If you inherited a stormwater permit or just took over compliance at your facility, stormwater sampling is usually the part that raises the most questions. When do you sample? What gets tested? What happens if it rains on a Sunday and nobody catches it? The rules are specific, and missing them quietly creates a gap that shows up later in your annual report.
This guide covers what stormwater sampling involves under California’s Industrial General Permit, how the timing works, and what the numbers mean when results come back. For the full picture of how the permit fits together, start with our SWPPP and stormwater compliance overview.
Stormwater Sampling Requirements in California
California regulates industrial stormwater under the Industrial General Permit (IGP), Water Quality Order 2014-0057-DWQ. The permit sets a minimum sampling schedule that every covered facility has to meet, tied to the reporting year that runs July 1 through June 30.
You collect samples from each discharge location during qualifying storm events. The minimum is four events per reporting year: two in the first half (July 1 through December 31) and two in the second half (January 1 through June 30). Results go to the State Water Board through SMARTS, the state’s online stormwater reporting portal.
| When | What | How Many | Where Results Go |
|---|---|---|---|
| First half of the reporting year (Jul 1 – Dec 31) | Sample each discharge location during a qualifying storm event | 2 qualifying events | Lab analysis entered into SMARTS, generally within 30 days of receiving results |
| Second half of the reporting year (Jan 1 – Jun 30) | Same: sample each discharge location during a qualifying storm event | 2 qualifying events | Same: results uploaded to SMARTS |
| End of reporting year (by Jul 15) | Annual report summarizing the year’s sampling and site evaluations | 1 annual report (the SWAR) | Submitted in SMARTS by the facility’s Legally Responsible Person |
“Each discharge location” matters. A facility with three storm drains that reach a public system has more sampling to do than one with a single discharge point. When we set up a monitoring plan, the first thing we map is where water actually leaves your property, because that determines how many samples each event requires.
What a Qualifying Storm Event Actually Is
This is where most facilities trip up. Not every rain counts. Under the IGP, a qualifying storm event (QSE) is a rain event that produces a discharge from your site and is preceded by 48 hours with no discharge from any drainage area. As a practical rule, that means a storm producing at least about a tenth of an inch of rain, after roughly two dry days.
Two more conditions decide whether your sample is valid:
- Timing. You have to collect the sample within four hours of when discharge begins, or within four hours of the start of your operating hours if the storm starts before you open.
- Operating hours. Sampling happens during scheduled facility operating hours. A storm that dumps an inch overnight and clears before your first shift does not give you a sample you can use.
The most common call we get mid-season sounds like this: it rained hard over the weekend, nobody was on site, and the rainy stretch has passed. That is the real challenge of stormwater sampling in California. Qualifying events do not wait for a convenient day, and you only get a handful of chances each season to bank the four you need.
This is also why we do not send a technician to collect your samples. A scheduled site visit often arrives on a day with no qualifying storm and still gets billed. We ship turnkey sampling kits instead, so the people already on site can collect the moment a qualifying storm hits. We cover that tradeoff in why CDMS doesn’t send a technician to collect your samples.
Not sure your facility is catching its qualifying storm events? Call (925) 551-7300 or request a consultation. Tell us your discharge points and your operating hours, and we will set up a monitoring plan and a kit schedule built around your reporting year.
What Gets Tested, and the Numbers That Matter
Two kinds of monitoring happen during a storm event. Visual observations come first: someone walks the discharge points and records what the runoff looks like, including color, odor, floating material, and any sign of a spill or illicit discharge. Then there is analytical sampling, where the collected water goes to a lab.
Every covered facility tests for three baseline parameters: pH, total suspended solids (TSS), and oil and grease. Beyond those, your facility tests for additional pollutants tied to your industry and the materials you handle. A metal fabricator samples for different metals than a food processor or a plastics manufacturer.
Results are measured against Numeric Action Levels (NALs), the thresholds in the permit. Going over an NAL does not mean an automatic penalty. It moves you up an escalation ladder of Exceedance Response Actions. If your results come back high, our stormwater exceedance and ERA guide walks through what Level 1 and Level 2 involve.
One detail that surprises people: additional requirements beyond the statewide NALs may apply to your facility. The permit’s standard numeric action levels use EPA benchmark values, such as a pH range of 6.0 to 9.0. But facilities in watersheds with approved Total Maximum Daily Loads (TMDLs) can face stricter limits for specific pollutants. Those limits can include tighter ranges for pH and significantly lower thresholds for metals like copper. A facility can read as compliant against the statewide NAL and still be in exceedance under the TMDL-specific limits that apply to its receiving water. Knowing whether your location carries additional requirements is part of reading your results correctly.
Missing an Event, and Earning Fewer of Them
If a qualifying storm passes and you cannot sample it for a legitimate reason, such as the event falling outside operating hours or unsafe conditions, you document why. If you simply miss events with no explanation, that absence shows up in your annual report and reads as a deficiency. The state expects either four samples or a defensible record of why you have fewer.
There is an upside for facilities with a clean track record. Once you have demonstrated consistent results within the NALs, the permit allows qualifying facilities to drop from four sampling events per year to two. It is one of the few places in the program where doing the work well lowers your ongoing burden.
After each event, lab results have to be entered into SMARTS, generally within 30 days of receiving them. SMARTS is not always intuitive, and its automated determinations are not always reliable, which is why we verify requirements independently rather than trusting the portal’s read. We cover the reporting side in SMARTS reporting for California stormwater permits.
The Bottom Line
Stormwater sampling comes down to catching the right storms, testing the right parameters, and getting results into SMARTS on time. The schedule is fixed, the qualifying-event rules are strict, and the benchmark that applies depends on where you are in California. Most facilities do not lose compliance because the work is hard. They lose it because a storm came on a weekend, or because no one was sure which events counted.
Want your stormwater sampling handled so you stop watching the weather? Call (925) 551-7300 or request a consultation. We will build your monitoring plan, ship sampling kits timed to your reporting year, coordinate the lab, and prepare your results for SMARTS. We will walk you through what it costs before any work begins.
