The PT module calculates the flow path of a hundreds, thousands, or even millions, of hypothetical particles, which are moved in the three-dimensional, saturated groundwater zone (SZ). The particles are displaced individually in a number of time steps. The movement of each particle is composed of a deterministic part, in which the particle is moved according to the local groundwater velocity calculated by the MIKE SHE water movement (WM) module, and a stochastic part where the particle is moved randomly based on the local dispersion coefficients.
Particle tracking is only calculated for the saturated zone (SZ) and particles that leave SZ are not traced any further. Initially, the user assigns a number of particles to each model grid cell. Particles are added during the simulation from sources, for example solutes in precipitation or from boundary or internal defined concentration cells. Particles leave SZ when they arrive at a boundary or an internal constant concentration cell or when they go to a sink. Possible sinks in the Particle Tracking are wells, rivers, drains, and exchange with the unsaturated zone or overland flow.
All particles are assigned a mass, which means that a number of particles within a specific volume correspond to a solute concentration. The Particle Tracking module can therefore be used for solute transport simulations and is in some cases superior to the conventional numerical solution of the advection-dispersion equation since numerical dispersion is negligible. However, the module is mostly used for delineation of abstraction well capture zones and upstream zones and for determination of groundwater age and conservative solute transport times.
The PT module uses the concept of 'registration' cells. This records particle data when particles enter certain model cells. Registration can be used to delineate capture zones or to observe particles passing through some region of interest, such as a redox layer.