The Plume function is based on an analytical approach for simulating the 3D transport of a contaminant plume. The solution integrates a point source over the specified horizontal and vertical source area (the Pipe and Aquifer have the solution for a source which is distributed longitudinally built in, so the Plume function only has to allow for transverse distribution and spreading). The point-source solution allows for transverse (horizontal and vertical) dispersion, and GoldSim automatically creates ‘mirror’ sources to represent the vertical reflections of the plume off the top and bottom of the aquifer.
The Plume function calculates transverse (horizontal and vertical) dispersion assuming that the observation point is at a (longitudinal) distance from the source equal to the total pathway length minus half of the source-length (i.e. at the center of the source-zone). If the source is distributed longitudinally over a distance that is a significant fraction of the total pathway length, this approximation will result in slightly underestimating the amount of transverse dispersion.
The details of the calculation are described in detail in Appendix B of the Contaminant Transport User’s Guide.