May 28, 2021

Examining Caltrans’ State & General NPDES Permit Interpretation

The department has developed a comprehensive storm water compliance program under the statewide NPDES Permit and the General NPDES Permits for storm water discharges from construction and industrial activities


The department has a storm water compliance program under the state NPDES Permit and the General NPDES permit for construction and industrial site discharges.

The California Department of Transporation (Caltrans) has developed a comprehensive storm water compliance program under their statewide NPDES Permit and the General NPDES permits for storm water discharges from construction and industrial activities as well as the regional Lake Tahoe General Construction Permit.

Many of Caltrans' interpretations of the construction general permit (NPDES No. CAS0000002) are included in its 2018 Standard Specifications Section 13 for Water Pollution Control 2018 Standard Plans and Standard Specifications | Caltrans or guidance manuals available on the website.

Since thick fog has been known to show up in the National Weather Service (NWS) forecast at 0.01 to  0.1% chance of precipitation, Caltrans has defined its storm event forecast triggering a rain event action plan (REAP) as 50% chance of 0.1 inches or more, whereas the CGP is not specific to the amount of rain but only 50% chance. This small addition of an amount of rain is expected to reduce the chance of preparing a REAP for fog. Caltrans requires the REAP to be prepared 72 hours prior to a predicted 50% chance of 0.1 or more inches of rain and requires the REAP to be submitted and implementation started within 48 hours prior to the forecast storm. This is 24 hours earlier than the respective 48 hours and 24 hours required by the CGP.

Caltrans has also determined the Risk Level 2 sampling and analyses requirements for pH and turbidity were meant to be representative. Therefore, Caltrans starts with sampling 20% representative discharge points (minimum of 5) per qualifying rain event selected based on drainage areas of highest percentages of disturbed area, areas with potential pH impact sources and at least one area where disturbed soil is stabilized. If the results of sampling these 20% exceed 200 nephelometric turbidity units (NTU) or are outside the pH range of 6.5-8.5, they then need to collect from 50% of discharge points during the next qualifying rain event. If samples from the 50% exceed 250 NTU or are outside the range of 6.2-8.8 pH, they need to sample 100% of discharge locations. Once they graduate to a higher percentage, they stay there for the duration of construction.


Exceeding numeric action levels (NALs), 250 NTU and 6.5-8.5 pH is still reported as required by the CGP regardless of samples collected. However, Caltrans evaluates the sample results at each location rather than averaging the entire site, differing from the CGP that specifies the average daily turbidity and pH be compared to the NALs to determine whether they are exceeded. Caltrans enters its own data into the Stormwater Multiple Application and Report Tracking System (SMARTS), and they submit averages for results for pH and turbidity as required by SMARTS for their online adhoc reports.

Caltrans has set the limit for dewatering storm water (e.g., accumulated water trapped in excavation or basin) or non-storm water (e.g., groundwater) at 200 NTU and within the range of 6.7 to 8.3 pH. 

The CGP wording for stockpile BMPs was cleared up in Caltrans Standard Specifications. Rather than say that stockpiled materials need a cover and berm when not being actively used, Caltrans states that active stockpile BMPs will be implemented before a forecasted storm event and inactive stockpiles according to the SWPPP schedule.

Additionally, CGP requires Risk level 2 projects to protect active areas with appropriate stabilization seemingly tied to a REAP. However, Caltrans does not limit to Risk Level 2 and includes a general requirement for all their construction projects to install soil stabilization and sediment control materials for water pollution control in all active areas or before any storm event. This includes Risk level 1 but also Caltrans projects that are not covered by the CGP (e.g., less than 1 acre soil disturbance).

Caltrans requires preparation of a Notice of Discharge for any discharge or evidence of prior discharge that could cause adverse conditions in the storm sewer system or receiving water. The resident engineer will decide if the discharge warrants an additional report to the Water Boards. 

Additional training is required for the person responsible for water pollution control work for the construction contractor, the Water Pollution Control (WPC) Manager. The WPC manager must be a QSP and must also have an 8-hour Caltrans-specific course. Among many others, the responsibilities of the WPC Manager include being at the job site within two hours of being contacted, mobilizing crews to make immediate repairs and stopping construction activities that are damaging WPC practices or causing water pollution.

Caltrans has a detailed process for complying with all the requirements of the CGP in the SWPPP as required by the CGP. The forms and their SWPPP templates serve to document compliance with every CGP requirement. Caltrans guidance documents include rationale and procedures. The department’s contract special provisions and standard specifications are used to get contractors to implement these requirements for compliance. 


About the author

John Gleason, PE, QSD, TOR, CPESC works for M.S. Hatch Consulting, LLC. Gleason can be reached at