A multi-species lattice-gas automaton model to study passive and reactive tracer migration in 2D fractures

We developed a numerical model based on a multi-species lattice gas cellular automaton to study passive and reactive tracer migration in saturated geological media. The model was made of multiple lattice gases interacting via a two-species collision rule. For a binary mixture, the model displayed a negative deviation from Raoult's law and therefore behaved as a real solution. By biasing the initial two-species collision rule, our model was made to obey the tracer assumption which requires that the tracer species does not affect the velocity of the vehicle fluid. In a 2D fracture, we checked the Taylor-Aris relation. An irreversible adsorption between the tracer and the solid phase was numerically added to perform filtration of the colloids. A good agreement was found with the solution of the filtration equation. An attachment efficiency was defined and was found to bear a linear relationship to the filtration coefficient. We added a third species to study the potential role of colloids in the transport of contaminants. Contaminant migration was enhanced when contaminants were bound to colloids and was slightly reduced when colloids were allowed to adsorb on the solid phase. Received 14 January 1999 and Received in final form 8 June 1999