Fate and Transport Modeling of Radionuclides and Chemicals in the Columbia River
It is important to assess the cumulative long-term effects of Hanford-derived contaminants to the Columbia River and the region after the site closes. The assessment will include impacts to human and ecological health as well as the region’s economy and cultures. To conduct this assessment, the site is in the process of developing the necessary tools and supporting data. The computational capability for performing an initial assessment is known as the System Assessment Capability (SAC Rev. 0).
Simulation of flow in a complex channel system in the Hanford Reach of the Columbia River. Click for a larger version.
As part of SAC, PNNL enhanced its MASS2 two dimensional river and water quality model to simulate chemical and radionuclide contaminants. MASS2 was enhanced to include the capability to simulate sediment transport, sediment-contaminant interaction, sedimentsorbed contaminant transport, and contaminant transport within the riverbed sediment layer. The simulated contaminant concentrations from the MASS2 model were compared to measurements for two separate time periods; 1964-1966 when radionuclides, including chromium-51 and zinc-65, were directly discharged to the river from once-through cooled plutonium production reactors and 1992-1996 when contaminants, including tritium and uranium, enter the river from groundwater sources and upstream inputs. The model was also modified to work in a linked system where contaminant inputs are received from a groundwater model and MASS2 outputs are passed on to risk assessment codes. The SAC assessment uses Monte-Carlo methods to provide an estimate of uncertainty. The uncertainty estimate requires performing 100 system realizations for a future time period of 1000 years. To complete the large number of simulations within a two-week time period, PNNL implemented a distributed computing environment on more than 25 workstations using the CONDOR software package to automate job execution.
Results and Benefits
MASS2 grids were developed from multiple sets of surveyed elevations using ARC/Info GIS and GRIDGEN. Model results were visualized using a variety of tools that included TECPLOT, GNUPlot, and ARC/Info. The assessment is currently on-going and the results to date illustrate that the MASS2 model predictions are within the range of field observations. By using in-house modeling tools, PNNL was able to make the necessary modifications to enhance the simulation codes.