Changes in acidity of dissolved organic carbon since 1990 revealed by modeling of 42,000 pH titrations from 110 streams

Sammendrag

Dissolved organic matter (DOM) is a natural source of acidity. DOM also binds to toxic metal ions, particularly aluminum. In recent years, research has focused primarily on changes in DOM concentrations and color in watercourses. Less attention has been given to DOM character related to its charge contribution, i.e., the density of negatively charged functional groups and their acid dissociation constants (pKa), which determine the acidity of DOM. The acidity of the DOM is often described using generic models that do not account for the possibility of changes in space and time. To test the assumption that the acid-base character of DOM is a constant, we examined 42,000 alkalinity titrations on surface water samples collected between 1990 and 2024 from 110 Swedish streams. Using the Stockholm Humic Model (SHM) implemented within the Visual MINTEQ chemical equilibrium model, we estimated dissociation of DOM acidity at the titration endpoint of pH 5.6. We found that the negative surface charge density of DOM at pH 5.6 has decreased since the 1990s. This calls into question the assumption that the acid-base character of DOM can be considered constant. Furthermore, despite considerable short-term variation in the values for each stream, a distinctive period of change shared by many streams was noted in the period 2000-2005. The relation of catchment factors and other water quality parameters related to this period of greatest change in DOM surface charge density have been explored.