Learn how California's Exceedance Response Action plans work, including NAL triggers, Level 1 and 2 requirements, compliance deadlines, and how to return to baseline status.
An Exceedance Response Action (ERA) is a required corrective process under California's Industrial General Permit (IGP). It triggers when your stormwater samples exceed Numeric Action Levels (NALs). The response is two-tiered: Level 1 uses operational source controls, while Level 2 requires advanced treatment and a new NAL.
What is an Exceedance Response Action?
An ERA is a mandatory corrective action process under the California IGP. It activates when your stormwater monitoring results exceed NALs. This is a response to data, not an immediate violation of the permit itself. The IGP requires dischargers to compare annual monitoring results to specified NALs to determine if BMP modifications are needed. An exceedance triggers response requirements but does not automatically mean a permit violation. This framework is specific to California's industrial stormwater regime and differs from federal MSGP requirements.
How are NAL exceedances calculated?
Exceedances are determined in two ways within a reporting year. An instantaneous maximum exceedance occurs if two or more samples for a parameter exceed the NAL. For example, this applies to TSS and O&G values or pH outside a specific range. An annual exceedance occurs if the average of all analytical results for that parameter exceeds the annual NAL value. The IGP defines these triggers clearly in its monitoring sections. Facilities must track each distinct sample against the corresponding instantaneous maximum NAL values in Table 2 of the permit.
| Feature | California IGP ERA | Federal MSGP AIM |
|---|---|---|
| Trigger Basis | Numeric Action Levels (NALs) | Benchmark Thresholds |
| Response Levels | Level 1 and Level 2 | Level 1, Level 2, Level 3 |
| Initial Status | Baseline for all facilities | Baseline for all facilities |
| Key Action | QISP Report and BMPs | Additional Implementation Measures |
What are the ERA compliance deadlines?
If an exceedance is found, Level 1 status begins on the following July 1. You must implement operational source control BMPs by August 30. A Level 1 ERA Report, prepared by a Qualified Industrial Storm Water Practitioner (QISP), is due by January 1. These dates are fixed in the IGP order and do not shift with your specific sampling schedule. Missing these deadlines can complicate your compliance record and delay any potential return to baseline status. Planning your site evaluation early in the year helps ensure the QISP has enough time to complete the technical report before the January deadline.
What is the difference between Level 1 and Level 2?
Level 1 is the first response to an NAL exceedance. It focuses on operational source controls and a QISP report. Level 2 is triggered by a second exceedance of the same parameter. This escalation requires more advanced source or treatment controls. You must submit a Level 2 ERA Report and provide a new NAL for that parameter. The shift from Level 1 to Level 2 signals that operational fixes were insufficient. It moves the focus toward engineering solutions that directly treat the pollutant in the water column, rather than just preventing it from entering the system.
How do you find the source of an exceedance?
The QISP must conduct a site evaluation and root-cause analysis. This determines if the exceedance is from industrial activity or background conditions. The process involves reviewing your SWPPP and inspecting current controls. You must evaluate if existing BMPs are addressing the specific pollutant source. For instance, if metals are elevated, you need to identify which operations generate them. If the exceedance is driven by the dissolved fraction, sediment controls will not help. You must trace the pollutant upstream from the outfall to identify surfaces and operations contributing to the load. This analysis justifies the corrective actions chosen in the ERA report.
Which BMPs fix exceedances?
Level 1 typically uses operational source controls like housekeeping and spill prevention. Level 2 requires treatment controls that address the specific pollutant. This often means filtration media for dissolved metals or hydrocarbons. Sediment controls often miss the dissolved fraction that causes many benchmark exceedances. For oil and grease, you might look at industrial stormwater oil and grease action levels to understand triggers. For nutrients, a phosphorus benchmark exceedance corrective action plan helps document the steps from lab result to installed BMP. The goal is documented, measurable pollutant reduction at the outfall.
How does ERA differ from MSGP AIMs?
The California IGP uses a two-level ERA system based on NALs. The federal MSGP uses a three-level Additional Implementation Measures (AIM) system based on benchmark thresholds. Both require escalating responses but have different triggers and reporting structures. Under the MSGP, an AIM is triggered by an exceedance of a benchmark monitoring parameter. This can happen if the four-quarterly annual average is exceeded or if a single sample exceeds the benchmark by more than four times. Facilities subject to both permits must track requirements for each separately. Confusing the two can lead to missed deadlines or incorrect reporting.
How do you return to baseline status?
You return to baseline by demonstrating that the exceedance was not from industrial activity. Alternatively, you must show you are using Best Available Technology (BAT/BCT) and no more BMPs are needed. This exit strategy is often the most difficult part to document. You need a clear record of your root-cause analysis and the effectiveness of your new controls. If you can prove the pollutant is background, you can exit Level 1 or 2 status. If not, you must maintain the advanced controls until your sampling data shows sustained compliance with the new NAL.
If you are facing an exceedance and need to match treatment media to your site's specific water quality, schedule a site assessment to review your options.