Managing stormwater turbidity in sensitive receiving environments is a persistent challenge in construction, particularly on sites with clay or silty soils that limit the effectiveness of conventional best management practices (BMPs). Chemical treatments such as flocculants are used widely to reduce turbidity by binding fine clays and silts into larger particles, or “flocs,” allowing them to settle or be captured.
A 20-acre residential development in California’s North Coast region provided an opportunity to implement a flocculant treatment that uses a passive dosing technology and an engineered mixing and monitoring system to meet stringent discharge requirements.
The project highlights the value of collaboration among treatment providers, manufacturers, and regulatory agencies, and demonstrates how integrating complementary approaches can safely and effectively meet water quality criteria in sensitive receiving environments.
Stormwater turbidity challenges on the Sonoma County, California, project were driven by highly erosive clay soils, intense seasonal rainfall, stringent regulatory requirements, and discharge to the Russian River, an ecologically sensitive system that supports salmonid species and the endemic Russian River tule perch (Fig. 1).
Site-specific regulations require all stormwater discharges to remain within 10% of background turbidity in the Russian River, which ranges from approximately 15 to 150 NTU depending on watershed conditions. This created a narrow and highly variable compliance window.
The standard statewide Numeric Action Limit (NAL) for turbidity in California is 250 NTU, but the protected watershed had a more restrictive rule that allowed only a 10% increase over the background turbidity level in the receiving stream. Average turbidity in the development’s nine ponds was 110–160 NTU; the stream was normally 10–30 NTU and spiked to around 60 during storm events.
Stormwater was routed through multiple sediment basins prior to discharge. While these systems provided initial solids capture, fine particle size limited settling efficiency, and turbidity in the ponds still exceeded discharge limits.
Although passive treatment flocculant technology has been used widely in the United States and Canada since the early 2000s, California waited to incorporate it into its Construction General Permit (CGP) guidance until 2022. As one of the first passive treatment applications in California’s North Coast Region, this project underwent a rigorous review process involving flocculant manufacturers, treatment specialists, the local municipality, and the North Coast Regional Water Quality Control Board.
Passive treatment typically applies anionic, environmentally safe flocculants within existing site infrastructure such as channels, sediment basins, and conveyance systems or in simple constructed systems. Treatment is delivered through gravity or pumping and works with conventional erosion and sediment control BMPs to promote floc formation and capture. This project’s strategy was defined as passive treatment under the CGP, although it incorporated select engineered components.
In California, passive flocculant treatment requires the development and submission of a formal Passive Treatment Plan prior to use. Flocculant plans provide a structured framework for chemical treatment, defining product selection, application methods, and required conditions prior to implementation. Such planning improves consistency, ensures safe use, and supports regulatory confidence, particularly in sensitive environments. The owner was quoted a cost of $400,000 for active treatment and $200,000 for passive treatment.
Only food- or drinking water-grade anionic chemicals were considered to ensure the protection of aquatic organisms and compliance with California regulations. A range of anionic polyacrylamide (PAM) products were laboratory performance-tested using site water and soils to identify the most effective formulation. In testing, turbidity was reduced from approximately 510 NTU to 13.8 NTU following application of the 703d#3 Floc Log—a greater than 97% reduction under controlled conditions (Fig. 2). Actual turbidity in ponds on the site ranged from 110–160 NTU, significantly exceeding discharge limits of 10% above background.
As with any chemical treatment, effectiveness and safety are governed by dose. Expected concentrations were calculated based on flow rates and product usage and evaluated against toxicological data to ensure application rates remained protective of the sensitive receiving environment.
The plan also defined system requirements for adequate mixing, reaction time, and capture of flocculated material prior to discharge. These criteria guided system design to ensure the selected chemistry performed as intended under field conditions.
Active Treatment Systems Inc. (ATS), Applied Polymer Systems Inc. (APS), and Centrifloc collaborated on the system to integrate operation, passive treatment design, and mixing equipment into a cohesive treatment approach.
Stormwater was pumped from ponding sediment basins to a centralized treatment system with a single basin, allowing flow rates to be controlled during operation. The system was designed to treat approximately 500 gallons per minute (1.1 ft3 per second), with flow split between two parallel, identical tank systems and mixing units (Fig. 3). The basin capacity was 58,860 ft3 (about 440,000 gallons).
Anionic PAM flocculant logs were installed within Centrifloc mixing units for passive dosing; pump rates controlled mixing and reaction times based on performance testing. Water was then directed to settling tanks where floc formation continued and solids separated from the water column.
A 10 HP centrifugal pump moved water from the main pond to a manifold valve system that split the flow between the CentriFloc dosing/mixing units on top of each tank. After the CentriFloc unit, the water gravity flowed through weir chambers in the tank. The pump was throttled back to pump at 100 gpm.
Initial use of sand filtration was discontinued after operation showed that the large flocs produced by anionic PAM caused rapid clogging. Sand filters were replaced with fibrous, high-surface-area jute matting in the tanks and at discharge points, improving floc capture and reducing maintenance (Fig. 4).
System performance was supported by modular control units (MCUs), which recorded turbidity, pH, and flow at 15-minute intervals. The local municipality required turbidity verification at hourly intervals during discharge events.
System performance was evaluated across two monitoring periods corresponding to active discharge events. During the first monitoring period (Feb. 11–17, 2026), the system treated approximately 1.32 million gallons of stormwater under controlled flow conditions. Effluent turbidity ranged from 5.3 to 78.5 NTU, with an average of 43.1 NTU. Flow rates and pH remained stable throughout the treatment period, averaging 391 GPM and 6.82, respectively.
During the second monitoring period (Feb. 18–March 1, 2026), the system treated approximately 3.05 million gallons of stormwater. Despite higher observed peak turbidity (maximum 97.9 NTU), average effluent turbidity decreased to 33.8 NTU, reflecting consistent treatment performance under varying influent conditions and extended operation. Flow rates and pH remained stable, averaging 390 GPM and 6.59, respectively.
Under the project’s background-based discharge requirements, all treated effluent remained within allowable limits relative to receiving water conditions, and no Numeric Action Level (NAL) exceedances were recorded during either monitoring period. Across both events, the system treated more than 4.3 million gallons of stormwater while maintaining reliable turbidity control under variable influent and receiving water conditions.
The project demonstrated that passive flocculant treatment can serve as an effective and reliable foundation for managing stormwater turbidity in clay-dominated systems where conventional controls alone are insufficient. Under stringent, background-based discharge requirements, the use of anionic PAM provided a consistent and effective mechanism for removing fine suspended particles and improving water quality. With appropriate mixing and sufficient residence time, passive systems can achieve reliable floc formation and solids capture across a range of influent conditions.
The residence time in the stormwater basins was months with no turbidity reduction. The residence time in the in-line tank system was approximately 180 minutes. Each tank held a volume of 18,000 gallons and the flow rate was 100 gpm, so the full volume of the tank was displaced in that time.
In this application, system components such as controlled flow, compact treatment infrastructure, and real-time monitoring were used to support performance and verify compliance under variable receiving water conditions. By combining passive treatment with targeted system enhancements and shared expertise, the team implemented an approach that was innovative, efficient, and cost-effective, resulting in positive outcomes for stakeholders and demonstrating a repeatable model for similar challenging environments.
Kyla J. Iwinski-Wood, Ph.D., is co-owner of Applied Polymer Systems Inc. She has more than 15 years of experience in erosion, sediment, and stormwater management. With expertise in polymer research, regulatory compliance, and environmental toxicology, Iwinski-Wood works to advance safe, effective, and sustainable polymer application in the construction, industrial, and municipal sectors.
Brian M. Free, CPESC, CPSWQ, QSD/P, is director of Business Development for Applied Polymer Systems Inc. He has helped clients in market segments including energy, transportation, homebuilding, solid waste management, mining reclamation, and post-fire remediation solve challenges in erosion control and stormwater management with holistic approaches.
