Since Aquifers and Pipes are meant to represent one-dimensional conduits, the concentration reported by these two pathways represents the average concentration discharging from the pathway, and any spatial variation in the concentration (perpendicular to the flow direction) is not represented.
In many cases, however, real world concentrations will vary spatially at the end of a flow path due to dispersion in the horizontal or vertical transverse direction and/or a source term which is not uniformly distributed across the area of the conduit being simulated.
In order to allow you to address this spatial variation, GoldSim provides a special function which returns a correction factor which can be used to multiply the Aquifer’s or Pipe’s concentration output in order to compute the actual (as opposed to spatially-averaged) concentration at an observation point at some distance perpendicular to the flow path at the end of the pathway.
The Plume function (which is analogous to other GoldSim functions like sin or max) has eleven input arguments which describe the properties of the pathway to which you wish to apply the correction, the size of the source, the dispersive properties of the pathway, and the exact location of the observation point. The source is assumed to be a point, rectangle or box centered on the midpoint of the flow path (the pathway), and the observation point is located at the end of the pathway some specified distance from the centerline.
The Plume function is described in detail in the GoldSim Contaminant Transport Module User's Guide. Neptune and Company has used the Plume funtion in various models, including at the Savannah River Site (SRS). To support that application, John Tauxe of Neptune and Company has written an excellent document describing use of the Plume function. Anyone using this function is encouraged to read this document.