This is one of six different example models (all developed by Ted Eary of Enchemica) that illustrate how GoldSim can be used to carry out mixing and geochemical calculations. The models proceed from the simplest to the most complex. You can see an overview of all six models here.
This model is an example of how to incorporate geochemical calculations into a GoldSim model indirectly through the use of Lookup Table elements. In this approach, the Lookup Table is constructed using water quality results from a series of PHREEQC model runs that encompass the range of compositions that might occur in the system. That is, rather than doing the geochemical calculations directly in GoldSim, they are carried out prior to running the model (in PHREEQC), and then represented in GoldSim as Lookup Tables. This allows much more complex chemical systems to be represented.
The model considers two streams (one acidic and one alkaline) flowing into a Pond where they mix. The input flow rates of the two streams can vary with time. However, the chemical compositions of the two streams that mix are constant with time (since these must be used to create the Lookup Table). Chemical equilibrium is assumed to be instantaneously achieved at the point of mixing of the two streams.