Journal of Hazardous Materials 136 (2006) 137–143 Stochastic analysis and prioritization of the influence of parameter uncertainty on the predicted pressure profile in heterogeneous, unsaturated soils Evan K. Paleologos a,∗ , T. Avanidou a , N. Mylopoulos b a Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA b Department of Civil Engineering, University of Thessaly, Volos 38221, Greece Available online 28 December 2005 Abstract This article utilizes a Monte Carlo stochastic framework to investigate the influence on the mean and variance of the predicted mean pressure head profile of statistical assumptions regarding the parameters that enter the mathematical description of the problem of infiltration in unsaturated, heterogeneous layers. The parameters are treated as random functions with an exponential auto-covariance function expressing their spatial continuity. Four different truncated distributions are taken to describe the parameters according to field observations and various phases of site characterization campaigns. The exponential distribution is seen to produce the largest (in absolute value) mean and variance in the pressure head profile. For all distributions the variance in pressure head increases with increasing mean pressure. A second topic of this article is to investigate, the relative importance of each parameter, in terms of the mean and the variance of the predicted pressure. For uniformly or triangularly distributed parameters the saturated hydraulic conductivity appears to dominate the mean-behavior and the uncertainty in the system’s solution. For lognormally or exponentially distributed parameters another parameter, the van Genuchten pore-size distribution index, is the dominant factor. © 2005 Elsevier B.V. All rights reserved. Keywords: Unsaturated flow; Heterogeneous soils; Monte Carlo simulations; Parameter ranking 1. Introduction One of the major tasks facing hazardous waste projects is to assess the critical components that influence the performance of a system and which affect predictive capabilities. Prioritiza- tion of the critical factors whose uncertainty reduction narrows a system’s output uncertainty is important at both the site char- acterization and the modeling level. Such prioritization allows the concentration of human and monetary resources on only those factors that critically influence a system’s predicted per- formance. The objective of this work is to illustrate, through the analysis of a physical problem of unsaturated flow in stratified, heterogeneous media, how such concepts can be utilized to guide site characterization and modeling efforts. The focus is the quan- tification of the effect of assumptions about the statistical struc- ture of data on the prediction of pressure head profile as well as the evaluation of the relative importance of three physical param- ∗ Corresponding author. Tel.: +1 803 777 8125. E-mail address: epal@geol.sc.edu (E.K. Paleologos). eters that enter the description of the problem. Field data from six hydrogeologic units at the US Department of Energy repository site of radioactive wastes at Yucca Mountain, Nevada, were uti- lized. The physical problem is characterized by three parameters exhibiting both spatial variability and uncertainty in their val- ues: the K s , the saturated hydraulic conductivity; the α, the van Genuchten air-entry scaling parameter; and β the van Genuchten pore-size distribution index parameter that enter into the govern- ing nonlinear equations. The main objective of this study is to investigate the effect of the parameters’ probability distribution functions on the prediction of the pressure head profile. This issue arises from the limitation in data for many earth studies, which as a consequence allows several probability distribution models to fit the data. In addition, for large projects where sev- eral site characterization campaigns are conducted over time, the statistical description of the data at each distinct site characteri- zation phase may vary and hence the modeling efforts (and pre- dictions) that correspond to these data collection phases may rely on different or evolving assumptions. The ranking of the three physical parameters relative to their influence on the mean and variance of pressure head is the second focal point of this article. 0304-3894/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2005.11.040