RW3D: An Open-Source Random-Walk Particle-Tracking Code for Reactive Transport under Complex 3D Conditions

We introduce RW3D, a random walk particle tracking (RWPT) method designed to simulate conservative and (some) reactive transport in saturated and unsaturated porous media within complex three-dimensional (3D) environments. Traditional methods such as Eulerian approaches often struggle to simulate efficiently and accurately transport and reactions under heterogeneous conditions. This often leads to a misrepresentation of macrodispersion and mixing, which can significantly deteriorate the performance of predictive models.

Our approach leverages a range of advanced modeling techniques to solve the advection-dispersion-reaction equation, which allow us to accurately capture the spatial and temporal variability of reactive solute transport under a large array of conditions.

Key features of RW3D include:

• 3D rectilinear grid: By allowing spatial variability in the horizontal and vertical discretization, our RWPT method can represent complex domain.
• Transient parameters: The code can read text or netCDF files to represent transient conditions in any transport and/or reaction parameter.
• Array of reactions: The code has been developed to accurately solve first-order decay networks, bimolecular kinetic reaction network, and retardation.
• Upscaling technique: Multirate-Mass Transfer, potentially coupled with reaction network, can be used to upscale transport parameters.
• Sinks: The code can handle mass removal by strong or weak sinks such as rivers and extraction wells.
• Multiple outputs: RW3D can computes (cumulative) breakthrough curves, snapshots of the plume of particle, spatial/temporal moments, particle paths. Kernel smoothing techniques can be used to mitigate subsampling effects due to a limited number of particles, enhancing the code’s efficiency.

Over the years, we have demonstrated the applicability of RW3D through a series of case studies, highlighting its robustness and versatility in addressing groundwater contamination scenarios. The code can handle any continuum spatial scale, from regional studies to site or lab specific assessments.

RW3D is now released under an open-source license, and contributions to its development and assessment are welcome.