Many facilities across California took their first (or even second) stormwater sample in October. With lab results rolling in, we wanted to provide our recommendations to prevent exceedances this stormwater season!

Regulatory Requirements

Any facility that discharges stormwater associated with industrial activity to waters of the United States is covered by the Industrial General Permit (IGP). The IGP requires the facilities to conduct stormwater sampling, monitoring, and reporting, among other requirements.

Sampling

Notice of Intent (NOI) facilities must collect and analyze stormwater samples from two (2) qualifying storm events (QSE) during the first half of the reporting year (July 2025-December 2025) and two (2) QSEs during the second half of the reporting year (January 2026-June 2026). A QSE is a precipitation event that produces a discharge for at least one drainage area and is preceded by at least 48 hours with no discharge from any drainage area. Samples from each discharge location shall be collected within four (4) hours of the start of the discharge; however, if a QSE starts while a facility is not operating, samples must be collected within four (4) hours of the start of facility operations provided the QSE began within the previous 12 hours. Stormwater samples need to be kept cool and should be sent to the testing laboratory as soon as possible.

All NOI facilities are required to analyze stormwater samples for total suspended solids (TSS), oil and grease (O&G), and pH. TSS measures the number of solid particles in stormwater. O&G measures the amount of oil residuals and contaminants in stormwater. pH is a measure of how acidic or basic stormwater is. Additional parameters may be required to be analyzed depending on your industry. All parameters except for pH are analyzed by a laboratory. pH must be measured in the field within 15 minutes of taking the stormwater sample using either litmus paper or a calibrated pH meter.

Monitoring

The IGP requires facilities to conduct Sampling Event Visual Observations at the same time that sampling occurs at each discharge location. Facilities are required to observe whether stormwater is discharged from containment sources, the presence or absence of floating and suspended materials, oil and grease, discolorations, turbidity, odors, trash/debris, and source(s) of any discharged pollutants.

Reporting

Facilities must submit sampling and analytical results for all samples via Stormwater Multiple Application and Report Tracking System (SMARTS) within 30 days of obtaining or receiving the results from the lab.

When You’ve Received Your Sample Results

Trinity’s stormwater experts recommend that you benchmark your stormwater sampling results once you receive them from the laboratory. Don’t wait until the end of the stormwater season!

After you receive your sampling results:

1. Track your stormwater sampling results on a spreadsheet so you can easily review results across the entire 2025-2026 reporting year.
2. Check for any NAL/TNAL exceedances. Based on your current sampling results, are you exceeding the annual NAL for any pollutant? Did you have 2 sample results above the instantaneous maximum NAL for any pollutant?
3. Check for any NEL exceedances, if applicable. Based on your current sampling results, did you have 2 sample results above the NEL for any pollutant?
4. Ensure sampling results are representative. If you notice any anomalous values in your sampling results, ask your lab if it is possible to re-analyze the samples. This will depend on the test method’s holding time for the sample.
5. Upload your sampling results in SMARTS. Don’t forget to submit your sampling results within 30 days of receiving them from the laboratory.

Tips To Prevent/Minimize Exceedances

Based on your stormwater sampling results for the first rain event, you could already have exceedances for one or more pollutants, or you might be in compliance. Regardless of what your sampling results show, preventing exceedances and minimizing additional exceedances is crucial to prevent Exceedance Response Actions (ERA) and water quality standard violations.

1. Check the forecast. Check the forecast weekly to make sure you know when rain is coming. If you’re expecting rain, check the forecast more frequently to ensure that you’re aware of potential QSEs. If you had annual NAL exceedances during your first sampling event, look for opportunities to sample additional QSEs to lower the annual average of your sampling results.
2. Focus on best management practices (BMPs) prior to rain events. Prioritize your BMPs, including good housekeeping, prior to sampling. Make sure that your structural BMPs are in good condition including downspout filters, wattles, catch basins, or treatment systems. If you had exceedances during your first sampling event, consider implementing additional BMPs or increasing the frequency of your BMPs to improve your sampling results in future QSEs.
3. Conduct training. Ensure your SWPPP team members are trained and confident in stormwater sampling and BMP implementation. At least one person per shift should be trained to sample stormwater. For those of you with Level 1 or Level 2 ERA status, training should be conducted by a QISP.

If your facility needs any assistance with stormwater sampling, reporting, benchmarking, or QISP training, please don’t hesitate to contact Trinity’s local offices in California.