Flying Blind: Geochemical Modeling and Thermodynamic Data Files

GroundwaterEarly View Guest EditorialFree Access Flying Blind: Geochemical Modeling and Thermodynamic Data Files Chen Zhu, Corresponding Author Chen Zhu chenzhu@indiana.edu orcid.org/0000-0001-5374-6787 Corresponding author: Indiana University, Bloomington, IN 47405, USA. Email: chenzhu@indiana.eduSearch for more papers by this authorD. Kirk Nordstrom, D. Kirk Nordstrom U.S. Geological Survey, Boulder, Colorado, USASearch for more papers by this author Chen Zhu, Corresponding Author Chen Zhu chenzhu@indiana.edu orcid.org/0000-0001-5374-6787 Corresponding author: Indiana University, Bloomington, IN 47405, USA. Email: chenzhu@indiana.eduSearch for more papers by this authorD. Kirk Nordstrom, D. Kirk Nordstrom U.S. Geological Survey, Boulder, Colorado, USASearch for more papers by this author First published: 23 July 2022 https://doi.org/10.1111/gwat.13223AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat For geochemical modeling, successful code development over the last 30 years has enabled wide applications. However, an explosion of applications has been accompanied by “institutionalized black boxing,” a term used by Anderson (1983) to warn of the detrimental consequences of the widespread use of solute transport models without adequate training. We refer to this practice as “flying blind.” For example, popular software packages such as Phreeqc, visual Minteq, TOUGHREACT, and The Geochemist's Workbench® have large user communities (any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government). However, prominent geochemists have published in premier geochemical journals with reference to the “Phreeqc model,” or the “MINTEQ model” of aqueous speciation or mineral solubility, without mentioning what thermodynamic data were used in the modeling. We remind readers that a computer “code” is NOT a “model” (Nordstrom 2012) and that many different models are used within the same code. For geochemical modeling, a thermodynamic data file (TDF) must accompany an input file and the executable code, comprising the “rule of three” (Zhu and Anderson 2002). A TDF provides equilibrium constants for reactions involving solids, aqueous species, gases, adsorbed surface species, and ion-exchange species. A TDF may also provide parameters for modeling kinetics of mineral dissolution and precipitation, activity coefficients of aqueous species, and fugacity coefficients of gaseous species. The quality of geochemical modeling results depends on the quality of the TDF. A basic requirement for a reliable TDF is internal consistency (Helgeson et al. 1978). Internal consistency implies, at a minimum, that the thermodynamic relationships, reference and standard states, and activity coefficient models are consistently applied to all data within a given TDF. Taking thermodynamic and kinetic data from one source and merging them with data from another source without ensuring they are internally consistent leads to degraded results (Nordstrom and Munoz 1994). As of June 2022, the Phreeqc package (v.3.7.3) contained 13 TDFs and the user can also develop their own TDF. This structure is helpful because different TDFs can serve different applications. However, calculated speciation, mineral solubilities, and gas concentrations may vary substantially. Each TDF also has its own limits of applicability in terms of temperature, pressure, salinity, and chemical composition. It is the user's responsibility to ensure the reliability of the TDF for the given application. The TDF used should be cited, documented, and justified, keeping in mind that current research continues to explore limitations and make improvements to TDFs. The proliferation of geochemical modeling applications coincides with the disappearance of thermodynamics courses in Earth Science curricula. It has been our experience that a change in student interest, faculty expertise, and enforcement of minimum enrollment at some universities have largely disabled the regular offering of these courses at most institutions. Automated software capabilities of computing and graphing cannot be a substitute for the user's basic background and training in thermodynamics, chemical kinetics, interfacial processes, and hydrogeochemistry. Journals like Groundwater can address these concerns by requiring authors to state clearly which TDFs were used with geochemical codes. Because most of these databases can be modified, another useful requirement would be to provide a listing of the key thermodynamic, kinetic, and surface data that are responsible for the modeling results. Such detail could be released in appendices or online Supporting Information files. For papers with complex geochemical models, putting the used TDF in a repository, consistent with Groundwater's data sharing policy, would be beneficial. Organizations such as NGWA and CUASHI could also offer training short courses and summer schools in thermodynamics and database evaluation to mitigate this problem. Flying blind with geochemical modeling is easy. Producing useful and high-quality modeling results takes time, dedication, and effort and is the least we can do to honor those modeling pioneers who showed us the path forward. References Anderson, M.P. 1983. Groundwater modeling - the emperor has no clothes. Ground Water 21, no. 6: 666– 669. Wiley Online LibraryWeb of Science®Google Scholar Helgeson, H.C., J.M. Delany, H.W. Nesbitt, and D.K. Bird. 1978. Summary and critique of the thermodynamic properties of rock forming minerals. American Journal of Science 278A: 569– 592. Google Scholar Nordstrom, D.K. 2012. Models, validation, and applied geochemistry: Issues in science, communication, and philosophy. Applied Geochemistry 27, no. 10: 1899– 1919. CrossrefCASWeb of Science®Google Scholar Nordstrom, D.K., and J.L. Munoz. 1994. Geochemical Thermodynamics, 2nd ed. UK: Blackwell Scientific Publications. Google Scholar Zhu, C., and G.M. Anderson. 2002. Environmental Applications of Geochemical Modeling. London: Cambridge University Press. CrossrefGoogle Scholar Early ViewOnline Version of Record before inclusion in an issue ReferencesRelatedInformation