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

Effective use of colloid transport models for heterogeneous subsurface poro us media requires the development of methodologies to identify the key mode l parameters. The inverse problem of a two-dimensional model for colloid tr ansport in geochemically heterogeneous porous media is systematically inves tigated in this paper. Sensitivity analysis prior to the parameter identifi cation 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 remainin g parameter identification. The four colloid deposition and release paramet ers, favorable (fast) colloid deposition rate coefficient, geochemical hete rogeneity, unfavorable (slow) colloid deposition rate coefficient, and coll oid release rate coefficient from the unfavorable surface fraction, were fo und to be highly interrelated, and the inverse solution of the four-paramet er set was not unique. When either the geochemical heterogeneity or favorab le colloid deposition rate coefficient is known, the other three colloid de position 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 affe cts the subsequent prediction of colloid transport.