Sensitivity analysis and parameter identifiability for colloid transport in geochemically heterogeneous porous media

Effective use of colloid transport models for heterogeneous subsurface porous media requires the development of methodologies to identify the key model parameters. The inverse problem of a two-dimensional model for colloid transport in geochemically heterogeneous porous media is systematically investigated in this paper. Sensitivity analysis prior to the parameter identification provided valuable insights into the identifiability of the six model parameters. The hydraulic conductivity and longitudinal dispersivity were identified from tracer breakthrough data and then were used in the remaining parameter identification. The four colloid deposition and release parameters, favorable (fast) colloid deposition rate coefficient, geochemical heterogeneity, unfavorable (slow) colloid deposition rate coefficient, and colloid release rate coefficient from the unfavorable surface fraction, were found to be highly interrelated, and the inverse solution of the four-parameter set was not unique. When either the geochemical heterogeneity or favorable colloid deposition rate coefficient is known, the other three colloid deposition and release parameters can be identified via the inverse solution if the duration of the colloid injection is sufficiently long. The colloid release rate coefficient, however, cannot be identified when using a short (pulse-like) colloid injection. Neglecting the colloid release rate results in estimation errors of the other model parameters and thus adversely affects the subsequent prediction of colloid transport.