Stability of alkalinity in the land-ocean transition zone: a geochemical CDR perspective for the Elbe River, Germany

[Geoengineering News]

https://iopscience.iop.org/article/10.1088/1748-9326/adeeab

Authors: Mingyang Tian, Jens Hartmann, Niels Suitner, Thorben Amann, Stephan Kempe, Carl Lim and Charly A Moras

Published 29 August 2025 

DOI 10.1088/1748-9326/adeeab

Abstract

Carbon dioxide removal (CDR) strategies like enhanced weathering and river/ocean alkalinity enhancement have been suggested to increase alkalinity in rivers, coastal areas, and eventually oceans. The effectiveness and sustainability of these CDR approaches depend on the persistence of added alkalinity, since exceeding certain Ω-thresholds for a given water composition may lead to carbonates formation, causing the loss of previously added alkalinity. In this research, stability of alkalinity was tested using incubation experiments with Elbe estuary freshwater from two seasons (March and August). Alkalinity was increased up to 4000 μmol kgw−1, with varying salinity from 0 to 16. This study shows that the stability of alkalinity depends on the presence and quantity of suspended particles, seasonality of water chemistry, and salinity. Based on the experimental data, the Elbe estuary may take up additional alkalinity in the freshwater part, corresponding to ∼3.0 MtCO2 yr−1 being transported towards the North Sea. Estimates of alkalinity additions from the 1970s to 2010s show a decreasing potential due to changes in pCO2 and pH. The upper geochemical limit for transporting additional alkalinity through estuarine systems serves as a critical boundary. Environmentally feasible levels may be lower than identified here and depend on environmental regulations.

Source: IOP Science