Learn what triggers MSGP AIM Level 1, what the Part 5.2 response requires, and how a parameter returns to baseline. Includes how to match a revised control measure to the pollutant fraction.
AIM Level 1 is the first tier of Additional Implementation Measures under Part 5.2 of the 2021 MSGP. AIM levels are a feature of EPA's 2021 MSGP. If your facility is covered by a state-issued industrial permit, check that permit's corrective-action section, because it may use different triggers. You enter it when a parameter's four-quarter annual average exceeds its benchmark, or when one early sample is high enough to make that exceedance certain. Once triggered, the AIM Level 1 response under Part 5.2 requires you to review and revise the stormwater control measures for that parameter and to document that review. You also keep sampling quarterly at the affected discharge points until four more quarters pass without a triggering event.
What triggers Level 1
Every operator subject to benchmark monitoring starts in baseline status. According to EPA's MSGP AIM data summary, each triggering event moves the facility up one level. The level is tracked separately for each discharge point and each parameter. A site can be at baseline for zinc and at Level 1 for phosphorus. It can also be at Level 1 at one outfall and at baseline at another.
There are two triggering events:
- The annual average. The four-quarter average for the parameter exceeds the benchmark.
- One high sample. You have fewer than four quarterly results, and a single sample, or the sum of your results so far that year, comes back at more than four times the benchmark (over 4B).
The second trigger settles the question before the year is out. Call the benchmark B. If one sample comes back over 4B, the annual average is at least that result divided by four, even if the other three quarters read zero. That average is greater than B, so the exceedance is already certain, and a parameter that was at baseline is in Level 1 now. You can begin the review this quarter; there's no reason to wait for the year to close.
What a Level 1 response requires
Review the control measures that apply to the triggering parameter and work out why they did not keep the discharge under the benchmark. Then implement whatever additional measures the review shows are needed, following the specific Level 1 requirements and deadlines in Part 5.2 of your permit. Then document what you found, what you changed and why you chose it. A benchmark exceedance is not by itself a permit violation, but that record is what regulators look for when they evaluate your response.
Start by asking whether your current BMPs address the actual pollutant or only sediment and trash. A sweeper and a sediment trap do little for dissolved zinc or dissolved phosphorus.
The history of exceedances also matters. On a first exceedance with an obvious source, tracing that source and correcting the operation may be a defensible response. A pattern of exceedances raises the bar. Each new triggering event moves the parameter up an AIM level. A Level 1 change that produces documented, measurable reduction at the outfall gives you the best chance of stopping that climb. For more on why source control alone often falls short once a site has a pattern of exceedances, see our post on industrial stormwater oil and grease action levels. If phosphorus triggered your Level 1, our phosphorus benchmark exceedance corrective action steps walk through the process from lab result to installed BMP.
Matching the revised control measure to the pollutant fraction
Pull the dissolved and total results for the triggering parameter, or add a dissolved analysis to your next sample if your lab reports total only. If most of it is dissolved, sediment controls won't move the number.
- Particulate-bound load. Metals, phosphorus or solids attached to sediment are a TSS capture problem. Smart Sponge® Sediment & TSS media is built to capture suspended solids and sediment while keeping water moving.
- Dissolved metals or phosphorus. Sediment controls let the dissolved fraction pass through. Smart Sponge® HM targets zinc, copper, lead, iron, arsenic and selenium, and it is also the media selected for phosphorus.
- Oil and grease or sheen. Standard Smart Sponge® is hydrophobic and binds oil, grease and hydrocarbons into a stable solid.
If your runoff carries more than one class of pollutant, such as organics behind elevated TOC or bacteria alongside metals, we can put several media in the same system.
The format can usually go into infrastructure you already have:
- Ultra Urban® Filter inserts in existing catch basins.
- Smart Pak® or Smart Bags in vaults and channels.
- Bulk media in existing pressure vessels.
Inserts, Smart Pak and Smart Bags are passive and run without power. A Smart Skid is packaged treatment with pumps, so it needs power where it sits.
Treatment can go at a single point or at several points in the drainage network. Where the pollutant originates, how much flow you have to treat, how much room there is and what the discharge has to meet all shape that choice.
Testing on your own water is the honest way to check media fit before you commit. On a larger site, we can add a column test on your own runoff and then a pilot to see how the media performs before a full installation. We start with a site assessment that looks at what is in your water, how much of it flows, where treatment can fit and how crews will reach it for maintenance. Schedule one and bring your lab reports.
How long monitoring continues and how you reset
Quarterly monitoring for the triggering parameter continues at all affected discharge points until four additional quarters pass without a triggering event. The parameter returns to baseline only if you also completed the benchmark responses within the required deadlines. Track those dates on the same calendar as your sampling, because the reset depends on meeting them.
| Stage | What happens | Scope |
|---|---|---|
| Baseline | Benchmark monitoring, no AIM obligation | Each discharge point and parameter |
| Triggering event | Annual average over benchmark, or one sample over 4× before four quarters are in | Moves that parameter up one level |
| Level 1 | Review and revise controls, document, continue quarterly sampling | Affected discharge points |
| Four more quarters with no trigger | Return to baseline, if response deadlines were met | That parameter at those points |
| Another triggering event | Advances to Level 2 | One level per event |
| Part 5.2.6 exception | Level set to baseline, no AIM corrective action | That discharge point and parameter |
What an exception does to your AIM level
Part 5.2.6 of the 2021 MSGP lists exceptions to AIM. While an exception is active, that discharge point and parameter are treated as baseline and are not subject to AIM corrective actions. Read the Part 5.2.6 conditions against your actual circumstances before you build your response around one.
What happens if the next quarters still exceed?
Another triggering event moves the parameter from Level 1 to Level 2, and the sequence continues to Level 3 after that. If your Level 1 fix targets the wrong fraction, you can spend another year monitoring and still end up at Level 2.
Take dissolved zinc as an example. If you respond by adding more sediment capture, the next four quarters will likely read much as the last four did.