  We implement the Multi-Rate Mass Transfer (MRMT) model for mobile-immobile
transport in porous media within the open-source finite volume library
\textsc{OpenFOAM}\reg \citep{Foundation2014}. Unlike other codes available in
the literature [Geiger, S., Dentz, M., Neuweiler, I., SPE Reservoir
Characterisation and Simulation Conference and Exhibition (2011); Silva, O.,
Carrera, J., Dentz, M., Kumar, S., Alcolea, A., Willmann, M., Hydrology and
Earth System Sciences 13, (2009)], we propose an implementation that can be
applied to complex three-dimensional geometries and highly heterogeneous
fields, where the parameters of the MRMT can arbitrarily vary in space.
Furthermore, being built over the widely diffused OpenFOAM\reg library, it can
be easily extended and included in other models, and run in parallel. We
briefly describe the structure of the < multiContinuumModels > library that
includes the formulation of the MRMT based on the works of [Haggerty, R.,
Gorelick, S.M., Water Resources Research 31, (1995)] and [F. Municchi and M.
Icardi Phys. Rev. Research 2, 013041, (2020)]. The implementation is verified
against benchmark solutions and tested on two- and three-dimensional random
permeability fields. The role of various physical and numerical parameters,
including the transfer rates, the heterogeneities, and the number of terms in
the MRMT expansions is investigated. Finally, we illustrate the significant
role played by heterogeneity in the mass transfer when permeability and
porosity are represented using Gaussian random fields.
