Quantifying potential carbon dioxide removal via enhanced weathering using porewater from a field trial in Scotland

Amy Louise McBride^1*Kirstine Skov^2*Peter Wade²Joey Betz^2,3Amanda Stubbs²Tzara Bierowiec²Talal Albahri²Giulia Cazzagon²Chieh-Jhen Chen²Amy Frew²Matthew Healey²Ifeoma Idah⁴Lucy Jones²Mike E Kelland⁵Jim Mann²David Manning⁶Callum Ward²Melissa J Murphy²Anežka Radkova⁷Marta-Villa de Toro Sanchez⁸Utku Solpuker⁹Yit Arn Teh⁶Rosalie Tostevin²Will Turner²Jez Wardman¹⁰Morven Wilkie²XinRan Liu²

• ¹Independent Researcher, Freiburg-im-Breisgau, Germany
• ²UNDO Carbon Ltd, London, United Kingdom
• ³Department of Physics and Astronomy, Faculty of Mathematical and Physical Sciences, University College London, London, England, United Kingdom
• ⁴3Keel, Long Hanborough, United Kingdom
• ⁵Weathering Industries Ltd, Sheffield, United Kingdom
• ⁶Newcastle University, Newcastle upon Tyne, North East England, United Kingdom
• ⁷Independent Researcher, Cambridge, United Kingdom
• ⁸Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland
• ⁹Independent Researcher, Lyon, France
• ¹⁰Independent Agronomist, Angus, United Kingdom

Enhanced weathering (EW) is cited as a promising carbon dioxide removal (CDR) strategy, and is being rapidly commercialized. Rigorous monitoring, reporting and verification (MRV) are essential to ensure carbon claims are accurate and carbon credits are not mis-sold. MRV protocols incorporate multiple approaches, including soil and porewater sampling. This paper calculates potential CDR (pCDR) from porewater (direct pCDR), via an alkalinity estimation calculated from charge balance, and from soil samples (inferred pCDR), via the accumulation of exchangeable cations on soil exchange sites. These pCDR estimations are then compared to the maximum theoretical CDR potential. Data were collected from a 1.5-year field trial, situated in south-east Scotland. Crushed basalt was surface-applied to plots at rates of 0 (control), 23, 78 and 126 t ha⁻¹. Application rates were increased relative to common agricultural spreading practices (78 and 126 t ha-1) to increase the chances of detecting a signal. To calculate direct pCDR from porewater, ion concentrations of porewater samples extracted from a depth of 5 and 10 cm were integrated with precipitation surplus to estimate the cation flux from each depth over c. two week periods, as water budgets allowed. Ordinary least squares model results identified a significant effect of treatment as an explanatory variable for potential CDR, both at 5 and 10 cm depth. Direct pCDR ranging from 0.33 to 0.53 tCO2 ha-1 after c. 1.5 years of weathering was calculated at 5 cm depth in the 78 and 126 t ha-1 application treatment relative to the control. The model prediction interval was overlapping between the control and the 23 t ha-1 treatment at 5 cm depth, as well as for all the treatments in the 10 cm treatments when evaluated relative to the control. Carbonate precipitation was also assessed, but remained below the detection limit (0.1 wt.% inorganic carbon). Inferred pCDR calculated from 30 cm-deep soil samples were not significant, possibly as a result of experimental design and sampling density. Overall, when direct pCDR is normalized to mass of rock applied and duration of weathering (e.g. mass-time-normalized-pCDR), the values fall within the mid-range of values published from other field studies.