What is a Qualifying Rain Event Under Stormwater Regulation?

This week’s article is going to be short; however, it is important given the topic causes a great deal of confusion. Several weeks back I wrote about stormwater permitting requirements which provides a more in-depth conversation over the regulations, click here to learn more. In every stormwater permit around the U.S., there is some form of stormwater sampling requirements. These can be samples that are utilized for visual analysis only, generally called “quarterly visual monitoring”, or samples sent to a laboratory for analysis. What one samples for, the frequency, and the analysis parameters vary greatly based on what state you reside in, type of operation, or if an impaired water body is nearby. However, the one piece that is consistent across the board is what is considered a “qualifying rain event.”

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The way I explain a qualifying rain event to my clients is as follows:

• Firstly, once the rain starts, it is important to confirm that there has been no rainwater discharge off the site in 72-hours. (Which is different than “has it rained in 72-hours?”). The sampling is contingent on water leaving your property. Therefore, if you have had no discharge due to the lack of rain volume then the rain event does not count towards your 72-hours.
• If the answer is “yes” then you must ask yourself “How much time has elapsed since the stormwater has started discharging from your designated outfall?” If the answer is less than an hour, you are in the clear. Most permits indicate immediately or within 30 minutes; however, a good guiding post is no longer than one hour. After one hour has elapsed, any useable sample has been washed offsite, thus rendering the sampling process moot.

Once confirmed, grab your sampling containers or transfer container and head out to your outfall for collection.

Here is an example from Texas’ general stormwater permit on how regulations generally state this:

Monitoring, sampling, examinations, and inspections of stormwater discharges that are required as a provision of this general permit must be conducted on discharges from a measureable storm event that results in an actual discharge from the site, and that follows the preceding measurable storm event by at least 72 hours (3 days). The 72-hour storm interval does not apply if the permittee is able to document in the SWP3 that less than a 72-hour (3-day) interval is representative for local qualifying storm events during the sampling period. In the case of snowmelt, the monitoring must be

performed at a time when a measurable discharge occurs at the site.

A facility that has retention ponds as BMPs will not always have a discharge from the pond(s) immediately following a qualifying storm event. If any storm events occurred prior to discharge from the outfall, regardless of the time period between the last storm event and the discharge, the permittee may consider the discharge to be the result of the previous qualifying storm event. For a complete listing of archived blogs and compliance insights, click here . Past blogs cover training requirements, clarification on additional confusing elements within the above rules, and much more.

Sampling should be conducted within the first 30 minutes of discharge using a grab sample. Sampling from retention ponds described in Part III, Section D.1.b. above should be conducted within 30 minutes of the initiation of discharge from the pond. If it is not practicable to collect the sample or to complete the sampling within the first 30 minutes, then sampling must be completed within the first hour of discharge.

If sampling is not completed within the first 30 minutes of discharge, the reason must be documented and attached to all required reports and records of the sampling activity.

In the case of snowmelt, samples must be taken during a period with a measurable discharge.

We are here to help solve your compliance questions and challenges. Need some compliance assistance, or just have a question? Please email John K. Carroll III ([email protected] ) Associate Managing Director – Compliance Services or call at +1 281-320-9796.

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