Comparing Potential Carbon Dioxide Removal Fluxes from Enhanced Rock Weathering with Baseline Fluxes in the UK

Enhanced Rock Weathering (ERW) - the addition of crushed alkaline rocks onto agricultural land - has emerged as a promising approach for atmospheric carbon dioxide removal (CDR). Evaluating the global and UK CDR potential and environmental implications of ERW prior to widespread implementation is essential. Accurate quantification of CDR via ERW requires an understanding of the baseline CO₂ flux due to existing natural and anthropogenic influences on weathering. Understanding these baseline weathering fluxes is also important for predicting the capacity of UK rivers to accommodate additional material from ERW, because natural weathering controls river geochemistry.  However, uncertainty exists regarding baseline values and their variability across UK catchments, which have varying lithological, climate, and anthropogenic influences. In this study, we quantify the annual baseline CO₂ consumption due to natural weathering in the UK using historical river geochemical data, and a geochemical inversion technique to separate fluxes derived from weathering of silicate and carbonate rocks.

Results reveal that baseline silicate and carbonate weathering contributes up to 6.3 Mt CO₂ yr^-1 as dissolved inorganic carbon (DIC) to UK rivers combined. Within this total, silicate weathering, vital for long- term carbon removal, contributes up to 1.3 Mt CO₂ yr^-1. Normalising the CDR by catchment area highlights significant variability across the UK, with Midlands and southeastern catchments exhibiting the highest weathering CO₂ yields. Increased DIC from baseline weathering in southeastern catchments brings riverine calcite saturation close to saturation thresholds. Consequently, these heightened weathering rates are expected to limit the rivers’ capacity to accommodate additional DIC from ERW. Conversely, our findings suggest that Midlands catchments may offer optimal conditions for ERW implementation- displaying favourable weathering conditions and increased riverine storage capacity to store ERW by-products. Therefore, the suitability of a catchment for ERW application hinges on achieving a balance between favourable weathering conditions and adequate riverine capacity for surplus weathering products. Consequently, a uniform approach to EW implementation may be unsuitable for widespread use in the UK. Instead, we propose a catchment specific approach, involving calculations of the potential river chemistry impacts based on intended spreading rate and arable land area.  Although more demanding, this ensures the safe implementation of ERW without compromising riverine chemical thresholds.

The baseline weathering CDR (6.3 MtCO₂ yr^-1) aligns with the lower end of that proposed achievable through widespread ERW implementation across the UK (6 -30 Mt CO₂ yr^-1)¹. If this anticipated CDR is achieved and evenly distributed within UK rivers as DIC, the background riverine DIC flux would at least double. However, given the heterogenous distribution of arable land, our findings suggest that catchments with extensive arable land may experience a substantial DIC flux from ERW. This flux, especially in regions with high baseline values, could trigger carbonate precipitation, potentially reducing CDR potential by 16- 27%².

References

¹Kantzas et al (2022) ‘Substantial Carbon Drawdown Potential from Enhanced Rock Weathering in the United Kingdom’, ²Harrington et al (2023), Implications for the Riverine Response to Enhanced Weathering to CO₂ Removal in the UK,