Evaluating the sensitivity of a subsurface multicomponent reactive transport model with respect to transport and reaction parameters

The input variables for a numerical model of reactive solute transport in g roundwater include both transport parameters, such as hydraulic conductivit y and infiltration, and reaction parameters that describe the important che mical and biological processes in the system. These parameters are subject to uncertainty due to measurement error and due to the spatial variability of properties in the subsurface environment. This paper compares the relati ve effects of uncertainty in the transport and reaction parameters on the r esults of a solute transport model. This question is addressed by comparing the magnitudes of the local sensitivity coefficients for transport and rea ction parameters. General sensitivity equations are presented for transport parameters, reaction parameters, and the initial (background) concentratio ns in the problem domain. Parameter sensitivity coefficients are then calcu lated for an example problem in which uranium(VI) hydrolysis species are tr ansported through a two-dimensional domain with a spatially variable patter n of surface complexation sites. In this example, the reaction model includ es equilibrium speciation reactions and mass transfer-limited non-electrost atic surface complexation reactions. The set of parameters to which the mod el is most sensitive includes the initial concentration of one of the surfa ce sites, the formation constant (Kr) of one of the surface complexes and t he hydraulic conductivity within the reactive zone. For this example proble m, the sensitivity analysis demonstrates that transport and reaction parame ters are, equally important in terms of how their variability affects the m odel results. (C) 2001 Elsevier Science B.V. All rights reserved.