From source to sink: Quantification of Riverine and Marine Carbonate System Losses for Alkalinity-Based Carbon Dioxide Removal

The efficacy of Carbon Dioxide Removal (CDR) methods that enhance alkalinity in natural waters is dependent on the successful transport of that alkalinity through river networks to the ocean for long-term storage. However, as alkalinity-rich water traverses these flow channels, reduction of net carbon dioxide removal can occur through several natural processes such as: abiotic or biotic carbonate mineral precipitation, CO2 outgassing from re-speciation of the carbonate system, and suppression of natural alkalinity fluxes. Existing Measurement, Reporting, and Verification (MRV) protocols recognize these losses, but lack a  rigorous, standardized quantification framework across different CDR methods.

In this work, we present a unified framework for quantifying riverine and marine carbonate system losses across diverse CDR pathways, including enhanced weathering, river alkalinity enhancement (RAE), and ocean alkalinity enhancement (OAE). This framework includes applicability criteria for eligible rivers, such as the maximum transit time of the river reach and hydraulic residence time for surface-water storage. We introduce the use of a total retention factor, calculated as the product of specific retention factors from relevant riverine and marine loss processes.We outline guidance for evaluating relevance, risk and quantification for each loss term. For implementation, we provide a simple PHREEQC-based geochemical model to calculate losses from carbonate precipitation and DIC re-speciation.  We also discuss standardized requirements for alternative acceptable models. .

Using scalable and standardized loss quantification calculations, we provide a transparent methodology to ensure that alkalinity-based CDR projects yield accurate quantification of durable carbon storage. It gives project developers clear direction for accounting for complex biogeochemical interactions between terrestrial discharge and the marine environment. This framework also provides the academic community with a shared, reproducible foundation for cross-study comparison and targeted research prioritization.