| Résumé: | <p>The UK’s route to net zero relies on employing carbon dioxide removal (CDR) strategies to offset hard-to-abate emissions from sectors such as transport and agriculture. Enhanced Weathering (EW) has emerged as a promising CDR technique that involves applying crushed alkaline silicate minerals onto agricultural land. Chemical weathering reactions involving these minerals within the soil transform atmospheric C to a dissolved ionic form, which is subsequently transported to the ocean via river systems for storage. It is essential to evaluate the feasibility of EW for CDR, both globally and in the UK, and a critical area of uncertainty concerns the role of rivers in transporting the atmospheric carbon removed via EW reactions. This thesis focuses on two key factors crucial to understand impacts of the widespread implementation of EW in the UK: i) the baseline CDR of natural silicate and carbonate weathering in the UK; ii) the potential release of CO2 resulting from secondary carbonate precipitation during the river transport of EW products.</p>
<p>To quantify the baseline flux of weathering across the UK, two approaches have been applied in this thesis that combine river geochemical data and modelling. First, an intensive spatial river sampling campaign was completed over two years in the Severn and Thames catchments. These two large rivers have contrasting bedrock geology and land use, providing a focused study on the spatial variations in natural silicate and carbonate weathering-based CDR at a catchment scale (Chapter 3). Second, the longer-term temporal variation in natural silicate and carbonate weathering CDR for 52 UK catchments was for the first time assessed using historical records from the Environmental Agency (Chapter 4). Building on these, the geochemical modelling results were compared alongside statistical methods to explore the relationship between weathering fluxes and various environmental drivers (Chapter 6).</p>
<p>The outcomes indicate that in the UK, natural weathering removes up to 6.3 Mt CO2 yr-1 of CO2 from the atmosphere. Approximately 79% of this removal is attributed to the dissolution of carbonate minerals. CDR by carbonate weathering increases towards the southeast, which is spatially linked to the large presence of carbonate rich bedrock. Conversely, silicate weathering yields are greatest in the midlands and in northern regions. The factors influencing silicate weathering rates are difficult to deconvolve in UK catchments, but likely involve a combination of lithology and water availability. Midlands regions exhibiting the highest silicate weathering yields are characterised by an optimal water supply; sufficient to sustain weathering reactions and permit the removal of dissolved products before reaching equilibrium in water. Additionally, in the rivers of most catchments, the proportion of solutes from silicates increases in the drier summer months which is presumed to be due to the increased contribution of groundwater to river water. This underscores the significance of prolonged water residence time in influencing natural silicate weathering fluxes in the UK. Finally, I account for the roles of strong acid weathering, secondary carbonate formation and cation exchange to decrease CDR by natural weathering. Only sulphuric acid- driven weathering may have the potential to significantly reduce the CDR resulting from natural weathering: from 6.3 to 4.9 Mt CO2 yr-1. However, further research is necessary to better understand sources of sulphate in UK catchments, and the potential role of sulphuric acid as a weathering agent.</p>
<p>The baseline quantity of CO2 removed through natural weathering in the UK calculated in this thesis is lower than the removal achievable through enhanced weathering in some previous studies (6-30 Mt CO2 yr-1). To address whether UK rivers can accommodate this extra EW derived influx on top of their baseline natural weathering flux (the second thesis objective), I estimated alterations in river chemistry of 36 major UK catchments by simulating the complete dissolution of 10-50 t ha-1 yr-1 of silicate rock within each catchment’s arable cropland (Chapter 5). Results indicated that if all this material dissolved and entered the river, most rivers would experience considerable changes in the dissolved inorganic carbon (DIC), causing most rivers to surpass calcite saturation. This indicates a high likelihood of carbonate precipitation, and I show that this riverine process has the potential to reduce the CDR potential of EW in the UK by up to 16% (10 t ha-1), 21% (20 t ha-1) and 27% (50 tha-1). If the flux carried by rivers is additionally constrained by silicate saturation, reductions in maximum CDR potential is decreased significantly further. Catchments in the southeast of England, characterised by extensive arable land and low river water flows, are likely to experiment higher influxes of DIC from EW, leading to increased carbonate precipitation.</p>
<p>This thesis contributes to understanding of the potential for EW by emphasizing the heterogenous nature of natural weathering in UK catchments and delineating specific zones which may be better placed for the adoption of EW. The southeast exhibits high natural weathering rates, alongside abundant arable land, seemingly making it an optimal location for EW. However, my research demonstrates that the high weathering rate in these regions causes higher chemical fluxes in rivers which drives waters to be saturated with respect to calcite naturally. Therefore, these rivers are unlikely to be able to transport the excess material. Consequently, caution is advised regarding EW application in southeastern catchments due to their limited water supply and the potential for substantial changes to river chemistry due to the significant portions of arable land. Conversely, the midlands regions, characterised by an optimal water availability, emerge as the most suitable areas for EW implementation. Based on the thesis findings a uniform spreading rate across all arable land may not be optimal, and a catchment- specific approach to EW implementation would be beneficial.</p>
<p>This work presents a first estimate for CO2 naturally consumed due to weathering in the UK, highlights the need for a catchment specific approach to determining suitable regions for employing enhanced weathering, and provides new scientific insights into the feasibility of EW for CDR in the UK.</p>
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