A Lagrangean Streamtube-Ensemble Approach in Reactive Transport Modeling in Natural Heterogeneous Porous Media

Understanding reactive transport in the subsurface in the neighborhood and downstream of an injection well is challenging in part due to the equal importance of radial and regional flow in the immediate neighborhood of the well, and due to uncharacterized heterogeneity in the governing properties. Conventional eulerian approaches require large computational resources and restrictive assumptions on heterogeneity in properties and on complexity of reactions. Here we describe a two-step lagrangean streamtube-ensemble approach, which deals with heterogeneity by separating micromixing from macromixing, and allows for multicomponent nonlinear reactions among mobile and immobile species. The reactive transport in the immediate neighborhood of the injection well, where both radial flow due to injection and linear (regional) flow are important, is solved in a streamline coordinate system, and the traveltime distribution function at an observation location is obtained. The transport process downstream of the observation location, where radial flow is insignificant, is then modeled via an ensemble of 1-D curvy streamtubes, along which advective-dispersive reactive transport occurs. Here the dispersion term represents intra-streamtube micromixing, while distributed streamtube fluxes (over traveltime) accomodates non-uniform arrivals, or macromixing, that appear in strongly heterogeneous porous media. Currently inter-streamtube mixing is ignored. This approach is applied to bacterial transport in a field experiment. Results of the application are presented and assumptions are discussed.