Sensitivity analysis of unsaturated flow and contaminant transport with correlated parameters

SummaryThis study conducts sensitivity and uncertainty analysis for predicting unsaturated flow and contaminant transport in a layered heterogeneous system. The objectives of this work are to: (1) examine the effects of parameter correlations on the sensitivity of unsaturated flow and contaminant transport and (2) assess the relative contributions of parameter uncertainties to the uncertainties of flow and transport at each hydrogeologic layer. Using the unsaturated zone (UZ) of Yucca Mountain (YM) in Nevada, USA, as an example, the study considers cases of independent and correlated parameters. A sampling-based regression method is used, when the model input parameters are independent, and a decomposition method is used for the correlated case. When the parameters are independent, the uncertainty in permeability has the largest contribution to the uncertainties in simulated percolation flux and mass of the reactive tracer arriving at the water table. For the percolation flux, the second largest contribution is from the van Genuchten α; the sorption coefficient of the reactive tracer is the second most important parameter for the tracer mass arrival uncertainty. The sensitivity to the sorption coefficient is larger in the layers of devitrified and zeolitic tuffs than in the layers of vitric tuff. Contributions of the uncertainties in van Genuchten n and porosity to the percolation flux and tracer transport uncertainties are larger in the case of correlated parameters compared with the case of independent parameters due to the correlations of n and porosity with the van Genuchten α and permeability, respectively. These results illustrate the significant effects of parameter correlations on the sensitivity and uncertainty of unsaturated flow and transport. The findings are of significance in facilitating future characterizations to reduce the parameter uncertainties and associated predictive uncertainties of flow and contaminant transport in unsaturated fractured porous media.

Research highlights
► Subsurface flow and transport modeling is uncertain due to parameter uncertainty.
► Sensitivity analysis is necessary to know which parameters are the most sensitive.
► Parameter correlations are important for sensitivity analysis of flow and transport.
► Findings can help reduce parameter uncertainty and subsurface transport uncertainty.