Projects & Programs
Sediment Recontamination Modeling, Washington
In response to regulatory pressures to reduce combined sewer overflow (CSO) discharges and to respond to a Natural Resource Damage Assessment, a CSO control and sediment remediation project was initiated at the Norfolk Site on the Duwamish River. The contaminants of concern at the site included benzoic acid, bis(2-ethylhexyl)phthalate, 1,4-dichlorobenzene, mercury and polychlorinated biphenyls (PCBs). As part of state sediment cleanup requirements, the potential for recontamination of remediated site sediments resulting from treated CSO and storm drain discharges was evaluated using a screening model.
The screening model was based on SEDCAM, a steady-state box model developed to evaluate natural recovery of contaminated sediments at cleanup sites. Screening-level models allow for a cost-effective demonstration that certain contaminants do not require expensive dynamic 3-D modeling studies. PNNL staff modified the SEDCAM model and applied it to the evaluation of potential sediment recontamination resulting from the discharge of treated CSOs and untreated stormwater runoff. Modifications included the use of a hydrodynamic mixing model (CORMIX3) to predict initial discharge dilution and conversion of the spreadsheet implementation of the model to an executable FORTRAN code. The executable program code allowed for shorter model time steps and longer simulations. The screening model of CSO discharge impacts was based on a 19-year synthetic hydrograph that contained flow data at intervals of approximately 10 minutes.
The screening-level model proved that sediment would not become recontaminated with benzoic acid, 1,4-dichlorobenzene, mercury or PCBs. Demonstrating that the site would not become recontaminated with PCBs was particularly important because PCBs were the primary target of the cleanup action, and there was concern that future discharges could result in recontamination.
The model did reveal that there was a potential for sediments to become recontaminated with bis(2-ethylhexyl)phthalate. This compound is discharged in untreated stormwater draining from nearshore commercial and industrial areas. The screening model indicated that a 3-D dynamic sediment recontamination model is needed to accurately assess the potential for recontamination of sediments with bis(2-ethylhexyl)phthalate before cleanup activities commence.
Project Highlights:
- Effluent dilution and mixing analysis showed that treated CSOs would comply with all water quality standards
- Model results showed that sediment would not become recontaminated with the primary target contaminant, PCBs
- Analysis showed that there was a potential for recontamination from bis(2-ethylhexylhthalate) from stormwater discharges.