PREDOMINANCE AND MINERAL STABILITY DIAGRAMS REVISITED

There is currently a ‘disconnect’ between the sophisticated models geochemists normally use to calculate the speciation and transport of chemicals in the environment and the simplified models used to calculate predominance and mineral stability diagrams. This need not be so. It is relatively easy for any general purpose chemical speciation program to calculate these diagrams in a rigorous way. All the chemical interactions involved (adsorption, ion exchange, solid solution, solution non-ideality etc) can then be included, not just a subset as in the usual, simplified approach. This leads to a consistent approach to geochemical modelling and, moreover, is more informative.

The easiest and most general way to do this is to use a numerical approach to identify the boundaries. This contrasts with the normal ‘analytical’ approach - manipulation of simplified equations - that is used in the construction of classical predominance and stability diagrams (as every student of geochemistry knows!). A ‘hunt and track’ algorithm is described that will do this. This is more efficient than the alternative grid or ‘brute force’ approach.

Unlike the classical diagrams where some prior simplification of the system under consideration is usually undertaken, the new method can retain all the complexity inherent in the original system and so can be customised for specific situations. It is precisely in these complex situations that the new approach can provide a useful insight into the many interactions occurring. It also shows how the system might respond to changing conditions. Critically, the diagrams can include adsorbed species which frequently dominate the behaviour of trace metals in environmental systems and so should not be ignored.

Various examples will be shown based on this new approach using the USGS PHREEQC geochemical speciation code.