Local and Global Sensitivity Analysis of Cr (VI) Geogenic Leakage Under Uncertain Environmental Conditions

We focus on the joint application of local and global sensitivity analyses to characterize propagation of model parameter uncertainties to outputs of subsurface water geochemical models. The latter typically involve uncertain inputs, including environmental conditions, mineral rock composition, and flow/transport features. In this context, implementation of sensitivity analysis techniques enables us to grasp the relative role of each model input. Here we focus on the application of several sensitivity approaches to the assessment of Cr (VI) geogenic leakage due to water-rock interactions. We specifically target the impact of uncertain environmental conditions on the chemical composition of spring waters following water transfer through a host rock system with given mineral composition. We employ a reaction path modeling approach and represent uncertainties of environmental conditions through three parameters, that is, oxygen fugacity (f(O2)), CO2 fugacity (f(CO2)), and temperature, which we consider as random quantities. We consider three diverse methodologies, that is, (a) the Scatter Plots sensitivity analysis, (b) the Distributed Evaluation of Local Sensitivity Analysis, and (c) a moment-based global sensitivity analysis. Our results suggest that (a) the relative importance of a given model parameter in driving the uncertainty of the spring water composition may display remarkable variations across the sampled parameter space and (b) parameter ranking through sensitivity metrics for geochemical applications in subsurface water resources requires a joint assessment of local and global sensitivity.