Development of a field method for measuring river alkalinity

Details

Supervisors

Delmelle, Pierre

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

enAlkalinity plays a fundamental role in aquatic ecosystems by buffering pH fluctuations, thereby maintaining the stable conditions required for aquatic life. While laboratory titration methods remain the reference standard, they require sample transport, which can induce CO2 degassing and subsequent carbonate precipitation, potentially altering alkalinity values before analysis. Field-deployable methods are therefore needed to enable rapid, in situ measurements. This work presents a preliminary methodological investigation into the applicability of the headspace method, originally developed for high-alkalinity oilfield produced waters by Wang et al. (2014), to natural freshwater systems characterised by lower alkalinity. The method is based on measuring the pressure increase generated in a sealed vessel following the addition of a strong acid, which quantitatively converts dissolved inorganic carbon into CO₂, thereby allowing carbonate alkalinity to be determined. The protocol and apparatus were progressively refined throughout this study, and several previously unrecognized factors influencing the pressure measurements were identified and addressed. Two analytical approaches, direct calibration and the standard addition method, were tested on synthetic solutions, bottled waters, and natural river waters. The dataset obtained remained too limited to draw definitive conclusions. The standard addition method proved more precise, while both approaches showed limited accuracy. A systematic opposing bias was observed between the two methods relative to reference titration values. Despite its practical advantages, including low cost, field portability, and avoidance of sample transport, the method currently suffers from limited sensitivity. Several improvements are proposed, including increasing the sample volume and developing a floating support system to improve thermal stability and ease of handling in the field. Further development and validation on a larger sample set are required before the method can be applied to freshwater alkalinity monitoring.