Uncertainty methods for groundwater flow and transport modelling

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Uncertainty analyses of groundwater systems are commonly used to derive the moments of the system dependent variables as a function of the uncertainty in the system parameters; usually only the first two moments are estimated.  For many problems however, it is not the first and second moment that are important but rather it is estimates at the tails of the PDF that are critical.  The classical problem of the first arrival of a contaminant at a biosphere point or receptor is one such example.  While analysts may estimate the full distribution of the concentration, the public, regulators and courts are often only concerned with the earliest possible arrival of a contaminant to a groundwater well.  Economic decisions often are made on these low-probability occurrences.  The difficulty for the analyst then is the robustness of the concentration PDF tails.  Important factors in the estimation of the probability at the tail of the distribution include parameter uncertainty, correlation, and the method used to determine the uncertainty.    The objective of the proposed research program is to develop and implement uncertainty methods that can be applied to detailed complex groundwater models.  The methods will be applied in the risk and safety assessment of deep geologic repositories for nuclear waste.  The methods will also be applicable for the assessment of groundwater protection zones and the forensic simulation of contaminant arrival at a biosphere point in either fractured or porous media.  For the forensic modeling, the case studies that will be explored include: the analysis of the arrival of a contaminant at a municipal well field that supplies water to a community with a statistically significant childhood cancer cluster; and the allocation of a bankruptcy trust to individuals with various health impacts (cancers) as a function of contaminant exposure concentration and duration.