Simulating Geochemical Processes By Linking GoldSim to a Geochemical Model

Some environmental applications involve consideration of complex geochemical processes. Although GoldSim can solve mass transport equations and can model simple chemical reactions (using the Contaminant Transport Module), it cannot solve complex geochemical problems by itself. However, it is possible to dynamically link GoldSim to geochemical equilibrium codes (such as PHREEQC).

Situations where linking GoldSim to a geochemical model may be valuable include the following:

• Mixing problems: In systems where the chemical effects of variable inputs to a mixture need to be modeled, linkage to a geochemical model may be warranted. Examples of such systems might be mixing of stream flows, mixing of groundwater inflows to a lake, or mixing of inputs to a batch chemical reactor.
• Chemical equilibration: Hydrochemical systems where chemical equilibration due to a change in conditions or reactants must be modeled may benefit from linking to a geochemical model. Examples may include systems where there are changes in gas partial pressure, temperature, or reactants as a function of time or space.
• Aqueous speciation: Systems where the speciation of dissolved species must be considered will likely need to incorporate a geochemical model. Examples may be adsorption-dominated systems where metal adsorption is controlled by the forms of the aqueous species, which in turn, are functions of the electrolyte composition and pH.

Ted Eary, a geochemist with Enchemica, and an experienced GoldSim modeler, has successfully linked GoldSim to PHREEQC. Before you try to do so, however, Ted suggests that the following points should be carefully considered:

• Is a geochemical equilbrium model really necessary? In many cases, careful examination of a geochemical problem may show that using the chemical capabilities already built into GoldSim (e.g., simple solubilility constraints, partitioning and first-order decay and ingrowth) is sufficient to model a system to a level of detail consistent with the actual level of understanding of the system. In other words, adding a complex geochemical model to represent a poorly understood system may not provide any better prediction than a simple mass balance model.
• Mass conservation may be lost. One of the advantages of GoldSim models is that there is a strict conservation of mass for chemical species and water. This advantage is lost when a geochemical model is linked to GoldSim because processes such as mineral precipitation or dissolution occur outside of the GoldSim model so GoldSim has no knowledge of what changes may have occurred. It is still possible to track changes in mass due to geochemical reactions in a model such as PHREEQC, but a separate set of tools must be used.
• Do you have a good understanding of the system and is it well constrained? If a geochemical system is poorly constrained, a geochemical model is likely to produce poor results. In most cases, a good understanding of what reactions can be expected to occur and what their effect on solution compositions should be is necessary in order to make sure that the results make sense (or even converge on a numerical solution).
• Do you have good working knowledge of the geochemical model? To use a geochemical model, you must have a very good understanding of how geochemical models work. You must also be familiar with the syntax used to setup the model's input files. This is not meant to warn users off from using geochemical models with GoldSim, but is meant to emphasize the fact that most models such as PHREEQC have their own cryptic syntax for inputs and display of results that is not easily learned without a great deal of trial and error.
• Programming experience is often needed. Approaches for linking geochemical models to GoldSim that involve DLLs or spreadsheet elements will require modification of the custom software code (C++ and/or VB6) to adapt the approach to new problems.

 A discussion of the various approaches for representing complex geochemical process in GoldSim can be found here.