Secondary containment is the engineered barrier, a berm, dike, or liner, that catches a spill before it reaches soil or water. For oilfield tanks it is both a federal requirement and one of the most frequently cited items during a field inspection, because a failed or undersized berm is easy for a regulator to spot. This guide covers when containment is required, how to size it, what materials qualify, and how the Texas RRC rules overlap.
What is secondary containment, and when is it required?
Secondary containment is a barrier around oil storage that holds the contents of a leak or rupture so it cannot escape to the environment. For Texas operators it sits under overlapping authorities: the EPA's SPCC rule (40 CFR Part 112), which sets the sizing standard for any facility with more than 1,320 gallons of aggregate aboveground oil storage, and the Texas Railroad Commission's rules, which require fire walls around certain oil tanks (Statewide Rule 21) and prohibit pollution of surface and subsurface water (16 TAC Chapter 4, which replaced Statewide Rule 8 on July 1, 2025). A facility can be subject to all of them at once, so the design has to work for each.
How do you size secondary containment?
The core rule is simple: containment must hold the full volume of the largest single container, plus enough freeboard to hold precipitation without overflowing. The two ways operators calculate that freeboard are the 110 percent rule of thumb and the 25-year, 24-hour storm event. Where several containers sit inside one berm, a separate RCRA-style test also applies: the greater of 10 percent of the total volume of all containers or the full volume of the largest container.
Volumetric requirements: EPA vs. RRC
| Requirement | EPA SPCC / RCRA | Texas RRC | Best-practice design |
|---|---|---|---|
| Base volume | Full volume of the largest container (110% rule of thumb), or 10% of all containers, whichever is greater | No general sizing standard. Rule 21(j) requires fire walls around permanent oil tanks in specified locations but sets no capacity. | Design to the SPCC standard: the full volume of the largest container |
| Freeboard / precipitation | Sufficient freeboard for a 25-year, 24-hour storm event | Not prescribed | Add the 25-year, 24-hour storm volume, which is often more than 10 percent |
| Governing rule | 40 CFR Part 112 (SPCC) and 40 CFR 264.193 (RCRA) | 16 TAC 3.21(j) (Rule 21, fire walls); 16 TAC Chapter 4 water protection (formerly Rule 8) | A single design that satisfies each |
What is the 110% rule?
The 110 percent rule is a fire-code convention that sizes containment to the full volume of the largest container plus 10 percent for freeboard. The EPA accepts it as a starting rule of thumb, but it is explicit that outdoors, 10 percent may not provide enough precipitation capacity. In wetter parts of Texas the 25-year, 24-hour storm event can require significantly more than 10 percent, and when it does, that documented freeboard need governs, not the 110 percent shortcut.
What do Texas RRC rules require?
The RRC's tank-specific requirement is Statewide Rule 21(j) (16 TAC 3.21(j)), part of its fire prevention rule. It requires fire walls to be built and kept around all permanent oil tanks, or batteries of tanks, that are within the corporate limits of a city, town, or village; closer than 500 feet to a highway or inhabited dwelling; closer than 1,000 feet to a school or church; or located where the Commission deems them an objectionable hazard. The rule does not set a capacity, so sizing is driven by SPCC. In RRC inspection data since 2015, dike or fire wall requirements under Rule 21(j) were cited 4,942 times, so an eroded or missing fire wall is a real inspection item.
Beyond fire walls, the RRC's oil and gas waste rules prohibit anyone from causing or allowing pollution of surface or subsurface water. Those rules were Statewide Rule 8 until July 1, 2025, when the RRC replaced Rule 8 with 16 TAC Chapter 4, Subchapter A. A release that escapes failed containment can therefore become an RRC violation as well as an SPCC one, which makes containment one of the clearest overlaps between federal SPCC work and state RRC compliance.
What materials qualify for secondary containment?
Whatever the geometry, the containment fails if the material cannot hold the product. Under 40 CFR 264.193 the floor and berms must be sufficiently impervious to prevent any migration of the fluid to soil, groundwater, or surface water. Material choice, and inspecting it for the specific ways it degrades, is where many otherwise-compliant systems fall down.
Containment materials and inspection points
| Material | Technical standard | Key inspection points |
|---|---|---|
| Compacted clay / earthen berm | Minimum permeability of 1 x 10-7 cm/sec, documented with soil and compaction testing | Erosion, animal burrows, root growth creating leak pathways, desiccation cracks |
| Poured concrete | Cured and sealed with a chemically compatible coating; joints need waterstops or sealant | Cracks, spalling, exposed rebar, sealant degradation, staining that indicates porosity |
| Geosynthetic liner (HDPE) | Thickness (30 to 60 mil) and material compatible with the stored fluid; seams professionally welded and tested | Punctures, tears, UV degradation, seam separation, settlement beneath the liner |
What gets secondary containment cited during an inspection?
Most containment citations are not exotic. Inspectors look for the visible signs a berm can no longer do its job: a firewall that has slumped or eroded below its required height, cracked or spalling concrete, a liner that is torn or lifting, vegetation and burrows in an earthen berm, standing product or heavy staining inside the containment, and a drain valve left open. Because these are easy to see and easy to document, containment is a routine focus of both RRC and EPA field inspections, and it is worth walking your own facilities with the same eye before an inspector does.
Designing containment that satisfies every rule at once
The requirements above come from three different authorities, and the cheapest way to fail is to design to only one of them. Tektite designs and audits secondary containment to satisfy EPA SPCC, the RRC's fire wall and water protection rules, and RCRA imperviousness standards in a single system, and documents the volume and freeboard calculations so they hold up under inspection. If you are unsure whether your existing berms hold the largest tank's volume or carry enough storm freeboard, a review as part of SPCC plan development will tell you before a discharge does.
Related reading
Frequently Asked Questions
How do you calculate secondary containment capacity?
Under EPA SPCC, containment must hold the full volume of the largest single container plus sufficient freeboard for precipitation. A common rule of thumb is 110 percent of the largest container (the extra 10 percent for freeboard), but for outdoor tanks the freeboard should be sized to a 25-year, 24-hour storm event, which often requires more than 10 percent. Where multiple containers share a berm, the RCRA-style test of 10 percent of the total volume or the full volume of the largest container, whichever is greater, also applies.
What is the 110% rule for secondary containment?
The 110 percent rule is a fire-code convention that sizes containment to the full volume of the largest container plus 10 percent for freeboard. The EPA accepts it as a starting rule of thumb, but explicitly notes that outdoors, 10 percent may not be enough precipitation capacity, and the documented freeboard need (typically a 25-year, 24-hour storm) controls.
What do Texas RRC rules require for tank containment?
The RRC does not set a general containment sizing standard the way SPCC does. Its tank-specific rule is Statewide Rule 21(j), which requires fire walls around permanent oil tanks located within a city, town, or village, closer than 500 feet to a highway or inhabited dwelling, closer than 1,000 feet to a school or church, or wherever the Commission deems them an objectionable hazard; the rule does not specify a capacity. Separately, the RRC's oil and gas waste rules (16 TAC Chapter 4, which replaced Statewide Rule 8 in July 2025) prohibit pollution of surface and subsurface water, so a release from failed containment is an RRC problem as well as an SPCC one. Sizing still follows the SPCC standard.
What materials qualify for secondary containment?
Floors and berms must be sufficiently impervious to prevent migration of the stored fluid to soil, groundwater, or surface water. Common options are compacted clay at a minimum permeability of 1 x 10-7 cm/sec, cured and sealed poured concrete, and chemically compatible geosynthetic liners such as HDPE.
