What To Do With Stormwater Samples AFTER Collecting Them: Quick Guide

So You've Collected a Sample for Your NPDES Permit... Now What?!

This is where most facilities start to second-guess themselves - not about how to collect samples, but what to do with them afterward. The moment those bottles are filled, everything that happens next determines whether your results will count... or get thrown out.

And unfortunately, your regulatory agency is probably about as helpful as a brick wall when it comes to answering:

• Now that I have my samples, what exactly do I do with them?
• How long can I hold on to them before they become invalid?
• Do I need to refrigerate them? Freeze them? Deliver them myself?
• Can the lab pick them up... and should they?

If you’re asking any of these, you’re not alone. These are the exact questions we hear from clients all the time. And if you get the answers wrong, you risk wasting all that time and effort… and having to do the whole thing again. Or worse, getting into trouble!

We’re here to make sure that doesn’t happen. And if you’ve still got questions when you’re done reading, just reach out to us!

First things first: keep your samples safe (and cold).

Let’s assume you’ve just taken your stormwater samples. If you're still not totally sure how to do that, check out this article that covers 9 tips on how to take a good stormwater sample! But for now, let's say you’ve collected them into the proper containers, and you’re ready to get out of the rain (don't blame you).

Step one: don’t leave them sitting out. Secure them immediately in a cooler, a heavy-duty box, or something safe enough to keep them from tipping over or getting tampered with. If possible, put them directly into ice or next to ice packs, right there at the sampling location.

If you didn’t have ice on hand, then your priority is to get the samples into a cold environment fast. We’re talking minutes, not hours.

Refrigeration is okay, freezing is not. You want to keep your samples at or below 4°C (40°F), but do not let them freeze. If they freeze, the water expands, the container breaks, and you lose everything. And yes, we’ve seen this happen. Don’t be that facility.

Got pH sampling to do? You’ve got 15 minutes, max.

If your NPDES permit requires you to analyze for pH, this is time-sensitive. You’ve got 15 minutes from the moment the sample hits the bottle to get your pH result. That’s it.

This part takes planning. Know where your pH meter is, have it calibrated and ready, and don’t get distracted. For more on this, check out: Does the pH of a stormwater sample change after 15 minutes?

Know your holding times and don’t miss them.

Every sample parameter you test for has something called a holding time - that’s the time you have between sample collection and when it must be analyzed by the lab.

For example:

• Fecal coliform: as little as 8 hours
• Total petroleum hydrocarbons (TPH): 28 days

These holding times aren’t suggestions. If you miss them, your sample is invalid. That usually means you have to start over, including waiting for another qualified rain event. Which, (understandably) you probably don't want to do.

If you’re not sure what the holding times are for your parameters, call your lab. Discuss your sampling needs, verify the time limits, and make sure you have a plan to get your samples delivered before they expire.

We go into more detail on stormwater sample holding times here!

Transporting samples to the lab: should you deliver, or should they pick up?

Either option is fine as long as the lab gets the samples on time.

But here’s what we’ve seen more than once: a facility waits on a lab courier… and the courier shows up after the holding time has expired. At that point, the lab can’t help you. You’re starting over.

Our recommendation? Take the samples to the lab yourself if you can. It’s the safest way to make sure nothing gets missed. If you do use a lab pickup service, confirm that they understand your holding time limits and that they’re scheduled well in advance.

Don’t forget your chain of custody form.

Every sample needs to be accompanied by a chain of custody (COC) form. This is a required document that tells the lab:

• What each sample is
• When and where it was collected
• Who collected it
• What it needs to be analyzed for

It also shows exact time of collection, which matters when calculating holding time. Fill this form out completely and correctly before dropping your samples off. If your lab provides you with a copy, be sure to hang on to it and file it for safekeeping.

What to Do With Your Samples: A Quick Recap

Here’s what you need to do once you’ve collected your stormwater samples:

1. Secure them immediately. Use a cooler or heavy box to protect them from spills or tampering.
2. Keep them cold, but don’t freeze them. Use ice packs or a refrigerator. 40°F or colder.
3. If analyzing for pH, do it within 15 minutes.
4. Know your holding times. Every parameter has a time limit.
5. Get the samples to the lab ASAP. Don’t rely on a courier unless you trust their timing.
6. Bring your chain of custody form. It’s required for submission and analysis.

And one more thing: check your specific permit. Some states may have additional rules or documentation requirements.

Still have questions?

Too often, facilities get vague guidance or none at all when it comes to stormwater sampling. Labs don’t always explain holding times. Regulators rarely walk you through the details. And what’s at stake is more than just a missed test, it’s your compliance record.

That’s where we come in.

At RMA, we help you understand your permit inside and out. We walk through your sampling requirements, help you plan your process, and make sure nothing falls through the cracks after the sample is taken. Whether you’re dealing with complex parameters, short holding times, or multiple outfalls, we’ll help you get it right the first time.

We’ve helped hundreds of facilities navigate stormwater sampling and NPDES compliance. If you’re confused about any part of the process - before or after sample collection - we’re happy to help. Reach out here for straight answers. No confusion, no fluff. Just clarity and a path forward.