Secondary Containment of Large Aboveground Storage Tanks
Posted / Last update: 01-08-1999
Meeting the volume requirements
NFPA recognizes two types of secondary containment structures. One type is a diked area that surrounds the existing tanks. The other is a remote impoundment area in which the liquid is drained into a pond area away from the tank field.
For diked areas, most state regulations require that secondary containment must also hold an amount of precipitation (usually a level of six inches) in addition to the volume of the largest tank in the tank field as required by NFPA, UFC and other codes. The SPCC program requires that the secondary containment area equal the volume of the largest tank plus ten percent. The safest approach to ensuring compliance with criteria in your area is to size the secondary containment areas according to SPCC program requirements.
When two or more tanks are permanently manifolded and hydraulically connected so that the tank levels move together, the sizing of the secondary containment area should be based on the combined volume of the connected tanks, plus 10 percent (unless there is a single tank in the field with a larger volume).
Remote impounding is an acceptable secondary containment method under NFPA 30 because the code primarily focuses on fire safety and emphasizes the importance of moving leaked or spilled flammable liquids away from the tank by adequate draining. A remote impoundment must be able to contain the contents of the largest tank. However, when this is not possible, partial impounding can be used in combination with diking to meet the largest-tank criterion.
For tank fields contained by diking (as shown in Figure 3), NFPA 30 requires that a slope of not less than one percent away from the tank shall be provided for at least 50 feet or to the dike base, whichever is less. This ensures that small spills will not accumulate against the wall of the tank. Also, if remote impounding is used, the drainage path to the pond should be designed so that if the drainage path is ignited, the flames will not pose serious risk to tanks or adjoining property. For an illustration of drainage to remote impoundments, see Figure 4.
Figure 4: Diagram of drainage to remote impoundment area, courtesy of NFPA.
It is not unreasonable to require the use of secondary containment to substantially reduce the risks of soil and groundwater contamination. However, requiring that secondary containment systems be completely leak-free would be unreasonable, because such a requirement is nearly impossible to meet (if this requirement could be tested).
One of the most significant debates taking place today has to do with defining an acceptable permeability rate for secondary containment areas. The correct definition, in my opinion, requires a case-by-case, cost-benefit analysis, as well as engineering analysis.
The amount of contamination that can escape into the ground below a spill in a secondary containment area depends on the:
• volume of leaked or spilled liquid;
• time that the liquid remains in the area before cleanup;
• permeability of the containment area, including any liner material;
• number and sizes of any cracks or breeches in the liner, if one is used; and
• depth of the liquid leaked or spilled.
The five factors cited above are interrelated. While the exact relationship is complex, changing any one of the variables will increase or decrease the amount of product released to the environment. For example, if a spill is immediately cleaned up, then even if the soil is relatively permeable, the oil will not have had time to permeate deeply. The removal of the soil to a shallow depth completely prevents a contamination. Of course, disposal of contaminated soil must be dealt with as well.
Of the factors cited, the volume of leaked or spilled liquid is probably the single most important one. If the volume is so small that the contaminated dirt is collected and disposed of, there is no release to the environment. This is usually the case when the volume is so small that the liquid does not form a “puddle” of some depth; only surface contamination occurs. This can be cleaned up before any hydrocarbon enters the environment.
The timing of the cleanup is also significant, as previously mentioned. Cleaning up a spill immediately after it occurs is the best way to prevent contaminants from escaping the secondary containment area. Most, if not all, companies have a policy to immediately clean up leaked or spilled product and correct whatever caused the leak or spill. In addition, most local regulations require that the leak or spill be stopped and cleaned up as soon as practical.
The permeability factor
The permeability of the secondary containment area, including any liner material used in the area, is an important and often controversial factor. While the concept of permeability is relatively simple (see definition on page 38), some state regulations tend to be vague on required permeability, often using such terms as “impervious” or “sufficiently impervious” in reference to secondary containment and liner materials.
Other states’ regulations specify a numerical value for the maximum permeability of secondary containment material. A review of these state standards indicates that the maximum permeability rates range from 10-6 to 10-7 cm/sec.
Even if a totally impermeable membrane were to be used, the problem of spill containment would not be solved. For example, the weak links in membranes used for secondary containment are the seams. Also, tears or punctures during installation, and settling after installation, represent possible vulnerable points of the membrane. The effect of the imperfections in the membrane may far outweigh its low permeability.
The American Petroleum Institute (API) Publication 315, Assessment of Tankfield Dike Lining Materials and Methods, has an excellent discussion of both vapor and liquid permeability and methods of testing. For practical purposes, simple rules of thumb may be sufficient to select the liner.
Generally, materials such as sand, aggregate or other open-grained soils have relatively high permeability, regardless of the liquid spilled. In these cases, a major spill would seep rapidly into the aquifer below. However, clay-like soils or other tight soils can prevent liquid penetration if the spill is cleaned up quickly. The adequacy of an earthen containment area depends not only on its permeability, but also on the type of liquid stored, the liquid’s hazardous properties and other factors.
The toxicity, mobility and persistence of the liquids stored at a facility play a major role in evaluating the need for, and appropriate type of, a secondary containment liner. Materials that can move quickly through soils and are highly toxic warrant special consideration.
Highly toxic, water soluble components with low viscosity and those that do not degrade easily are more likely to need a leak-proof liner than substances with high viscosity, low toxicity and lower degrees of water solubility. Examples of the former are pure benzene, methanol and some halogenated hydrocarbons. Examples of the latter might be typical fuels, heating oils, crude oil or other hydrocarbons.
Many different types of liners have been used for secondary containment areas. Some of the common materials are:
• Native soil
• Bentonite and soil-bentonite admixtures
• Synthetic flexible membranes
• Spray-on applications
The most commonly used liner material has been compacted native soil. In assessing the options for liner material, it may be useful to do a “what-if” analysis—a risk assessment based on a hypothetical spill, its consequences and remediation efforts and costs. For example, a spill of methanol requires a substantial cleanup effort, because it has a low viscosity, is toxic and water soluble. All this means that methanol can contaminate large quantities of groundwater quickly. Because of this, remediation efforts would be costly and spread over large areas. In addition, methanol is toxic to bacteria and, in concentrations greater than a few hundred parts per million, would prevent the use of organic methods of remediating the soil.
Some regulated chemicals pose unique handling problems if spilled, because hazardous waste management requirements would be triggered. Examples are benzene and toluene in concentrated form.
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