Modelling of pH and inorganic carbon speciation in estuaries using the composition of the river and seawater end members

An equilibrium model based on the CO2 system was developed to model pH throughout the estuarine salinity range using the composition of the river and seawater end members. In order to validate the model, a spectrophotometric method was used to measure pH in an estuary and laboratory mixing experiments. The model successfully represented the measured pH values (±0.1 pH units) for three different river water end member compositions. The pH decreased at low salinities (S ≈ 0–2), increased at intermediate salinities (S ≈ 2–15), and then stabilised for higher salinity values. Changes in the inorganic carbon speciation in the low salinity region of estuaries result in assimilation of river borne CO2(aq).