Temporal variations in carbon fluxes in European rivers and their implications for River Alkalinity Enhancement (RAE) scenarios - Thesis

[Geoengineering News]

https://ediss.sub.uni-hamburg.de/handle/ediss/12487

Authors: Tian, Mingyang

Abstract 

River Alkalinity Enhancement (RAE) is a carbon dioxide removal (CDR) strategy that uses river flow to transport captured CO2 to the ocean as carbonate alkalinity, promoting carbon sequestration on geologic timescales while mitigating ocean acidification. However, current knowledge cannot adequately assess the feasibility of RAE due to limited understanding of river chemistry dynamics across spatial and temporal scales. This thesis develops a framework integrating historical, experimental, and regional analyzes to evaluate RAE from a geochemical perspective, providing a scientific foundation for implementation and development of monitoring, reporting, and verification (MRV) protocol.

The historical analysis examined continuous carbon and nutrient fluxes in the Elbe River from 1984 to 2018, demonstrating that riverine carbonate systems undergo substantial changes in response to human intervention. Improved water quality following German reunification, particularly reductions in organic matter loads from enhanced wastewater treatment, was associated with a 66% decrease in annual CO2 emissions (from 3.8 ± 1.7 to 1.3 ± 0.6 Tg C yr−1). The long-term trends in the Elbe River underscores the importance of water quality management for mitigating CO2 emissions from polluted rivers around the globe. Additionally, the high chemical sensitivity identified in this study suggests that targeted interventions such as RAE could substantially alter riverine carbon fluxes but also underscore the importance of precise characterization of river chemistry before the deployment of such CDR methods.

The effectiveness of RAE depends on the persistence of added alkalinity: exceeding the saturation thresholds of carbonate minerals for a given water composition may lead to carbonate formation, causing partial loss of the added alkalinity. In the second study, the 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. Results show that the stability of alkalinity depends on the presence and quantity of suspended particles, seasonal variations in water chemistry, and salinity. Based on these findings, the Elbe estuary has the potential to accommodate additional alkalinity. Under idealized conditions, this would correspond to 3.0 ± 1.1 MtCO2 yr−1 being transported towards the North Sea. Estimates of alkalinity transport potential based on historical river chemistry indicated that this potential has declined from the 1970s to 2010s 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.

In the third study of this thesis, the modeling work extended the RAE analysis to a selection of 40 major European rivers representing approximately 26% of the total European continental discharge, using the GLOWACHEM water chemistry database. Three alkalinity addition scenarios were simulated: river-CO2-equilibrated, non-equilibrated, and air-CO2-equilibrated. The carbon transport potential (CTP), defined as the capacity of rivers to transport added alkalinity to the coastal ocean without CaCO3 precipitation losses due to carbonate minerals supersaturation, was calculated under idealized geochemical conditions established in the preceding studies. River-CO2-equilibrated alkalinity additions yielded an annual CTP of 472.9 ± 23.5 MtCO2 yr–1. Non-equilibrated alkalinity additions yielded a CTP of 8.4 ± 0.5 MtCO2 yr–1 and a reduction in riverine CO2 emissions of 67.0–99.7 MtCO2 yr–1. Air-CO2-equilibrated alkalinity additions yielded a CTP of 51.9 ± 2.0 MtCO2 yr–1 and a reduction in riverine CO2 emissions of 37.5–55.9 MtCO2 yr–1. These large CTP differences between the scenarios reflect the distinct carbonate chemistry responses to each alkalinity source. Under natural conditions, the non‑equilibrated scenario will shift toward the air-CO2-equilibrated outcomes as the system approaches atmospheric equilibrium, with the extent depending on reaction time.

In summary, this thesis evaluates the feasibility of RAE from the perspective of carbonate geochemistry. It documents the historical development of riverine carbonate chemistry, establishes experimental carbonate precipitation thresholds for estuarine systems, and estimates regional RAE potential under idealized conditions. European rivers possess significant RAE capacity; however, realizing this potential requires addressing geochemical, ecological, and operational complexities. In practice, environmental regulations would likely permit only lower CDR levels to ensure the safety of aquatic ecosystems. Future research should account for more detailed biogeochemical processes to refine the findings presented here, thereby supporting the development of robust MRV protocols.

Source: Hamburg State and University Library