Summary of Limitations on the Use of Pipe Pathways

While Pipe pathways are versatile and powerful, they do have several limitations, which are worth summarizing here:

• Unlike Cell and Aquifer pathways, GoldSim does not apply solubility limits within Pipe pathways.

• If the properties of a Pipe (e.g., partition coefficients, flow rates) change as a function of time, GoldSim has to use an approximation. The approximation is that species are discharged from a pathway based on the properties of the pathway (and the outflow rate) at the time the species (or their parents) entered it.

• Pipe pathways with very low dispersivities (<<1% of the pathway length) can produce unstable and inaccurate results (even if using a High Solution precision).

Two additional limitations (that has not been discussed previously) are as follows:

• Pipe pathways with transit times which are much less than a single timestep can produce unstable and inaccurate results. In such a case, a High Solution precision may be necessary to keep the solution stable and accurate.

• “Looping” reaction chains (A decays to B and B decays to A) are not permitted. Defining such reaction loops in a model that uses a Pipe will result in a fatal error message.

If any of these limitations applies, then the Aquifer pathway may be preferred over the Pipe pathway.