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Chemical and isotopic processes occur in every segment of the hydrological cycle. Hydrogeochemistry—the subdiscipline that studies these processes—has seen a transformation from "witch's brew" to credible science since 2000. Going forward, hydrogeochemical research and applications are critical to meeting urgent societal needs of climate change mitigation and clean energy, such as (1) removing CO2 from the atmosphere and storing gigatons of CO2 in soils and aquifers to achieve net‐zero emissions, (2) securing critical minerals in support of the transition from fossil fuels to renewable energies, and (3) protecting water resources by adapting to a warming climate. In the last two decades, we have seen extensive activity and progress in four research areas of hydrogeochemistry related to water‐rock interactions: arsenic contamination of groundwater; the use of isotopic and chemical tracers to quantify groundwater recharge and submarine groundwater discharge; the kinetics of chemical reactions and the mineral‐water interface's control of contaminant fate and transport; and the transformation of geochemical modeling from an expert‐only exercise to a widely accessible tool. In the future, embracing technological advances in machine learning, cyberinfrastructure, and isotope analytical tools will allow breakthrough research and expand the role of hydrogeochemistry in meeting society's needs for climate change mitigation and the transition from fossil fuels to renewable energies. [ABSTRACT FROM AUTHOR]

It is a challenge to make thorough but efficient experimental designs for the coupled mineral dissolution and precipitation studies in a multi-mineral system, because it is difficult to speculate the best experimental duration, optimal sampling schedule, effects of different experimental conditions, and how to maximize the experimental outputs prior to the actual experiments. Geochemical modeling is an efficient and effective tool to assist the experimental design by virtually running all scenarios of interest for the studied system and predicting the experimental outcomes. Here we demonstrated an example of geochemical modeling assisted experimental design of coupled labradorite dissolution and calcite and clayey mineral precipitation using multiple isotope tracers. In this study, labradorite (plagioclase) was chosen as the reactant because it is both a major component and one of the most reactive minerals in basalt. Following our isotope doping studies of single minerals in the last ten years, initial solutions in the simulations were doped with multiple isotopes (e.g., Ca and Si). Geochemical modeling results show that the use of isotope tracers gives us orders of magnitude more sensitivity than the conventional method based on concentrations and allows us to decouple dissolution and precipitation reactions at near-equilibrium condition. The simulations suggest that the precise unidirectional dissolution rates can inform us which rate laws plagioclase dissolution has followed. Calcite precipitation occurred at near-equilibrium and the multiple isotope tracer experiments would provide near-equilibrium precipitation rates, which was a challenge for the conventional concentration-based experiments. In addition, whether the precipitation of clayey phases is the rate-limiting step in some multi-mineral systems will be revealed. Overall, the modeling results of multi-mineral reaction kinetics will improve the understanding of the coupled dissolution–precipitation in the multi-mineral systems and the quality of geochemical modeling prediction of CO2 removal and storage efficacy in the basalt systems. [ABSTRACT FROM AUTHOR]

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. 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. [Extracted from the article]

Carbon capture and storage (CCS) in basalt is being actively investigated as a scalable climate change mitigation option. Accurate geochemical modeling prediction of the extent and rate of CO 2 mineralization is a critical component in assessing the local and global feasibility and efficacy of this strategy. In this study, we review basalt-CO 2 -water interaction experimental studies conducted during the last two decades to determine whether they provide useable information for geochemical modeling. Most of the cited experiments generate data on the temporal evolution of water composition, and a few provide identification of secondary precipitates and their compositions, offering information about the reactivity of basalts and the likelihood of secondary carbonate mineralization at various temperatures, pHs, and p CO 2 conditions. However, most experiments provide insufficient information on the properties and quantity of secondary minerals formed, prohibiting accurate mass balance calculations and hence more quantitative geochemical modeling studies. Primary Ca, Mg, and Fe-bearing minerals in basalt control the availability of major ions released into aqueous solution for carbonate precipitation, and many secondary minerals, i.e., smectites, Ca-Mg-Fe carbonates, and zeolites, provide sinks for the same major ions, some of which are difficult to quantify experimentally. Thus, we have a "multi-source and multi-sink" inverse mass balance modeling problem with insufficient constraints on the bulk system in which the temporal evolution of major ions does not provide sufficient information on which mineral(s) dissolve or the sequence of dissolution and precipitation reactions. Going forward, this review highlights the need for providing better-constrained experimental data to calibrate basalt-CO 2 -water geochemical models. Hence, we propose that future experimental work should focus on trace elements and multiple isotopic tracers as well as emphasize the need for better characterization of the solid reaction products with modern analytical instruments. [ABSTRACT FROM AUTHOR]

There are several Li-mica and spodumene deposits in Central Europe. The Cínovec deposit on the border of the Czech Republic and Germany is the largest known hard-rock Li deposit in Europe and is currently being explored. Weathering and mining of such deposits could release Li and F to the environment. Both elements are associated with potential health effects when ingested via drinking water, yet little is known about the aqueous geochemistry of streams and creeks around Li hard-rock deposits. In this study, we sampled surface waters (n = 47) near three Li-mica and one spodumene deposit to examine a potential public health risk resulting from the weathering of Li-minerals. At the Cínovec and Homolka (also in the Czech Republic) sites, several water samples have elevated Li contents relative to those in typical surface waters, but they are all below the U.S. EPA-recommended threshold of 0.7 mg/L. Three of the ten samples from Cínovec contain F in concentrations that are above World Health Organization's 1.5 mg/L drinking water limit, and thus these waters (max. concentration: 3.8 mg/L) may represent a public health hazard. Based on geochemical modeling and statistical analysis, the lithium in the water comes from Li-micas and the fluoride is derived from a combination of Li-micas and fluorite, depending on the specific site analyzed. Lithium seems to be scavenged by clay minerals. • Weathering of hard rock lithium rocks could release Li and F into the environment. • Lithium comes from Li-micas. • Li is likely scrubbed by secondary minerals in the streams. • Fluoride comes from a mixture of Li-micas and accessory fluorite. [ABSTRACT FROM AUTHOR]

Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (Δ f G ° 298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of Δ f G ° 298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the Δ f G ° 298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the Δ f G ° 298 values of both REE3+ and REEOH2+ by ∼ 2–10 kJ/mol and ∼ 15–31 kJ/mol, respectively. For xenotime, adjustments of Δ f G ° 298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems. • A comprehensive REE phosphates solubility experimental database ThermoExp_REE was compiled. • Thermodynamic properties of REE3+ and REEOH2+ were optimized at 25−300 °C using the program GEMSFITS. • The solubility of monazite is controlled by REE3+ and REEOH2+ and xenotime by REE3+ at the studied pH and temperature. • The updated REE aqueous species provide a framework for modeling the stability of REE phosphates in hydrothermal fluids. [ABSTRACT FROM AUTHOR]

PHREEQC is a geochemical computer code developed by the United States Geological Survey and used to model mineral solubilities, aqueous solutions, and water-rock interactions over a large range of conditions. This easy-to-use and open-source program is widely used in the fields of geochemistry, environmental science and engineering, petroleum industry, mining and chemical engineering and biogeochemistry. However, the quality of the modeling depends on the experience of the modeler, and the quality of thermodynamic data available for the code, which is addressed in this review. The authors of the PHREEQC program clearly stated that the choice of thermodynamic data files (TDFs) is the responsibility of the user. Thirteen, mostly "third party", TDFs are distributed along with the software release without detailed documentation or critical evaluation. The quality of the TDFs varies with a wide spectrum of the internal consistency of the thermodynamic data, comprehensiveness of the chemical components and species, the temperature-pressure coverage of log K values, activity coefficient models suitable for ionic strength, and traceability of the sources of data. In this review, we provide some basic documentation, evaluate and compare these TDFs, and attempt to outline the limits of their applicability. We also used the TDFs to model examples of river water, seawater, groundwater, oil-field brine, and selected mineral solubility data for some oxides, oxy-hydroxides, hydroxides, aluminosilicates, carbonates, and sulfates. These comparisons clearly demonstrate that, with the same PHREEQC computer code but a different TDF, significantly different modeling results can be produced. It is imperative that users specify which TDF they used when referring to their modeling results. [ABSTRACT FROM AUTHOR]