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.