Particle tracking can be an important tool in numerical groundwater modeling. Particles can be used for conceptualizing and quantifying source areas for water entering a groundwater flow system and the discharge areas for flow exiting the groundwater system and all points in between. Visual MODFLOW uses MODPATH, a semi-analytical particle tracking scheme, to calculate 3D particle tracking pathlines from the simulation output obtained by MODFLOW.

Like all numerical and analytical models, there may be times when MODPATH returns an error or does not calculate appropriate results. For example, you may find that your particles do not move at all after a succesful MODFLOW run. This issue was recently discussed on the Visual MODFLOW Users Group on LinkedIn. To ensure you get the best results from a MODPATH particle tracking simulation, consider the following five guidelines:

  1. MODPATH calculates based on MODFLOW head calculations. If a cell has become dry (inactive for flow) during a model run, the particles will not be able to move.
  2. Particles entering weak sinks are not handled correctly. Weak sinks are discharge boundaries where not all the flow entering the boundary necessarily discharges. There are three options in Visual MODFLOW to handle weak sinks: particles can pass through cells with weak sinks; particles are stopped in cells with weak sinks; and particles are stopped when the discharge to the sink is a user specified discharge percentage.
  3. The assigned circle diameter assigned reverse particles can introduce errors if it is too large. Often the circle should be slightly larger than the zone it is being assigned to.
  4. The grid discretization can impact the velocity field, fine grids can provide a better definition so scenario analysis is important when selecting an appropriate grid discretization.
  5. Forward and reverse particle track results can be very different. Both should be considered when determining a capture zone but starting with the reverse simulation and following with a forward simulation using forward particles placed at the source area defined by the backward particles.

Learn more about MODPATH particle tracking in the Visual MODFLOW Online Users Guide

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