tanks for home heating oil and vehicle fuel, not exceeding two 550-gallon tanks, may be exempted from this requirement in situations where neither a high seasonal water table is within 15 inches of the surface, nor rapidly permeable sandy soils are involved.