Mass Balance of Transport Media in Pathway Networks

The GoldSim Contaminant Transport Module is a mass transport model. That is, it tracks the masses of the species as it moves them through the system. It is important to understand, however, that GoldSim does not impose a mass balance on the transport media. Mass balances for transport media must be specifically imposed by the user.

For example, if two pathways discharge water into a third pathway, the volumetric flow rate out of the third pathway does not have to be equal to be the sum of the discharges from the two preceding pathways. The inherent assumption here is that if the flows do not sum to zero, it is because you know of an additional source (or sink) of flow that does not warrant explicit modeling.

If the flows of a particular medium out of the pathway are greater than the flows into the pathway, it implies that additional quantities of “clean” medium (containing no species concentrations) also enter the pathway. If the flows out of the pathway are less than the flows into the pathway, it implies that a quantity of the transport medium is leaving the system without carrying any species with it. This is generally not physically possible, although there are several special processes by which this could indeed occur (e.g., evaporation of water from a pathway; movement of a medium through a filter as it leaves the pathway).

Because this latter case is only physically reasonable under special circumstances, GoldSim will provide some limited warning messages. In particular, if the sum of the discharges for the Reference Fluid from a Cell, Aquifer or Pipe is less than the sum of the recharges (advective mass flux links into the pathway) for the Reference Fluid, GoldSim will issue a (non-fatal) warning message. (Warning messages are not provided for External pathways or Network pathways.)

Note: Although this is a helpful warning, every model you build will have at least one Cell in which the Inflows exceed the Outflows. This is because all models have an “edge” or an “sink” where the model ends and you are no longer tracking mass. Hence, to avoid the warning message in this case, if the name of the Cell starts with the letters “sink” (case-insensitive), GoldSim will not issue the warning message for that Cell.

Note: For Cells, if the amount of the Reference Fluid has been defined in a maner that allows it to change (e.g., by defining the amount using a Pool or a Reservoir), GoldSim will not issue this warning message. This is because under these circumstances, it is assumed that you are properly tracking the volume of the fluid directly (and hence inflow/outflow imbalances are being explicitly accounted for). Moreover, the warning message is only written for the Reference Fluid. That is, if the Inflows to a Cell exceed the Outflows for any other medium in the Cell whose quantity is constant, the warning message is not written.

Note: GoldSim only provides a warning message if the absolute difference between the inflow and the outflow exceeds 1E-10 m3/sec AND the ratio of the absolute difference to the inflow exceeds 1E-6.

Note: If you want to disable these warning messages completely, you can do so by clearing the Log warning messages checkbox in the Contaminant Transport tab of the Options dialog. Note, however, that this is not recommended.

It is strongly recommended that you specifically enforce media flow balances by specifying pathway flow rates and media volumes and masses in a manner that is physically consistent with the mechanics of the system being modeled.