Calibration and Reliability in Groundwater Modelling: Credibility of Modelling

(Proceedings of ModelCARE 2007 Conference, held in Denmark, September 2007). IAHS Publ. 320, 2008, 64-69.

Application of maximum likelihood Bayesian model averaging to groundwater flow and transport at the Hanford Site 300 area

Philip D. Meyer¹, Ming Ye², Shlomo P. Neuman³ &
Mark L. Rockhold⁴

1 Pacific Northwest National Laboratory, 620 SW 5th Ave, Portland, Oregon 97213, USA

philip.meyer@pnl.gov

2 Florida State University, Tallahassee, Florida 32306, USA

3 University of Arizona, Tucson, Arizona 85721, USA

4 Pacific Northwest National Laboratory, Richland, Washington 99352, USA

Abstract A methodology to systematically and quantitatively assess model predictive uncertainty was applied to saturated zone uranium transport at the 300 Area of the US Department of Energy Hanford Site in Washington State, USA. The methodology extends Maximum Likelihood Bayesian Model Averaging (MLBMA) to account jointly for uncertainties due to the conceptual–mathematical basis of models, model parameters, and the scenarios to which the models are applied. Conceptual uncertainty was represented by postulating four alternative models of hydrogeology and uranium adsorption. Parameter uncertainties were represented by estimation covariances resulting from the joint calibration of each model to observed heads and uranium concentration. Posterior model probability was dominated by one model. Results demonstrated the role of model complexity and fidelity to observed system behaviour in determining model probabilities, as well as the impact of prior information. Two scenarios representing alternative future behaviour of the Columbia River adjacent to the site were considered. Predictive simulations carried out with the calibrated models illustrated the computation of model- and scenario-averaged predictions and how results can be displayed to clearly indicate the individual contributions to predictive uncertainty of the model, parameter, and scenario uncertainties. The application demonstrated the practicability of applying a comprehensive uncertainty assessment to large-scale, detailed groundwater flow and transport modelling.

Key words groundwater; modelling; uncertainty