Geochemical speciation modeling, based upon chemical thermodynamic relationships between aqueous species, mineral phases, and gases in closed as well as open systems, can be used to study a large variety of problems in earth and environmental sciences. A random sampling of topics includes ore formation processes, petroleum reservoir or playa lake brine chemistries, soil pedogenesis, geologic storage of hazardous and radioactive wastes, and the environmental chemistry of shallow aquifer environments, the latter potentially including trace element mobility, bioremediation, and agricultural/nitrate impacts. Geochemical models simulate reactions in such systems by solving mass-, charge-, and valence electron-balance equations among the relevant species and components, constrained by databases of equilibrium constants. These reaction modeling capabilities can be extended to include irreversible reactions, water-rock and water-gas mass exchanges, reaction kinetics, solid solution chemistry, and other phenomena.
Geochemical models can be used in a “batch” or zero-dimensional mode to simulate a variety of scenarios, such as mixing of two solutions with different chemistries, equilibration of water chemistry with a mineral assemblage, or the geochemical response of a water + mineral assemblage + gas system to the addition of one or more external reactants (e.g., a waste package, or an additive to promote changes in oxidation-reduction conditions for remediation of a contaminated site). In addition, a geochemical model can be used as a component of a more complex, multi-dimensional reactive transport model, where dissolved constituents are transported from one location to another in response to hydraulic gradients and diffusion, while subject to spatially-variable chemical reaction processes.
An understanding of geochemical and reactive transport modeling, using well-vetted simulation packages available in the public domain, will provide practicing hydrogeologists with a capability to quantitatively explain, and predict, the chemical evolution of aquifer environments in a variety of contexts.
This course will provide practical instruction in the use of PHREEQC and PHAST, two public-domain models available for download from the United States Geological Survey, to simulate aqueous geochemical processes and reactive transport in groundwater systems. After the key concepts for understanding and running both models are covered, techniques for extending the applicability of both packages using Python will also be explored.
At the conclusion of the course, students will be able to:
- Construct PHREEQC simulations for a variety of situations, including water-rock and water-gas systems, reactions involving mineral surfaces (ion exchange, surface complexation), and oxidation-reduction processes. Students will also understand how to apply more complex features, such as using solid-solution formulations for variable-composition mineral phases, and how to implement reaction kinetic expressions in a model.
- Construct PHAST simulations of multispecies reactive transport in groundwater, including systems entailing hydrologic as well as geochemical/mineralogical heterogeneities.
- Use Python scientific programming tools, together with PHREEQC and/or PHAST, to address additional problems; examples will include reactive transport through partially-saturated porous media and conducting sensitivity analyses to parameter values. Also, Python scripts to process model input (e.g., specifying mineral assemblages) and output (e.g., Stiff diagrams) will be developed.
The course will be presented through the use of multimedia explanatory exercises, instructional videos, and a webinar involving working through a specific PHAST application for an environmental remediation example problem.