Transport and fate of wollastonite weathering products through soil and subsoil under realistic irrigation/rainfall conditions

Deemed an inexpensive and low-energy method for mitigating atmospheric CO2 levels, enhanced rock weathering offers a long-term stable sink of soil carbon by converting alkaline earth metals into stable carbonates. Several silicate-rich minerals (e.g., basalt, olivine, and wollastonite) have been a matter of particular interest of researchers investigating the applicability of this approach for sequestrating atmospheric CO₂ in agricultural and urban soils. Several field-scale and laboratory-scale experiments have been conducted in our research group investigating the impact of the wollastonite amendment on the agricultural soil of Ontario. This includes monitoring pedogenic carbonate formation and migration in soil and subsoil systems (through collecting shallow and deep samples down to 1-meter profiles) as well as the effect on various plant growths in soils amended with crushed wollastonite.

The water (e.g., rainfall or irrigation water) infiltrating the porous medium of soil could transport and relocate solid particles over the vertical profile of the soil. Accordingly, when crushed silicate minerals (e.g., wollastonite) is applied to topsoil, the recurring introduction of water leads to the dissolution of mineral as well as downward migration of weathering products which could be settled in subsoil layers. Furthermore, the dissolution of wollastonite alters the chemical properties (e.g., pH, EC, etc.) of migrating water, which finally find its way to the water table below the soil medium. In the present study, we have looked into evidence of the vertical distribution of weathering products in soils amended with crushed wollastonite, whose relatively rapid weathering rate helps in the shorter-term to inform what occurs in the longer-term with slower weathering minerals. The experimental setup includes soil columns with and without wollastonite enrichment, located under two situations of lab environment (with regular hand-operated irrigation) and outdoor (with natural rainfall-fed). We also investigated the leachate collected from the bottom of columns in term of physiochemical properties.

The current study is part of the analytical and modelling framework we are developing in order to account for newly formed pedogenic carbonate as a qualified implementation for carbon capture credits. Such verified methods would encourage private and governmental entities to contribute to meeting emissions reduction goals and encourage the adoption of enhanced rock weathering as a reliable negative emissions technology.