Determining freshwater pCO₂ based on geochemical calculation and modelling using PHREEQC

Fossil fuel combustion results in rising atmospheric carbon dioxide (CO₂), which is known to impact the global climate and the oceans. Latest insights indicate that rising atmospheric CO₂ levels also affect CO₂ partial pressure (pCO₂) in freshwaters, where pCO₂ is controlled by a multitude of parameters. However, up to date there is no standardized method, which allows the determination of current and past freshwater pCO₂ levels. Ideally methods should incorporate numerous hydrogeochemical and -physical factors to reflect the interplay of all interacting components and their effect on pCO₂. We here describe the application of the geochemical program PHREEQC. This freeware serves as an easy method enabling a plausible and comprehensive analysis of pCO₂ for field, laboratory, and especially long-term data. We present the use of the different input parameters of a laboratory- and a field long-term monitoring dataset including dissociation constants of carbonic acid measured as total inorganic carbon (TIC) and total CO₂ concentration (TCO₂) or total alkalinity (TA), together with hydrogeochemical and -physical parameters. Based on current literature and our analyses PHREEQC appears a solid strategy to determine freshwater pCO₂ that can moreover be used for long-term datasets.•Comprehensive analysis of pCO₂ for field, laboratory, and long-term data.•PHREEQC is not dependent on just one sampling method or parameter scheme.•PHREEQC includes testing the plausibility of a water analysis and enables the assessment of the quality of the laboratory analysis, as well as automatic calculation of all relevant aquatic complexes.