The central concept of the Laplace transform solution technique used by GoldSim to simulate transport through Pipes is that the discharge history is a linear (and time invariant) function of the input history (for details on the Laplace transform solution, see Appendix B, "Details of Pathway Computations").
Because the Laplace transform approach requires that the transport equations be linear and unchanging in time, if any of the properties of a Pipe (e.g., partition coefficients for solid media, area, perimeter, storage zone properties) change during a realization, GoldSim must use an approximation to represent this change.
In particular, when a change in the properties of a Pipe occurs, the change has no effect on mass that was already in the pathway prior to the change. Mass which is already in the pathway continues to be transported based on the properties at the time it entered the pathway. Only mass that enters the pathway subsequent to the change is affected and utilizes the updated properties.
Hence, if you are simulating a Pipe which is long (i.e., has long travel times for some species) and has properties that vary significantly over time, this approximation can result in errors. In such a case, the way to minimize these errors is to discretize long pathways into a series of shorter pathways (such that travel time through each Pipe pathway is relatively rapid).
Note: If you do discretize a
long pathway into a series of shorter Pipes, the dispersivity (which is
typically defined as a fraction of the length of a pathway) should be defined in
terms of the total length (not the length of each individual Pipe).