Acid neutralizing capacity, alkalinity, and acid-base status of natural waters containing organic acids

Abstract

The terms acid neutralizing capacity (ANC) and alkalinity (Alk) are extensively employed in the characterization of natural waters, including soft circumneutral or acidic waters. However, in the presence of organic acids, ANC measurements are inconsistent with many conceptual definitions of ANC or Alk and do not provide an adequate characterization of the acid-base chemistry of water. Knowledge of Gran ANC and inorganic carbon concentrations does not by itself allow calculation of the pH of a water containing organic acids. Neither is measured ANC invariant upon changes in organic acid concentration. The result is a significant, but hidden, problem for policy makers and regulators, since ANC is considered a fundamental index of natural water acid-base status. ANC is the main output of many of the watershed simulation models now used in acid precipitation assessment programs, and considerations of present or expected ANCs apparently will play a major role in regulatory decisions. It is proposed to model such natural waters by (1) independently specifying the organic acid concentration and (2) adopting a definition of alkalinity that is invariant with changes of organic acid concentrations. Alkalinity, when so defined, can be both measurable and useful and possesses the conservative chemical properties commonly attributed to the term. A simple computational scheme, amenable to graphical presentation, is proposed to express the relationship between alkalinity, pH, organic acid concentration and inorganic carbon content.