SPCC Secondary Containment: Sizing, Freeboard, and Inspections

The SPCC secondary containment requirements in 40 CFR Part 112 come down to a simple sizing rule with a few strict design details: the containment around your bulk oil storage must hold the full shell capacity of the largest single tank inside the diked area, plus enough extra height (freeboard) to catch rainfall without overtopping.¹ Walls and floors have to be impervious enough to hold oil, drainage valves have to be manually operated, and accumulated rainwater has to be inspected before it’s released.

Does the Rule Apply to Your Facility

The SPCC rule reaches non-transportation-related facilities that store oil and could reasonably be expected to discharge into navigable waters or adjoining shorelines. The trigger is aggregate storage capacity: more than 1,320 gallons aboveground, or more than 42,000 gallons buried underground.² Cross either threshold and you need a written SPCC plan that addresses secondary containment.

“Oil” is defined broadly. It covers crude oil, diesel, gasoline, hydraulic fluid, lubricating oil, and even vegetable oil. Facilities that are often surprised to find themselves subject to the rule include farms with large diesel tanks, hospitals with backup generator fuel, and commercial buildings with heating oil.

How Big the Containment Must Be

The core sizing standard is the full shell capacity of the largest single tank inside the diked area, plus sufficient freeboard for precipitation.¹ If the largest tank in a shared dike holds 20,000 gallons, containment must be able to capture all 20,000 gallons, whatever else is in the dike.

You may have heard of a “110% rule.” That figure is not in the federal regulation. It became an industry convention because engineers assumed the largest tank volume plus a precipitation allowance came out to roughly 110%, and many older facility plans treated it as if it were the standard. The real requirement is the largest tank’s shell capacity plus enough freeboard for rainfall, which depending on your location may work out to more or less than 110%.

Shared Dikes and Tank Displacement

When several tanks sit inside one dike, the math has to account for the floor space the other tanks occupy. Those tanks displace containment volume, so a dike that looks adequate on paper can fall short once you subtract what the smaller tanks take up.³ The calculation runs in four steps:

Measure interior length, width, and wall height of the containment area to find gross dike volume.
Calculate the volume displaced by every tank in the dike except the largest one, which is assumed to have ruptured.
Subtract that displacement from gross dike volume. The remainder must equal or exceed the shell capacity of the largest tank.
Confirm there is still enough height above the largest-tank volume to hold the precipitation allowance.

Skipping the displacement step is one of the most common sizing errors.

Drums, Totes, and Portable Containers

Mobile and portable oil containers follow the same principle. Containment must hold the capacity of the largest single container or compartment, plus freeboard for precipitation.¹ Mobile refuelers and non-transportation-related tank trucks are the exception; they are exempt from the containment mandate but still must be positioned to prevent discharges.

Oil-Filled Operational Equipment

Transformers, circuit breakers, and similar oil-filled operational equipment do not have to follow the strict largest-tank formula. Under 40 CFR 112.7(k), they can qualify for alternate requirements that weigh discharge likelihood and proximity to waterways instead of demanding containment sized to the full oil volume.⁴ A professional engineer usually designs containment based on the largest internal oil compartment.

Freeboard for Rain and Snow

Outdoor containment collects precipitation, and that precipitation eats into the space you reserved for a spill. The regulation requires “sufficient freeboard to contain precipitation” but does not name a specific storm event in the regulatory text.¹ The EPA has stated that it considers the proper benchmark to be enough capacity to hold a 25-year, 24-hour storm event, though it declined to codify that as a hard rule because gathering local storm data can be difficult and expensive.⁵

In practice, inspectors evaluate freeboard against that 25-year, 24-hour storm. You can pull the rainfall depth for your location from NOAA’s precipitation frequency atlas. Multiply that depth by the surface area of your dike and the result is the freeboard volume you need above the largest-tank capacity. Many engineers add another six inches of wall height as a buffer against wave action, splashing during a spill, or settlement of earthen dikes.

Wall and Floor Construction

Dikes, berms, and retaining walls must be “sufficiently impervious to contain oil.”⁴ Reinforced concrete is the most common choice because it resists petroleum degradation and stands up to the hydrostatic pressure of a full spill. Compacted clay, high-density polyethylene liners, and steel barriers can also work, provided they are engineered to keep oil out of the subsoil and groundwater.

Joints, penetrations, and porous surfaces need seals or liners so the barrier stays liquid-tight. Walls have to be designed for the lateral pressure of the contained liquid at full depth. Thin or under-reinforced walls can fail catastrophically under the weight of thousands of gallons of oil mixed with rainwater.

Drainage Valves and Pumps

How you get rainwater out of the dike matters as much as how you keep oil in. The regulation requires manual, open-and-closed drain valves. Flapper-type valves are explicitly prohibited because they can let oil escape uncontrolled.¹

Before opening any drain valve, staff must visually inspect the accumulated water for sheens, discoloration, or odor. If oil is present, that water cannot go to a storm sewer or open watercourse. It has to be pumped out for treatment or disposal. Pumps and ejectors used to empty diked areas must be manually activated after inspection, not left on automatic.¹

This is where facilities get cited most often. Drain valves left open, automatic pumps running without a pre-drain inspection, and draining water with a visible sheen are among the most frequent SPCC violations.

Inspection and Recordkeeping

The regulation requires frequent inspection of the outside of bulk storage containers for signs of deterioration, leaks, or oil accumulation inside diked areas.¹ The rule does not set a specific interval like “weekly” or “monthly.” Your facility develops written inspection procedures, and the frequency has to be appropriate for the risk.

Inspection and testing records must be signed by the appropriate supervisor or inspector and kept on-site with the SPCC plan for at least three years.⁴ Records kept under your usual business practices satisfy the requirement, so no specialized form is needed. Something written down with a name and date on it, however, is not optional.

Who Signs the Plan

Most SPCC plans must be certified by a licensed Professional Engineer. The PE reviews the technical content, including the secondary containment design, and certifies that it meets the regulation. Whether the PE must apply a seal or just sign a certification statement depends on the licensing laws of the state where the facility sits.⁶

Smaller facilities can skip PE certification by qualifying as Tier I or Tier II:

Tier II qualified facility: aggregate aboveground oil storage of 10,000 gallons or less, with no reportable spill history (no single discharge over 1,000 gallons, and no two discharges over 42 gallons each within 12 months in the prior three years). The owner or operator may self-certify the plan.⁷
Tier I qualified facility: meets all Tier II criteria and has no individual aboveground container larger than 5,000 gallons. Tier I facilities may self-certify and use the EPA’s streamlined plan template in Appendix G of Part 112.⁷

Self-certification does not relax the containment standards. Tier I and Tier II facilities have to meet the same capacity and structural requirements. The exemption is only about who signs the plan.

What Noncompliance Costs

The Clean Water Act gives the EPA authority to pursue both civil and criminal penalties for SPCC violations. Civil penalties under Section 311(b)(6) are adjusted periodically for inflation and can exceed $20,000 per day per violation.⁸ Criminal penalties for negligent violations run between $2,500 and $25,000 per day and up to one year in prison, doubling on a second conviction; knowing violations carry significantly higher penalties.⁹ The EPA actively pursues enforcement actions against facilities with inadequate or missing secondary containment, and the per-day structure makes even a short lapse expensive.

1
eCFR. 40 CFR 112.8 – Spill Prevention, Control, and Countermeasure Plan Requirements for Onshore Facilities (Excluding Production Facilities)
2
eCFR. 40 CFR Part 112 – Oil Pollution Prevention
3
Environmental Protection Agency (EPA). Example – Multiple Horizontal Cylindrical Tanks Inside a Rectangular or Square Dike or Berm
4
eCFR. 40 CFR 112.7 – General Requirements for Spill Prevention, Control, and Countermeasure Plans
5
United States Environmental Protection Agency. What Are the Specifications for Bulk Storage Secondary Containment Systems
6
U.S. Environmental Protection Agency. PE Certification and Applying PEs Seal
7
U.S. Environmental Protection Agency. Difference Between an SPCC Tier I and Tier II Qualified Facility
8
Environmental Protection Agency. 2023 Spill Prevention, Control and Countermeasures Expedited Settlement Agreement
9
Office of the Law Revision Counsel. 33 USC 1319 – Enforcement