mf6rtm: Reactive Transport Model with MODFLOW 6 and PHREEQCRM

Integration Overview

mf6rtm couples the modular capabilities of MODFLOW 6 (MF6) with the geochemical modeling power of PHREEQCRM (PHREEQC-3). The workflow is as follows:

• Initialization: PHREEQCRM and arrays are initialized using a phinp.dat file.
• Model Setup: Grid dimensions and transported components are derived from the MF6 dis or disv object and PHREEQCRM respectively. MF6 model files are written for each component.
• Transport Loop: At each time step, MF6 solves the transport system. Convergence iterations and failures are tracked.
• Reaction Calculations: If enabled, concentrations are passed to PHREEQCRM for reaction calculations, and results are fed back into MF6.
• Output: MF6 handles UCN outputs. Custom outputs can also be saved as CSV.

Current Capabilities

• Structured grids (dis object) and unstructured grids through discretization by vertices (disv)
• Single simulation setup (flow + transport)
• Supported PHREEQC reactions:
  • Solution
  • Equilibrium phases
  • Cation exchange
  • Surface complexation
  • Kinetics

Benchmarks

Example 1: Engesgaard & Kipp (1992)

A 1D domain with calcite and dolomite equilibrium reacting to a chemical change in inflow. Simulated using the WEL and SSM packages. Compared against PHT3D.

Example 2: Walter et al. (1994) — 1D Redox Problem

Simulates acidic mine tailings leaching into a carbonate aquifer. Includes 17 aqueous components, 6 minerals. mf6rtm uses larger time steps than PHT3D for similar accuracy.

Example 3: Walter et al. (1994) — 2D Redox Problem

Extension of Example 2 to 2D. Uses CHD and CNC in MF6 for compatibility with the original PHT3D setup.

Example 4: Cation Exchange — Na/K/NO₃ flushed with CaCl₂

Simulates ion exchange in a flow-through column. Benchmarked against PHT3D and PHREEQC-3. mf6rtm uses double the time step due to numerical dispersion.

Example 5: Oxidation of Marine Pyritic Sediment (Appelo et al. 1998)

Includes pyrite oxidation, calcite dissolution, CO₂ sorption, ion exchange, and organic matter oxidation. Simulates three experimental phases involving MgCl₂ and H₂O₂ solutions.