PyGeochemCalc: A Python package for geochemical thermodynamic calculations from ambient to deep Earth conditions

Abstract

Over the past half century, techniques for evaluating the thermodynamics of water-rock interactions from ambient to deep Earth conditions have advanced incredibly and in myriad directions. As these tools for analyzing the thermodynamic states of geochemical species as a function of temperature, pressure, and composition have multiplied, so too have the possibilities for tracing water-rock interaction from ambient to deep conditions on Earth and beyond. Yet, the aqueous geochemical community still lacks a centralized platform for incorporating this constantly updating thermodynamic data into aqueous geochemical models. Here, we introduce PyGeochemCalc (PyGCC), a community-driven, open-source Python package that meets this need by providing a consolidated set of functions for calculating the thermodynamic properties of gas, aqueous, and mineral (including solid solutions and variable-formula clays) species, as well as reactions amongst these species, over a broad range of temperature and pressure conditions. The PyGCC package utilizes the revised Helgeson-Kirkham-Flowers (HKF) equation of state, and newly proposed density-based extrapolations based upon it, to calculate the thermodynamic properties of aqueous species; a choice of equations of state and electrostatic models (including the Deep Earth Water (DEW) model) to calculate thermodynamic and dielectric properties of water; and heat capacity functions to calculate thermodynamic properties of minerals and gases. Additionally, PyGCC integrates these functions to generate thermodynamic databases for various geochemical programs, including the Geochemist's Workbench (GWB), EQ3/6, TOUGHREACT, and PFLOTRAN, with straightforward possibilities for extension to other simulators. The various functions in the package can also be modularly utilized, and introduced into other modeling tools, as desired. In this paper, we detail the capabilities of PyGCC and the equations it relies on for calculating thermodynamic properties of water, aqueous species, and gases. Although the fundamental thermodynamic data necessary for state-of-the-science PyGCC calculations will necessarily evolve as our collective geochemical knowledge base expands, PyGCC's open source, community-driven design will allow for users to keep pace via rapid implementation of these advancements in this modern geochemical tool.