Evaluation of different approaches to quantify strong organic acidity and acid–base buffering of organic-rich surface waters in Sweden

Abstract

The role of organic acids in buffering pH in surface waters has been studied using a small brownwater stream (26 mg L −1 TOC) draining a forested catchment in Northern Sweden. Under the conditions of elevated pressure of CO 2 stream field pH was changed between 3.5 and 6.1 during the acidification and alkalinization experiment. Acid–base characteristics of the natural organic matter were also determined using a high precision potentiometric method for a concentrated sample from the same stream. We compared the predictions from the Windermere Humic Aqueous Model ( WHAM Model V), a model derived from the potentiometric titration (diprotic/monoprotic acid model) and a previously derived triprotic acid model which only uses alkalinity and TOC as input variables. The predicted buffering characteristics of all three models are very similar in the pH range 4.5–7 which suggests that during routine analysis alkalinity and TOC are sufficient to give a good estimate of organic acid anion charge contribution in a large range of surface waters. A slightly adjusted version of WHAM V successfully describes the organic charge contribution in a large number of sampled surface water lakes, which were previously used to calibrate the triprotic model.