Evaluating Total Cation Accounting (TCA) as an MRV Approach for Enhanced Rock Weathering - Insights from a trial in Ontario, Canada

Enhanced rock weathering is a promising carbon dioxide removal (CDR) technology that involves the dissolution of silicate minerals (e.g., wollastonite). This process releases elements such as calcium, which can remain in solution and be charge balanced by bicarbonate, or be stored as pedogenic carbonate or on soil exchange sites. To verify carbon removal credits, robust monitoring, reporting, and verification (MRV) approaches are essential. In this study, we explore total cation accounting (TCA) as a novel method for MRV. TCA involves conducting a total digest of a soil-feedstock mixture in the near-field zone (NFZ; here defined as 15 cm) and analysing the major cation content via Inductively Coupled Plasma Optical Emission Spectroscopy. The resulting data include baseline cations in the soil (pre-spread), residual feedstock (post-spread), and weathered cations bound to exchange sites or forming carbonate minerals (post-spread). The major cations (Ca, Mg, K, Na) are summed to calculate total cations, and net cation loss from the NFZ is used to determine CDR.

This study uses data from a small plot monitoring site (SPMS; 4x10 m) in Ontario, Canada, where Canadian Wollastonite feedstock was applied at four different densities (0, 5, 50, and 100 t/ha) in November 2023. Soil samples were collected across the SPMS using a 20:1 composite of 15 cm soil cores. The soils were finely crushed to preserve the distribution of larger feedstock particles, ensuring homogeneity and maintaining a representative soil-to-feedstock ratio.

Prior to spreading, cation concentrations in both treatment and control groups were clustered around a similar mean, representing the baseline soil composition. Samples collected post-spread show an increase in cation content on treatment plots, demonstrating that the addition of our feedstock is resolvable, even in settings with unusually high background soil Ca content. All subsequent samples show cation content decreasing relative to the post-spread levels, indicating cations have been exported into the far-field zone (FFZ) or lost via plant uptake or solid transport. The results are consistent with other MRV approaches applied on the same field trial, although our results suggest that other methods can underestimate CDR. One advantage of TCA over competing MRV methods is that the results are time-integrative, meaning the signal-to-noise ratio improves with longer sampling intervals. These first field trials using this approach demonstrate its potential as a scalable, robust methodology for MRV in ERW trials.