A Particle Tracking Model, IGSM_PT, as a Post-Processor to Finite-Element Groundwater Flow Model, IGSM

The purpose of this paper is to provide the necessary information to understand the theory of the Particle Tracking Model (IGSM_PT). IGSM_PT is a particle tracking model that was developed to compute two-dimensional flow paths using output from steady-state or transient (as small as daily simulation time steps) ground-water flow simulations by Integrated Groundwater and Surface Water Model (IGSM), a finite element flow model (Taghavi et al., 2003). Once the head solutions from IGSM have been obtained at every node, the principal velocity components for each element can be computed using the finite element shape functions and Darcy's Law. In order to compute the path lines, IGSM_PT uses constant velocity throughout the element to compute particle paths. IGSM_PT uses a technique (in-element particle tracking) that traces particles on an element-by-element basis. Given a velocity field, a particle is traced element by element until either a boundary or an internal sink/source is encountered, or the available time is completely consumed. The paper also presents verification of the model with 2 example cases. First, the comparison of the model results with a simple example with an analytical solution. The second example is a moderately complex sample problem from the MODPATH User Manual, which is used to verify IGSM_PT and compare its results with MODPATH.