A process-based reactive hybrid transport model for coupled discrete conduit–continuum systems

SummaryA process-based reactive hybrid transport model, RUMT3D, was developed to quantify the fate of dissolved contaminants and their interaction with solid phases in domains where discrete conduit networks are embedded in a permeable matrix. Such hybrid systems can be found, for example, in flooded underground mines, discrete karstic and fractured systems as well as in aquifers with intersecting boreholes. RUMT3D specifically takes into account the two distinctly different transport regimes within such hybrid systems: (i) rapid transport within the network of the highly conductive conduits, shafts, drifts, ventilation raises or roadways and (ii) the low velocity regime within the considerably less permeable matrix. RUMT3D is an extension of the existing reactive transport model PHT3D. Compared to the original model, which combines the multi-species transport simulator MT3DMS with the geochemical model PHREEQC-2, RUMT3D additionally incorporates a conduit transport model. A benchmark problem simulating the principal processes occurring at contaminated sites affected by acid mine drainage (AMD) was used to evaluate the model. The results suggest that for the simulation of contaminant transport in a hybrid system the consideration of rapid transport pathways is necessary. Conduits can strongly affect groundwater hydraulics and therefore become responsible for rapid hydrochemical changes.