Sec. 599.4. Secondary containment for underground tanks

(a) General requirements.

(1) All new underground tanks must have a secondary containment system. This must consist of one of the following:

(i) a double-walled tank;

(ii) a vault;

(iii) a synthetic liner; or

(iv) any other comparable system designed and installed in accordance with a consensus code, standard or practice developed by a nationally recognized association or independent testing laboratory that meets the criteria of this section.

(2) The secondary containment system must:

(i) be designed and constructed with a permeability rate to the substance stored of 1 × 10^-6 cm/sec or less;

(ii) be designed, installed and operated to prevent any migration of hazardous substances out of the system to the environment;

(iii) allow for detection and collection of releases, spills and accumulated liquids until the collected material is removed;

(iv) be constructed of, or lined with materials that are compatible with the hazardous substances to be placed in the tank system. It must have sufficient strength and thickness to prevent failure owing to pressure gradients, physical contact with the materials to which it is exposed, climatic conditions, and the normal stress of operation;

(v) be placed on a foundation or base capable of providing support to the secondary containment system, and preventing failure due to settlement, compression or uplift; and

(vi) be provided with a leak detection system that is designed and operated in accordance with section 599.5 of this Part.

(b) Standards for double-walled tanks.

Double-walled tanks must be designed, constructed and installed in accordance with the following:

(1) the outer wall must contain a release from any portion of the inner wall and must enclose the entire primary tank;

(2) the tank must be designed so that monitoring of the interstitial space for tightness can be readily performed;

(3) there must be no penetrations of any kind through the outer wall into the tank, except top entry manholes and fittings required for filling and emptying the tank, venting the tank, or monitoring the tank;

(4) the outer wall must be resistant to punctures and protected from corrosion in a manner consistent with section 599.3(d) of this Part; and

(5) the outer wall must be designed to contain an inert gas or liquid at a pressure greater than the maximum internal pressure of the inner wall.

(c) Standards for vaults.

Vaults must be water tight, impervious to leakage of hazardous substances, compatible with the substance in storage, and able to withstand chemical deterioration and structural stresses from internal and external causes. The vault must be a continuous structure with a chemical resistant water stop used at all joints. There must be no drain connections or other entries through the vault except that there may be top entry manholes and other top openings for filling and emptying the tank, venting and for monitoring and pumping of hazardous substances which may leak into the vault. The tank or tanks within the vault must be supported, backfilled or bedded in a manner consistent with generally acceptable engineering practices.

(d) Standards for synthetic liners.

Synthetic liners must be compatible with the substance in storage, be at least 60 mils in thickness and not deteriorate in an underground environment. The life expectancy of the liner must be specified in the spill prevention report. All punctures, tears or inadequate seams in the liner must be repaired prior to backfilling. The liner must be installed with a slope to the sump of at least one quarter of an inch per foot. Since some chemicals will readily diffuse through a synthetic liner, the liner used must have been tested and found resistant to diffusion of the substance stored.