Secondary containment is the last line of defense between an oil storage failure and a discharge to navigable waters. Under 40 CFR 112.7(c), every SPCC plan must describe the secondary containment for each oil storage area, demonstrate that containment capacity is adequate, and establish procedures for drainage and maintenance. Getting containment wrong is one of the most common SPCC violations and one of the most consequential: if a tank fails and containment is inadequate, the resulting discharge can trigger cleanup liability, EPA enforcement, and penalties under the Clean Water Act.
Common Containment Compliance Challenges
The Capacity Rule (40 CFR § 112.7(c))
Largest Container Plus Freeboard
The fundamental SPCC containment requirement is that secondary containment must be sized to hold:
- The capacity of the largest single container within the containment area
- Plus sufficient freeboard to contain precipitation from a 25-year, 24-hour rainfall event (or the maximum precipitation event recorded for the area, whichever is greater)
When multiple containers share the same containment area, the net available volume must account for the displacement of all other containers, piping, foundations, and equipment within the containment footprint.
Containment Types
Dikes and Berms
The most common form of secondary containment for aboveground storage tanks. Constructed from earth, concrete, steel, or a combination of materials. Dikes surround the tank(s) and create a basin to capture any release.
- Earthen dikes must be compacted and may require a synthetic or clay liner to be sufficiently impervious to oil
- Concrete dikes provide reliable imperviousness but must be inspected for cracks and joint failures
- Steel containment walls are common in smaller installations and prefabricated containment systems
- Dike walls must be capable of withstanding the hydrostatic pressure of a full release from the largest container
Double-Walled Tanks
A double-walled tank provides built-in secondary containment: the outer wall contains any release from the inner tank. Double-wall construction is common for smaller tanks (under 12,000 gallons) and is widely used for generator fuel tanks, day tanks, and transformers. The interstitial space between walls should be monitored for leaks, typically with interstitial monitors or periodic inspections.
Drainage Trays and Drip Pans
Used for smaller containers such as drums, totes, and oil-filled equipment (transformers, hydraulic units). Drainage trays sit beneath the container and capture drips, leaks, or small releases. Must be sized to contain the volume of the largest container placed on the tray.
Retention Ponds and Remote Impoundments
Some facilities use engineered retention ponds or remote impoundments connected to tank areas by drainage channels. Oil flows by gravity to the impoundment where it can be recovered. These systems require careful engineering to ensure drainage paths are reliable and the impoundment is adequately sized and lined.
Precipitation and Freeboard Calculations
Accounting for Rainfall
Containment must accommodate both a full release from the largest container and accumulated precipitation. The standard design basis is the 25-year, 24-hour rainfall event for the facility's geographic location, though some facilities use the maximum recorded precipitation event if it exceeds the 25-year value.
- Obtain local precipitation data from NOAA Atlas 14 or equivalent state rainfall atlas
- Calculate the volume of rainfall across the entire containment footprint for the design storm event
- Add this precipitation volume to the largest container capacity to determine the total required containment volume
- Subtract displacement volumes of all containers, piping, foundations, and equipment within the containment area
- The resulting net available volume must equal or exceed the total required volume
Drainage Valve Management
Stormwater Management in Containment
Accumulated precipitation must be removed promptly to maintain containment capacity. Options include:
- Manual drainage through supervised valve operation after visual inspection
- Sump pumps with oil-water separators or oil-stop valves that prevent oil from being discharged
- Oil/water separators that treat accumulated water before discharge
Whichever method is used, it must be described in the SPCC plan and the facility must maintain records of drainage events and inspections.
Containment Inspections
Secondary containment must be inspected regularly to ensure it remains functional. Inspection items include structural integrity of dike walls, liner condition, drainage valve operability, accumulation of debris or vegetation, erosion or settlement, and any evidence of oil contamination. Findings must be documented and deficiencies corrected promptly through tracked corrective actions.
Frequently Asked Questions
What is the SPCC secondary containment capacity requirement?
Under 40 CFR 112.7(c), secondary containment must be sized to hold the capacity of the largest single container within the containment area, plus sufficient freeboard to contain precipitation. The containment volume must account for the displacement volume of all other containers within the same containment area. This is commonly called the largest-container-plus-freeboard rule.
What does the 110 percent secondary containment rule mean?
The 110 percent figure is a widely used design convention, not the wording of the federal SPCC rule. It comes from taking the largest container's capacity and adding roughly 10 percent as freeboard for precipitation. 40 CFR 112.7(c) itself requires the capacity of the largest single container plus sufficient freeboard for precipitation, without naming a percentage. Some state and local programs and some tank standards do specify 110 percent explicitly, so check the requirements that apply to your site rather than assuming the convention satisfies them.
What is sized secondary containment?
Sized secondary containment is containment engineered to a calculated volume for a specific bulk storage container, as required by 40 CFR 112.8(c)(2) for bulk storage. It is distinguished from general secondary containment under 112.7(c), which must contain the most likely discharge from an activity such as a transfer or a piece of oil-filled equipment but is not sized to a container's full capacity. Bulk storage containers require sized containment; transfer areas, mobile containers, and oil-filled operational equipment generally require general containment.
What types of secondary containment are acceptable under SPCC?
Acceptable containment types include dikes and berms (earthen, concrete, or steel), double-walled tanks, drainage trays and drip pans, retention ponds, curbing, and other equivalent containment methods. The containment must be sufficiently impervious to contain oil until it can be cleaned up, and the method chosen must be appropriate for the container size, contents, and site conditions.
How do you calculate secondary containment volume?
Calculate the gross volume of the containment structure based on its dimensions and fill height. Then subtract the displacement volume of all containers, piping, and equipment within the containment area. The resulting net volume must equal or exceed the capacity of the largest single container plus a freeboard allowance for precipitation. SPCC software and containment volume calculators automate this calculation.
Must drainage valves in secondary containment be open or closed?
Drainage valves must be maintained in the closed position at all times unless actively draining accumulated stormwater under supervised conditions. Before draining, the accumulated water must be inspected to ensure it is not contaminated with oil. If oil is present, it must be recovered before drainage. The valve must be returned to the closed position immediately after draining is complete.
How often must secondary containment be inspected?
The SPCC rule does not fix a single universal interval. Containment must be inspected on a schedule described in the facility's plan and frequently enough to confirm it will function when needed, which in practice usually means monthly for routine visual checks with more frequent checks after significant rainfall. Inspections should confirm structural integrity, available freeboard, drainage valve position, liner condition, and the absence of sheen or oil in accumulated water. Findings and corrective actions must be documented and retained for three years.
Is secondary containment required for all oil containers?
The SPCC rule requires appropriate containment or diversionary structures for all bulk storage containers. For qualified facilities with total capacity of 10,000 gallons or less, general secondary containment requirements apply. For larger facilities, the rule provides specific containment requirements for different container types including bulk storage, mobile containers, oil-filled equipment, and transfer areas.
Ecesis SPCC Software
SPCC Software
Automated containment calculations, container management, and plan generation.
Inspection Software
Schedule and document containment inspections with mobile checklists.
Preventive Maintenance
Track containment repairs, liner replacements, and structural maintenance.
Task Management
Assign and track corrective actions from containment inspection findings.
Incident Management
Log spill events and trigger regulatory reporting workflows automatically.
Mobile EHS App
Complete containment inspections in the field with photos and GPS.
