The Contaminant Transport Module is a mass transport model. That is, it tracks the mass of the species as it moves them through the pathway network. However, GoldSim does not automatically impose a mass balance on the transport media that it is moving between pathways (e.g., water). Mass balances for transport media must be specifically imposed by the user.
In practice, this simply means that when specifying the media amounts (typically volumes of water) and media flow rates for Cells (and other pathways), you must be careful to ensure that there is a flow balance. For some systems, ensuring a flow balance may be quite straightforward. For example, if the Cells represented portions of a saturated aquifer, the volume of water in each Cell remains constant, and you need to only ensure that the flows between Cells are consistent. In many cases, you may not even require a separate calculation for the flow rates, as they may be simply specified (or perhaps read from a Time Series).
However, if your Cells represent surface water bodies, or other compartments in which the quantity of media (e.g., water) varies with time, then you will need to build a parallel calculation in your model that tracks the water balance. This will almost always be done using Pool or Reservoir elements.
In this case, it is necessary to couple the network of Pool or Reservoir elements (representing the flow system) to the network of Cells (representing the mass transport system). This can be done as follows:
•The Amount of each medium in a Cell (e.g., water) should be defined using the corresponding element (e.g., a Reservoir or Pool) that is being used to track that medium:
•The Outflow Flow Rates for each Cell should be defined using the appropriate rates for the corresponding element (e.g., outflow/withdrawal rates and/or overflow rates associated with a Reservoir or Pool).
Note: It is highly
recommended that you use Pools, rather than Reservoirs when building a flow
network that will link to a Cell network. This is because Pools have a
number of distinct advantages (in particular, they have separate output for each
outflow and they have total inflow and total outflow outputs) that make the
coupling process much simpler.
Warning: If the media
amounts (e.g., volumes of water) within your flow system )that you are modeling
using Reservoirs or Pools and linking to Cells) change very rapidly over a
timestep or go to zero (e.g., due to evaporation), Cells can compute incorrect
concentrations and mass transfer rates. In these cases, GoldSim will write
warning messages to the Run Log, and there are actions you can take to minimize
such errors.
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