What is a Qualifying Storm Event? - Industrial General Stormwater Permitting

Last week’s article was a reminder to get your Discharge Monitoring Reports (DMRs) in before the March 31st deadline. You can read the article if you are unfamiliar with DMRs. The backbone of DMRs in relation to industrial general stormwater permitting is stormwater runoff sampling. The term sampling seems straightforward: grab some stormwater in a container and ship it off to be tested. However, industrial general stormwater permits around the United States have specific definitions for sampling, and these distinctions can cause confusion. Every state has a slight variation on how the sampling requirement is worded within their state-issued permit. Outside of these small wording differences, however, they all follow the guidelines set forth in the Environment Protection Agency’s (EPA) Multi-Sector General Permit (MSGP).

As always, let’s first look at the rule:

3.2.2.1 Make the assessment of a stormwater discharge sample in a clean, colorless glass or plastic container, and examined in a well-lit area;

3.2.2.2 Make the assessment of the sample you collected within the first 30 minutes of an actual discharge from a storm event. If it is not possible to collect the sample within the first 30 minutes of discharge, the sample must be collected as soon as practicable after the first 30 minutes and you must document why it was not possible to take the sample within the first 30 minutes. In the case of snowmelt, samples must be taken during a period with a measurable discharge; and

3.2.2.3 For storm events, make the assessment on discharges that occur at least 72 hours (three days) from the previous discharge. The 72-hour (three-day) storm interval does not apply if you document that less than a 72-hour (three-day) interval is representative for local storm events during the sampling period.

The EPA’s MSGP does a much better job simplifying what sampling is than some states do when they issued their state-specific permits.

Here is how Witt O’Brien’s explains it to our clients: if you have not had a stormwater discharge off your property within 72 hours, you should take a sample within 30 minutes, and no later than one hour, once the stormwater starts to leave out of your outfall. We always emphasize that you should do your sampling once a discharge event has begun out one’s outfall, not focusing on when it started to rain. NOTE: we won’t address frequency and what to do with the sample in this article. You can refer to your permit for this guidance.

Let’s break this down. Why should you sample after 72 hours of no discharge and not after no rainfall? First, it may have just sprinkled for the past two days resulting in no stormwater discharges off your site. As such, everything that you are sampling for is still on site. Also, for various reasons, it may take four hours of steady rain before stormwater starts to run out of your outfall. As such, in stormwater sampling, we say discharge from the site, not a rain event when talking about the 72-hour timeframe.

Why is this important? Stormwater sampling under this type of permitting is done to determine if there are any site concerns. If you sample after stormwater has run off your property continuously over a period of time your sample will not be representative of onsite conditions, as the rain essentially acted like a dishwasher. That may be a good thing for some properties, but it is not a true reflection of what is happening on-site.

Outside of when to sample, another big mistake companies make when sampling is in how they collect the sample. Many simply grab a clean clear container go out to their outfall and lay the container directly on the ground to allow stormwater to run into the container. This poses the risk of stirring up debris which can cause issues with the water sample. Sample containers should be slightly elevated off the ground. If there’s not enough flow for water to enter the container, it’s more than likely that there’s not enough qualifying rainfall.

What do you sample for? That’s too large of a discussion for any single article, as states are allowed to expand upon what the EPA establishes in the MSGP. Further, sampling is industry-sector specific. If you have any questions about what is required for your company, shoot me an email.

I hope this article simplifies sampling. If you still have questions, reach out to me. I’ve been involved in thousands of stormwater permits.

For a complete listing of archived articles and compliance insights, click here. Past articles cover training requirements, clarification on additional unclear elements within the above rules, and much more.

We are here to help solve your compliance questions and challenges. If you need compliance assistance or have a question, please email John K. Carroll III (jcarroll@wittobriens.com), Associate Managing Director – Compliance Services, or call +1 954-625-9373.

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