Uncertainty analysis of radionuclide transport in a fractured coastal aquifer with geothermal effects

Groundwater flow and radionuclide transport at the Milrow underground nucle ar test site on Amchitka Island are modeled using two-dimensional numerical simulations. A multi-parameter uncertainty analysis is adapted and used to address the effects of uncertainties associated with the definition of the modeled processes and the values of the parameters governing these process es. In particular, we focus on the effects on radioactive transport of unce rtainties associated with conduction and convection of heat relative to the uncertainties associated with other flow and transport parameters. These i nclude recharge, hydraulic conductivity, fracture porosity, dispersivity an d strength of matrix diffusion. The flow model is conceptualized to address the problem of density-driven flow under conditions of variable salinity a nd geothermal gradient. The conceptual transport model simulates the advect ion-dispersion process, the diffusion process from the high-velocity fractu res into the porous matrix blocks, and radioactive decay. For this case study, the uncertainty of the recharge-conductivity ratio con tributes the most to the output uncertainty (standard deviation of mass flu x across the seafloor). The location of the freshwater-saltwater transition zone changes dramatically as this ratio changes with the thickness of the freshwater lens and the location of the seepage face changing as well. In t he context of radionuclide transport from the nuclear test cavity that is l ocated in the area where the transition zone is uncertain, travel times of radionuclide mass from the cavity to the seepage face along the seafloor ar e significantly impacted. The variation in transition zone location changes the velocity magnitude at the cavity location by a large factor (probably an order of magnitude). When this effect is combined with porosity and matr ix diffusion uncertainty, the uncertainty of transport results becomes larg e. Although thermal parameters have an effect on the solution of the flow p roblem and also on travel times of radionuclides, the effect is relatively small compared to other flow and transport parameters.