Learn how to meet CII stormwater sampling requirements under the new LA permit and federal MSGP, including collection methods, testing parameters, and reporting obligations.
CII stands for Commercial, Industrial, and Institutional. Under the new Los Angeles Regional Water Board permit and the federal Multi-Sector General Permit, covered sites must collect grab or composite samples during qualifying storm events. You then test for sector-specific pollutants and report results via Discharge Monitoring Reports and annual filings.
What does CII mean in stormwater permitting?
CII covers private properties with one acre or more of impervious surface classified as commercial, industrial, or institutional land uses. The 2024 Draft CII General Permit Fact Sheet defines these categories using state tax codes. Facilities in these classes must obtain NPDES permit coverage for stormwater discharges to waters of the U.S. This requirement stems from Section 402(p) of the Clean Water Act, as noted by the Los Angeles Regional Water Quality Control Board.
Which facilities must sample under the new LA CII permit?
Facilities in the Dominguez Channel and Los Cerritos watersheds with five or more acres of impervious surface must sample. Some cases require five or more acres of total site area instead. The Los Angeles Regional Water Board adopted this permit on July 23, 2026. It becomes effective around October 31, 2026. Covered sites include certain facilities currently using No Exposure Certifications or Notices of Non-Applicability. Campus-style developments may aggregate contiguous parcels for coverage purposes.
How often must you collect stormwater samples?
Under the 2021 MSGP, you sample during each measurable storm event that follows the previous one by at least 72 hours. The 2021 MSGP Permit Parts 1-7 sets this interval rule. ADEQ requires sampling twice per year, once per rainy season, for certain sectors under its state permit. The LA CII permit may specify different frequencies depending on the compliance pathway you choose. Check your specific permit conditions for exact annual event counts.
What counts as a qualifying storm event?
A measurable storm event results in actual discharge and follows the previous sampled event by at least 72 hours. You may use shorter intervals if you document that they are representative of local conditions. For snowmelt, sample when measurable discharge occurs. This flexibility ensures your data reflects actual local storm patterns.
How do you collect a stormwater sample correctly?
Take a grab sample during the first 30 minutes of discharge for most parameters. Flow-weighted composite samples are generally preferred for TSS, dissolved metals, phosphorus, and TOC because stormwater concentrations shift sharply between the first flush and the tail of a storm. The 2021 MSGP Fact Sheet specifies the 30-minute grab window. Composites capture concentration shifts between first flush and storm tail. Use the same method for both influent and effluent in a given storm to maintain comparability. Mixing grab influent with composite effluent breaks the data comparison.
Which pollutants must you test, and what are the benchmark values?
Pollutants and benchmarks vary by sector under the MSGP or by compliance pathway under the LA CII permit. Common parameters include TSS, pH, oil and grease, dissolved metals like zinc and copper, phosphorus, and bacteria. The Los Angeles Regional Water Board notes copper and zinc harm aquatic life in these watersheds. Check your permit's sector-specific tables for numeric benchmarks. These values trigger corrective action if exceeded. Do not rely on general industry averages; your permit defines the exact thresholds for your site.
What happens when results exceed benchmarks?
A benchmark exceedance triggers corrective action requirements, including continued monitoring for the exceeded parameters. You must submit a corrective action plan within specified deadlines. ADEQ added Additional Implementation Measures (AIM) to the Non-Mining MSGP, which includes a three-level response approach to action level exceedances, and established a 30-calendar day period for the submittal of a corrective action report following a corrective action trigger in Part 3.1.1 of the MSGP. The Arizona MSGP update details these timelines. Continued monitoring ensures you track whether corrective measures reduce pollutant levels over time.
What reporting and recordkeeping are required after sampling?
Submit Discharge Monitoring Reports for each sampled event and file an annual report summarizing all monitoring results. Maintain SWPPP documentation that reflects your current BMPs and any corrective actions taken. The LA CII permit also requires annual fees and compliance with one of three pathways. Keep lab reports that separate dissolved from total fractions, as this split determines treatment options. Retain records for inspection periods specified in your permit.
How do you choose between grab and composite sampling?
Use grab samples for bacteria, pH, and rapidly degrading parameters. Use flow-weighted composites for TSS, dissolved metals, phosphorus, and TOC when storm duration exceeds 30 minutes. Stormwater concentrations shift sharply between first flush and tail, so a single grab can misrepresent the event. Document your method selection in your SWPPP to show regulatory consistency. Review our guide on influent and effluent sampling for stormwater treatment pilots for detailed protocol planning.
What are the three compliance pathways under the LA CII permit?
The permit offers three pathways, some involving substantial capital expenditures. Each pathway specifies different sampling frequencies, parameters, and BMP requirements. Choose based on your site's pollutant sources, available budget, and long-term maintenance capacity. The Stoel Rives analysis outlines these options. Your choice affects both upfront costs and ongoing operational burden.
| Pathway Type | Sampling Focus | Reporting Obligation |
|---|---|---|
| Pathway 1 | Standard parameters per sector | Annual DMRs and SWPPP updates |
| Pathway 2 | Enhanced monitoring for high-risk sites | Quarterly DMRs and corrective action plans |
| Pathway 3 | Performance-based with capital upgrades | Annual performance verification and fee payment |
What if your site is in Arizona or another state?
State-specific MSGPs like ADEQ's add requirements beyond the federal baseline. These include Additional Implementation Measures for major storms and hardness-dependent metal action levels. ADEQ requires sampling twice per year for certain sectors, once per rainy season. The Arizona MSGP update details these state-specific obligations. Always check your state's modified permit for local deviations from the federal MSGP.
How do you plan sampling for a treatment pilot?
Influent sampling points should sit immediately upstream of the treatment media, and effluent points belong immediately downstream of the media. Choose grab or composite methods based on pollutant type and storm duration. Define how percent removal will be calculated before the first storm arrives. Use enough storm events to produce a defensible average that reflects your site's actual conditions. This data validates whether the treatment system meets your discharge objectives before full-scale deployment.
If you need help matching sampling protocols to your site's specific pollutant profile, contact us for a site assessment.