Model Validation and Decision Making: An Example Using the Twin Lakes Tracer Test
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MODEL VALIDATION AND DECISION MAKING: AN EXAMPLE USING THE TWIN LAKES TRACER TEST* NATALIE E. OLAGUEa, PAUL A. DAVIS', DARRLA SMITHa, AND TOM FEENEYb
aSandia National Laboratories, Department 6331, Albuquerque, NM 87185 bGRAM Inc., 8500 Menaul Blvd, Suite B370, Albuquerque, NM 87112
ABSTRACT
An approach to the validation of ground-water flow and transport models that are used in support of licensing a radioactive waste site is proposed and demonstrated. The approach attempts to interpret modeling results in light of their intended purpose. Instead of trying to find the "best" fit model to the experimental data, we attempt to determine the conservativeness or bias invoked by applying different models to simulate the same experiment. In this way we are able to provide guidance to the regulatory community on where they should expect model results to fall relative to actual data and provide some evidence that certain model approaches lead to conservative results while other modeling approaches lead to non-conservative results. Applying this approach to the Twin Lakes Tracer Test demonstrated that a simple one-dimensional flow and one-dimensional dispersion model consistently over-predicts the maximum concentration. A one-dimensional flow with three-dimensional dispersion and two dimensional flow with two dimensional advection-dispersion are conservative only if they employ laboratory scale dispersivities. Therefore, if a decision-maker were to accept a similar site based on results from these types of models, there would be a relatively low chance that he has accepted an unsafe site. However, if decision-makers were to reject a similar site based on these types of results, there would be a relatively large chance that they rejected a safe site. INTRODUCTION
Background Validation has been defined as "the process of obtaining assurance that a model as embodied in a computer code, is a correct representation of the process or system for which it is intended".' The overall purpose of modeling, with respect to the disposal of radioactive and hazardous wastes, is to aid in the decision-making process with regard to the safety of such disposal sites. Models are used to simulate quantities related to safety (e.g., dose, travel-time) that have been defined by the relevant regulations. These quantities are usually estimated over relatively large spatial (meters, kilometers) and temporal (1,000 years, 10,000 years, 100,000 years) scales. However, the purpose of modeling is not to precisely predict the quantity(s) of interest. This cannot be done because of the large uncertainties associated with the time and spatial scales. Rather, the purpose is to simulate the expected site conditions and estimate the quantity of interest so that it can be used as an indicator of site safety and provide the basis for a decision. In light of this purpose, the seemingly insurmountable uncertainties associated with assessing
" This work performed at Sandia National Laboratories which is operated for the U.S. Department of Energy under contrac
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