There is one aspect of the solution algorithm that you may need modify in order to increase the accuracy of simulations using Pipes. This feature is accessed via the Contaminant Transport tab in the Options dialog (accessed from the main menu items Model | Options…):
GoldSim uses a particular algorithm (the de Hoog algorithm) to carry out the Laplace transform calculations. This algorithm includes a parameter which controls the number of terms used in an approximation within the solution. The number of terms used is determined by the Solution precision field in the Options dialog (note that this field also controls the precision of the Cell pathway algorithm). There are three possible choices for the solution precision (Low, Medium, High). If Medium is selected (the default), GoldSim uses 15 terms in the Laplace transform solution. If Low is selected, it uses 5; if High is selected, it uses 25. You only need be concerned with this value if you are trying to simulate Pipes with very low dispersivity values. In such a case, you may need to select High precision (i.e., 25 terms) in order to reduce oscillations in your computed fluxes. The solution algorithm for Pipes is discussed in detail in Appendix B of the Contaminant Transport Module User’s Guide (in a section entitled Solution Technique for Pipe Pathways).