Colloid transport in physically and geochemically heterogeneous porous media: Modeling, measurements, and parameter identification

There is a growing interest in studying colloid transport and fate in groundwater because many problems of groundwater contamination and remediation are directly related to colloid transport. To date, there is no applicable model to accurately predict colloid transport in heterogeneous subsurface porous media.; Systematic theoretical and experimental investigations have been carried out in this work to study colloid transport in heterogeneous porous media. A one-dimensional model was first developed to describe colloid transport in geochemically heterogeneous porous media and applied to column experiments. The good agreement between the experimental measurements and model predictions delineated the importance of geochemical heterogeneity and dynamics of particle deposition in controlling colloid transport in porous media. A more sophisticated two-dimensional model was then developed based on some of the principles used in the one-dimensional model. Subsurface geological formations were modeled as physically and geochemically heterogeneous porous media. Both layered and random distributions were used to describe heterogeneities of porous media. The colloid transport behaviors in such porous media were simulated by the model. The simulation results indicate that physical and geochemical heterogeneities can result in preferential flow of colloids in the layered porous media, while random distributions of the two heterogeneities, especially the physical heterogeneity, lead to a random behavior of colloid transport.; Parameter identification of our two-dimensional model was carried out. Sensitivity analysis was performed to quantitatively determine the important model parameters and to provide insights into parameter identification. The identifiability of model parameters was discussed by using simulated observations. The results show that hydraulic conductivity and longitudinal dispersivity can be identified from the tracer breakthrough data; however, the inverse solution for a group of four particle deposition and release parameters is not unique. It was also found that by knowing either the favorable particle deposition rate or geochemical heterogeneity and injecting colloids with long durations, a group of three parameters--the geochemical heterogeneity (or favorable particle deposition rate), unfavorable particle deposition rate, and simultaneous particle release rate--can be identified. The sensitivity analysis and parameter identification results can help researchers to design technically and economically efficient experiments to obtain the necessary observations to predict colloid transport in subsurface environments.