IGP off-hours sampling: what if the storm hits at night?

IGP off-hours sampling comes down to one rule. Under California's Industrial General Permit (IGP), facilities collect stormwater samples from a Qualifying Storm Event (QSE) during scheduled facility operating hours, within four hours of when discharge starts or when operations begin. A night or weekend storm often means sampling at the next shift start.

Clocks fell back on November 1, so the dark comes earlier. The first half of the IGP reporting year ends December 31. If your facility has not caught its samples yet, the next storm matters, whatever time it arrives.

Key takeaways

• Under the IGP, a QSE is a storm that produces a discharge from at least one drainage area after 48 hours with no discharge from any drainage area.
• Samples are collected during scheduled facility operating hours, within four hours of the start of discharge or the start of operations.
• Facilities sample two QSEs in each half of the July 1 to June 30 reporting year, so the July 1 to December 31 half closes soon.
• A plan for who samples, who drives to the lab and who tests pH on site is what turns a surprise night storm into a normal workday.

What counts as a qualifying storm event?

A Qualifying Storm Event (QSE) is a storm that produces a discharge from at least one drainage area and is preceded by 48 hours with no discharge from any drainage area. That is the definition under the IGP.

Many people shorten this to 48 hours of dry weather. That is not quite right. What matters is whether water was leaving any drainage area at your site in the previous 48 hours. A sprinkler, a wash-down or a small drizzle that did not cause a discharge does not reset the clock in the same way a real discharge does.

Keep a simple log of when discharge started and stopped. An inspector may ask how you decided a storm qualified.

When do you have to collect the sample?

Under the IGP, you collect the sample within four hours of the start of the discharge. If the storm starts outside scheduled operating hours, you collect within four hours of the start of the next operating period.

This is why your written facility hours matter. Your Storm Water Pollution Prevention Plan (SWPPP) lists them, and your sampling plan should match them. If your hours say 7 a.m. to 3 p.m. and the rain starts at midnight, the clock for that storm usually starts at 7 a.m.

Two things to keep in mind:

• Storms that start during hours: The four-hour window starts when you first see discharge, so someone needs to be watching.
• Storms that start before hours: Someone must be on site, trained and ready when the shift begins, with bottles and a cooler already staged.

How do you get ready for a night or weekend storm?

The best preparation is a short, written plan that names people and times. Work through these steps before the next forecast.

1. Name the sampler and a backup: Pick two trained people who can start at the beginning of operations, including after a weekend.
2. Stage the kit: Keep clean bottles from your lab, labels, a cooler, ice packs, gloves and the chain-of-custody form in one place.
3. Confirm lab hours: Ask which days and hours the lab accepts samples, and whether it can take them early or on a holiday.
4. Check the forecast daily: Look at your site's drainage areas and the 48 hours before the expected rain.
5. Walk the site: Make sure your Best Management Practices (BMPs), like covers, berms and drain protection, are in place before rain arrives.
6. Set up a call tree: Write down who gets the first call when rain starts and who decides that a storm qualifies.

For a wider list, see our rainy season checklist for CGP and IGP sites.

What goes wrong with samples after you collect them?

Most lost samples fail because of timing or temperature, not because of the storm. Under 40 CFR Part 136, each test has a hold time, and the hold time for pH is 15 minutes. Most samples are kept at or below 6°C without freezing.

Common mistakes include:

• Sending pH to the lab: With a 15-minute hold time, pH is usually measured on site with a calibrated meter or test paper that meets the method.
• Warm coolers: Ice packs that are not frozen, or too few of them, can put a sample out of range.
• Freezing samples: Ice that touches the bottles can freeze them. Freezing is not allowed.
• Closed lab on delivery day: A sample collected Friday may sit past its hold time if the lab is closed.
• Missing paperwork: Without time, date and sampler on the chain-of-custody form, the result is harder to defend.

If a sample misses its hold time, tell your QISP right away. We can help you decide what to do before the next storm.

Frequently asked questions

Do I have to sample a storm that happens at night or on a weekend?

Not at the moment it happens. Under the Industrial General Permit (IGP), sampling happens during scheduled facility operating hours. If a Qualifying Storm Event (QSE) starts outside those hours, the sampling window runs from the start of the next operating period. Confirm the exact wording with your Qualified Industrial Stormwater Practitioner (QISP) or us.

How many storms do I need to sample each year?

Under the IGP, the reporting year runs July 1 to June 30. Facilities sample two QSEs in each half of the year, July 1 to December 31 and January 1 to June 30. The Annual Report is due July 15.

Does 48 hours of dry weather make a storm count as a QSE?

No. Under the IGP, a QSE produces a discharge from at least one drainage area and is preceded by 48 hours with no discharge from any drainage area. A dry forecast does not decide it. What counts is whether your site was discharging in the 48 hours before.

How fast do stormwater samples have to reach the lab?

It depends on the test. Under 40 CFR Part 136, pH has a hold time of 15 minutes, so it is usually measured on site. Most other samples are kept at or below 6°C without freezing and delivered within the hold time for each test. Ask your lab for its schedule before the storm.

How Water Environmental can help

Water Environmental helps industrial facilities and construction sites in the greater Sacramento area stay in compliance with California's stormwater permits. For sampling, we can build a plan around your real operating hours, stage your kit and handle the lab runs through our field sampling and laboratory services.

We can also review your SWPPP and sampling schedule through industrial SWPPP and QISP monitoring. For the official rules, see the State Water Board's Industrial Stormwater Program (IGP) page and 40 CFR Part 136. If you want a second set of eyes before the next storm, talk to us.