Abstract Floating treatment wetlands (FTWs) have been increasingly studied and applied over the last 15 years, yet the conditions for their effective implementation are still not fully understood. To refine current understanding and identify conditions that maximize particulate pollutant removal, a flume experiment with a real FTW was conducted to investigate how the mean flow velocity ( U 1 = 0.018 m · s − 1 ; U 2 = 0.083 m · s − 1 ; and U 3 = 0.171 m · s − 1 ) and root-network characteristics (i.e., R = ratio of the water column occupied by the FTW over the total pond water depth) influence the flow structure and associated suspended solids (SS) retention within FTWs. Flow acceleration (up to twice the mean flow velocity) and deceleration occurred underneath and downstream, respectively, of the FTW having a rooting depth ratio R of 0.56 ( FTW 0.56 ). A substantial fraction of the flow was diverted toward the flume bottom—particularly under U 1 —reducing root–flow contact and particle entrapment. Under higher velocities, the flow could penetrate deeper into the root network, increasing opportunities for particles’ entrapment and increased treatment benefits. The results indicated that FTW 0.56 could enhance pond treatment efficiency by up to ∼ 35 % , with greater benefits at higher velocities. This is partly due to reduced performances of conventional ponds to settle particles under these higher flow velocities ( U 2 and U 3 ). Increasing the rooting depth ratio (e.g., 0.87) would further improve efficiency, especially under low flow (up to three times higher removal efficiency), where FTW 0.56 offered limited additional benefit because of greater short-circuiting around the root network. Overall, the results of the present study suggest that careful consideration should be given to the flow the FTWs are subject to, the original pond design and its performance toward natural settling, to evaluate whether inclusion of FTWs would provide sufficient additional treatment benefits.
Abstract Municipalities traditionally deliver stormwater infrastructure through a design–bid–build (DBB) model, in which planning, engineering design, and construction are executed sequentially and contracted separately. However, given the substantial investment required and the significant consequences of ineffective stormwater management systems, the public sector is prompted to seek alternative approaches to the delivery and maintenance of these assets. This study empirically assessed the community-based public–private partnership (CBP3) model for stormwater asset delivery and maintenance, as implemented in the Clean Water Partnership (CWP), focusing on its governance structure, management mechanisms, and socioeconomic outcomes. A qualitative approach was adopted to enable an in-depth examination of the model through participant perspectives and documented evidence. A thematic analytical method was employed to analyze data collected from 49 sources, including the published literature, technical reports, fact sheets, permits, contract documents, and news articles related to the CWP. In addition, semistructured interviews were conducted with nine principal actors directly involved in structuring the partnership and defining its milestones and performance objectives. Findings show that the CBP3 model enables a collaborative, performance-driven governance structure that integrates public agencies, private partners, third-party auditors, and community stakeholders. The model demonstrates enhanced implementation efficiency, strong alignment with regulatory objectives, and significant socioeconomic benefits, including high levels of participation by local and minority businesses. This positions CBP3 as an effective and scalable framework for delivering resilient, equitable, and community-centered green stormwater infrastructure (GSI) assets.
Abstract Ground-mounted solar arrays, or solar farms, are being implemented for renewable energy generation. Solar panels can alter local hydrology, making accurate modeling essential for optimal site selection and management. We developed a modeling framework leveraging open-access software OpenHydroQual, since this model can represent the unique way that solar panels redistribute water on solar farms, and has explicit representation of unsaturated zone hydrologic processes. This study focuses on a solar farm with a steep (20%) slope within central Pennsylvania, USA. The model was calibrated and validated using one year of soil moisture data collected in prior research by the study authors; Nash–Sutcliffe efficiencies varied from 0.74 to 0.79 for different zones of the solar farm. Comparison of the solar farm model with a model of pre-development conditions indicates redistribution of soil moisture, with concentration of water at the dripline and reduced moisture under panels. The hydrologic changes increased runoff, with annual runoff generation changing from 2 cm to 3.5 cm. Additional scenarios performed to explore implications of management options indicated that increasing space between solar panel rows from 3 m to 4 m led to a decrease in runoff depth by 28.6%. Reduction of Manning’s n (representing reduction in grass height) from 0.06 to 0.03 led to a 2.9% increase in runoff depth. The comparison of different scenarios for 2- and 100-year return period storm events also indicated that solar panel implementation led to an increase in runoff volume and peak flow rate. Over the entire solar farm area, the implementation of solar panels resulted in a 21.5% decrease in annual evapotranspiration. Though this work demonstrated increases in runoff due to solar panel implementation, it also showed how certain management practices can counter these increases. Though these specific results apply to one site, this modeling framework can be customized for other solar farm sites. Resulting insights on anticipated runoff changes are critical for appropriate selection and sizing of stormwater management practices on solar farms.
Abstract Bioretention performance data for polychlorinated biphenyls (PCBs) and mercury (Hg) remain limited, yet are critically important to guide stormwater managers in places with PCB and Hg regulations. This study monitored four bioretention systems in San Francisco, California, with varying ages ( < 1 to 11 + years) and influent concentrations over two wet seasons. To help interpret the San Francisco findings, the data were integrated with a larger regional data set including nine other systems (13 sites total) and investigated using concentration-dependent efficiency curves and reference–effluent–influent (REI) plots, along with a Best Management Practice (BMP) Performance Index (PI) tool for objective assessment of water quality goals. The results of the synthesis show that influent concentration and system maturity played key roles; more mature systems outperformed newly installed ones, and sites with higher influent concentrations had higher percentage reductions in concentrations. A clear, irreducible concentration was apparent for both PCBs and Hg, and the study found significant capture of PCB-11 (dichlorobiphenyl), a lighter congener often found in the dissolved phase, despite past assumptions regarding its mobility. Mercury concentration reductions were more variable, consistently exhibiting reductions only when influent concentrations were greater than 32 ng / L . Additionally, hydrologic modeling of an undersized system [2% green stormwater infrastructure to drainage management area (GIS:DMA) ratio] demonstrated that even systems that do not meet standard sizing conventions can achieve substantial annual mass capture (e.g., 59% for PCBs). Soil profile analysis confirmed that pollutants accumulate primarily in the upper 50–100 mm of media and near inlets, with concentrations generally well below EPA regional screening levels (RSLs). The study shows that integrating efficiency curves to support parameterizing load-reduction models with REI plots for regulatory compliance supplies a robust framework for stormwater managers. The study findings suggest prioritizing bioretention placement in highly contaminated areas to maximize mass capture and allow for a multiyear stabilization period before formal performance assessment.
