SPCC Secondary Containment Requirements for Oil Storage
Secondary containment is one of the most inspected parts of an SPCC plan, and one of the most common places where the document looks fine on paper but the field reality doesn’t match.
Secondary containment is one of the most inspected parts of an SPCC plan, and one of the most common places where the document looks fine on paper but the field reality doesn’t match. If your facility stores oil aboveground, the SPCC rule requires you to provide a way to catch a release before it reaches the environment. The plan has to show how, where, and how much.
This piece walks through what secondary containment means under SPCC, how capacity is calculated, what counts as acceptable containment, and what inspectors check when they walk your site. For a broader look at the program, see our SPCC plan development and certification overview.
What the SPCC Rule Requires
Under 40 CFR Part 112, any facility subject to SPCC must provide containment for oil storage that could reasonably be expected to discharge to a navigable water or adjoining shoreline. The regulation addresses this in three places:
Bulk storage containers (40 CFR 112.8(c)(2)):
Containment must hold the entire capacity of the largest single container in the system, with sufficient freeboard to contain precipitation. Many plans use a conservative 110% design target to account for rain, but the plan should document the actual containment basis for the site’s precipitation conditions.
Tank-car and tank-truck loading/unloading racks (40 CFR 112.7(h)(1)):
Where rack drainage does not flow into a catchment basin or treatment facility designed to handle discharges, the containment system must hold at least the maximum capacity of any single compartment of a tank car or tank truck loaded or unloaded at the facility.
General containment (40 CFR 112.7(c)):
“Appropriate containment” sized to the typical failure mode and most likely quantity of oil discharged. This applies wherever oil is handled, transferred, or stored: drum storage, transformer pads, oil-filled equipment, generator base tanks.
The plan document has to show all of this. Calculations, photos, drawings, and a description of how each containment area meets the requirement. For the full structure of what your plan must contain, see our breakdown of SPCC plan requirements under 40 CFR 112.
How Containment Capacity Is Calculated
For a dike or berm around a single tank, the math is straightforward: the volume of the diked area, minus the volume displaced by the tank itself, must be at least equal to the tank capacity plus freeboard for precipitation.
For multi-tank containment, you also subtract the volume of every other tank in the dike up to the level of the containment wall. Tank piers, ladders, columns, and any other fixed equipment inside the dike also displace volume and must be subtracted.
A simple example for a single 5,000-gallon tank inside a rectangular concrete dike:
| Element | Calculation |
|---|---|
| Dike interior footprint | 15 ft × 12 ft = 180 sq ft |
| Dike wall height | 2 ft |
| Gross dike volume | 180 × 2 = 360 cu ft (≈ 2,693 gal) |
| Volume displaced by tank base inside dike | 50 cu ft (≈ 374 gal) |
| Net containment volume | 2,693 − 374 = 2,319 gal |
| Required (largest tank + precipitation freeboard, using 110% as conservative check) | 5,000 × 1.10 = 5,500 gal |
| Result | Insufficient. Dike must be raised or expanded. |
The math isn’t difficult. The hard part is measuring everything accurately and accounting for what’s actually inside the containment area. When we walk a facility, the first thing we do at each storage area is measure the dike interior, the wall height, and the footprint of every object inside the wall, then run the calculation on site before we leave.
Types of Secondary Containment
Different methods work for different situations. What the rule cares about is whether the method actually contains the oil before it leaves the area.
Dikes, berms, and retaining walls.
Concrete, earthen, or block walls around a tank or group of tanks. The standard solution for outdoor bulk storage. Must be sufficiently impervious to hold oil for the time it takes to clean up a release.
Double-walled tanks.
The outer wall serves as containment. Common for smaller tanks, generator base tanks, and indoor installations. The interstitial space must be monitored for leaks.
Spill pallets and drip pans.
Used for drum and tote storage. Pallet capacity has to equal at least the largest container, with freeboard.
Building containment.
Indoor oil storage can use the building itself as containment if floor drains are sealed or routed to a holding area and the floor and curbs hold the required volume.
Active containment systems.
Quick-closing valves on floor drains, deployable berms, or shutoff systems. Allowed under SPCC but must be documented with response procedures and inspection records showing the system works.
Drainage Is Part of the Containment System
A diked area that holds the required volume but drains through an open valve to a storm drain isn’t containment. The rule requires that drainage from a containment area be controlled. If your containment drains toward a storm drain, stormwater rules may also apply; our SPCC vs SWPPP comparison explains which plan covers what.
Three approaches the rule accepts:
- Manual valves, normally closed. Operators inspect accumulated rainwater for oil sheen before opening the valve and discharging. Requires a documented procedure and trained personnel.
- Sumps and pumps. Accumulated water collects in a sump and is pumped to a holding tank or treated before discharge.
- No drainage. Smaller containment areas may be designed with no outlet. Accumulated rainwater is removed manually after inspection.
Whichever method is used, the plan has to describe it and the inspection log has to show it’s being followed.
What California Inspectors Check
In California, your local CUPA (which may be a county environmental health department, fire department, or another local agency depending on jurisdiction) enforces the Aboveground Petroleum Storage Act, the state program that runs alongside federal SPCC. APSA covers petroleum products only, and was updated under the Title 19 regulations effective December 17, 2024. SPCC, by contrast, covers all oils, including vegetable, animal, and synthetic.
When a CUPA inspector reviews containment, they typically work in this order:
Compare the plan’s containment calculation to the actual containment area. They will pull out a tape measure if anything looks off.
Check that drainage valves are closed and that the inspection log shows when accumulated water was last released.
Look for cracks, gaps, or unsealed joints in the dike. A dike with cracking at the base is not impervious.
Confirm that drum and tote storage has a spill pallet sized correctly.
Check test and inspection records for any active containment system.
The most common deficiency we find when we walk a facility for the first time isn’t undersized containment, it’s a containment area that was sized correctly when the plan was written but no longer matches the current tank layout. Tanks get added, removed, or relocated. A dike that held the required volume at 5,500 gallons is no longer adequate after a new 3,000-gallon tank is dropped inside the same area.
This is one reason monthly visual inspections and annual comprehensive reviews matter, and why CDMS builds the SPCC inspection program into plan delivery rather than leaving it to the facility to set up afterward. Tank integrity and containment are related obligations. For more on the integrity side, see our hazardous waste tank certification overview.
Get Your Containment Sized and Documented
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