Projects & Programs
Effluent Solids Transport and Deposition Using Coupled Near-Field and Far-Field Dilution Models
The focus of this investigation was to define a sediment impact zone (SIZ) associated with the Rayonier, Inc. (Rayonier) outfalls to the Straits of Juan de Fuca. The SIZ was used to design the sediment sampling plan for cleanup of the Rayonier Mill site in Port Angeles, Washington. From 1937 through 1997, the ammonia-based, acid and sulphite-dissolving grade pulp mill produced approximately 36 gal/day of process water. The total suspended solids (TSS) loading to Port Angeles Harbor from the mill outfall was approximately 36,000 lb/day. Environmental Protection Agency investigated the former mill site and subsequently prioritized the site for cleanup. Consequently, an agreed order with the Washington State Department of Ecology initiated a remedial investigation/feasibility study. A management plan, which included a sampling and analysis plan for the marine environment, was developed for Rayonier. Before joining PNNL, our staff coupled a near-field plume dilution model with a far-field three-dimensional (3-D) circulation and sediment transport model to predict sediment deposition patterns and to delineate the SIZ of the Rayonier outfalls.
In 1972, Rayonier installed a deepwater outfall with a diffuser to consolidate treated effluent. Previously, raw effluent was discharged to the Port Angeles Harbor from five near-shore outfalls. The PNNL project modeling team, investigated the possibility that the plumes from both the near-shore and deepwater outfalls could have migrated with the tidal circulation well beyond the immediate mixing zone region. The project modeling team applied a near-field initial dilution model (UDKHDEN) and a 3-D hydrodynamic and far-field transport model (EFDC) to conduct effluent TSS transport and deposition analysis. Historical current, tide and drogue data were used to calibrate the hydrodynamic model, and multiple particle sizes corresponding to typical secondary pulp mill effluent were used for the sediment transport and deposition models. The results showed that within 8,000 m from the outfall, the far-field transport coupled with near-field dilution reduces the effluent concentrations by a 1:1000 factor. This is well away from the nearest sensitive area, which is over 15,000 m away from the outfall. Also, the results showed that there would be little chance for particulate matter to deposit in the sensitive areas beyond the well defined as a 4,000 m by 1,000 m area around the outfall.
Project Highlights:
- UDKHDEN and EFDC models were used to conduct effluent total suspended solids transport and deposition analysis
- Sediment deposition and transport were modeled using multiple particle sizes corresponding to typical secondary pulp mill effluent
- Long-term deposition rates from the outfall were less than the natural background sedimentation at the site
- Results showed that there was little chance for effluent particles to impact sensitive areas
- The predicted sediment impact zone was used to design a focused sediment sampling program.