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.
Note: This approach is based on an analytical solution initially provided by Domenico and Robbins (1985). Note, however, that the Plume function actually goes beyond Domenico and Robbins in two key ways: Domenico and Robbins assumes 1) that the source always extends down from the water; and 2) that the aquifer is very thick (the plume does not ever "feel" the bottom). But GoldSim can allow the top of the source to be below the water table; and 2) it also allows for a very thin aquifer. To represent this, GoldSim creates "reflections" off the top and bottom of the aquifer. It should also be noted that the Domenico and Robbins approach is an approximate solution and can introduce significant error, depending on the parameter values (West et al., 2007).
P.A Domenico and G.A. Robbins, 1985, "A New Method of Contaminant Plume Analysis" Ground Water, Vol 23(4), 476-485.
M.R. West, B.H Kueper and M.J Ungs, 2007, "On the Use and Error of Approximation in the Domenico (1987) Solution" Ground Water, Vol 45(2), 126-135.
The details of the calculation are described in detail in Appendix B of the Contaminant Transport Module User’s Guide.