The aquifer is divided into discrete elements using the Cartesian coordinate system. The material properties and solute concentrations are assumed to be constant throughout the element, but are allowed to differ between different elements. In this way a non-homogeneous aquifer is approximated as a collection of different homogeneous regions.

Figure 3.3.1 illustrates a "5 x 4" aquifer mesh.

Figure 3.3.1 Illustration of a 5 x 4 Aquifer Mesh

Elements are node centered. Boundaries separating elements are located at an equal distance from each node. For boundary nodes, the exterior boundary is located the same distance from the node as its opposite boundary. Unless otherwise specified, boundary nodes are assumed to be a no-flow no-flux boundary.