A variety of particulate-bound and dissolved chemicals may be present in stormwater discharges. Of particular interest here are those chemicals that are associated with settleable particles that can result in recontamination of near-field bed sediments in receiving waters. Sediment recontamination can lead to negative impacts on the receiving waters and sediment and also limit the recovery or effectiveness of remediation efforts in those sediments. This research project was conducted to evaluate the performance of existing stormwater control measures (SCM) in terms of their potential for reducing sediment recontamination risk. The objective of the project was to gain a better understanding of how stormwater best management practices or SCM can influence the physical and chemical characteristics of stormwater loads to receiving waters and thereby affect the potential for sediment recontamination. This understanding was used to adapt the existing modeling tools to develop a planning tool that can be used to support the evaluation of SCM based on a quantitative assessment of performance and reduction in sediment recontamination risk.

The technical approach of the research was the evaluation of the performance of SCM in terms of their potential for reducing sediment recontamination risk. The performance evaluation considered the mass and particulate strength (or chemical concentration per mass of suspended sediment) of size segregated chemical loads of stormwater discharges, and the change in toxicity and sediment recontamination risk resulting from various practical, cost-effective SCM implementation scenarios. Site-specific stormwater pollutant fractionation data from several locations was used to validate the Source Loading and Management Model for Windows (WinSLAMM) and extend it to size fractionated stormwater load prediction. The model was then used to develop a family of related spreadsheet tools specific to several locations.

All of the SCM studied should be able to effectively remove particulates and particulate-bound chemicals and were observed to do so in many of the sampled storms. Low concentration inflows and maintenance and design limitations, however, did lead to significant particulates being observed in the effluent of some systems or limited the ability to see differences between influent and effluent stormwater. There were significant differences between removal of dissolved chemicals versus solid associated chemicals with the latter being more effectively removed. Dissolved chemicals, however, are unlikely to pose a significant sediment recontamination risk.

WinSLAMM enhancements with the collected data continue to make WinSLAMM an excellent tool for the evaluation of stormwater management systems. WinSLAMM was employed to develop site specific stormwater management tools in spreadsheet format that allow the performance of individual SCM to be evaluated. A sediment recontamination likelihood assessment tool (SRLAT) was developed in spreadsheet form to evaluate the potential for the evaluated SCM to lead to sediment recontamination. Likely sediment recontamination requires both significant solids release from SCM in particles sizes likely to settle in the receiving waters and environmentally significant chemical concentrations on those particles. Assessment of all of the monitored SCM indicated no significant likelihood of sediment recontamination.

Prior WinSLAMM modeling efforts have focused on specific metals commonly of concern for stormwater, and were not designed to provide size segregated stormwater chemical load predictions. By expanding the model to size segregated stormwater chemical loads and expanding the chemical understanding to other common sediment chemicals (including those often responsible for sediment toxicity, such as polycyclic aromatic hydrocarbons), the model can be better used to evaluate specific management strategies to directly compare cost and effectiveness in terms of preventing (or significantly delaying or reducing the likelihood of) the occurrence of sediment recontamination. An SRLAT spreadsheet tool was developed that can be used to assess the potential for sediment recontamination due to SCM discharges. Additional spreadsheets were developed to evaluate performance of SCM at three regional locations (Southern California, Pacific Northwest and Virginia Mid-Atlantic) and provide a convenient tool for assessment of SCM based upon local climatic conditions. Successful implementation of this research holds profound implications for improving the Department of War's management of stormwater, directly addressing mission readiness by increasing the availability of testing and training ranges. (Project Completion - 2024)

Hussain, T., D. Athanasiou, B. Rao, M. Bejar, M. Rakowska, I. Drygiannaki, D.B. Chadwick, M.A. Colvin, N.T. Hayman, G.H. Rosen, M. Otto, B. Steets, R. Pitt, and D. Reible. 2023. Sediment Recontamination Potential and Biological Impacts of Hydrophobic Organics from Stormwater in a Mixed-use Watershed. Science of The Total Environment, 906:167444. doi.org/10.1016/j.scitotenv.2023.167444.