Projects & Programs
Sensitivity Analysis of a Fate and Transport Model for Sediment Contamination in the Thea Foss Waterway, Washington
Sediment contamination in the Thea Foss Waterway, located near Tacoma, Washington has led to a remedial investigation and feasibility study analysis and accompanying sediment recovery modeling. A sensitivity analysis using a toxic fate and transport model of the Thea Foss Waterway, based on the WASP model originally developed for the City of Tacoma, was conducted. Our experience with a wide variety of models allowed us to identify limitations and recommend improvements. The contaminants considered were bis(2-ethylhexyl)phthalate [BEP], dibenz(a,h)anthracene [DIB], phenanthrene and pyrene. Examination of the input data, the data sources and the methods used for data development showed 1) the predicted sediment contaminant concentrations are very sensitive to the partition coefficient, resulting in a high level of uncertainty for the predicted sediment contaminant concentrations, 2) using the upper end of the range of measured partition coefficients, the need for an "undocumented" load to calibrate the model is small, 3) analysis of the sediment balance to the Thea Foss Waterway suggests the open water boundary conditions in the model may not represent the effect of the Puyallup River sediments, 4) the model does not consider tidal and seasonal dynamics of the Waterway and its storm water inputs, and 5) experience indicates that the grid resolution may be too coarse, causing high numerical diffusion and lower predicted contaminant concentrations in the sediment.
Evaluation of the sensitivity of the model to contaminant and suspended sediment loads was conducted by modifying the boundary conditions. Results are as follows: 1) removal of municipal and "undocumented" loads reveals that only BEP would exceed the sediment quality objective near the head of the Thea Foss Waterway, 2) use of the upper confidence limit of the whole-water loads dramatically increased the sediment concentrations of BEP and DIB, 3) use of average total suspended sediment and sediment trap contaminant data caused little change for each constituent, and 4) sediment trap loads calculated using the upper confidence limit of both the total suspended solids data and sediment trap contaminant data produced a dramatic increase in sediment concentrations for BEP.
Recommendations for addressing the limitations listed above include: 1) use the partition coefficient as the primary calibration parameter, 2) make the model truly dynamic by using tidal boundary conditions and seasonally varied inputs of suspended sediment and contaminants, 3) consider the effect of the Puyallup River sediments transported into the waterway, 4) use thinner and more sediment layers (1-2 cm) to track the location of the Cs 137 peak using an inert tracer, thus ensuring the correct sediment mass balance; and 5)increase the model’s grid resolution.
Project Highlights:
- Conducted sensitivity analyses and a toxic fate and transport model of the Thea Foss Waterway using the WASP model originally developed for the City of Tacoma to demonstrate existing calibration could be significantly improved
- Identified the improvements to the model that could lead to completion of calibration and better results.