Potential accumulation of toxic trace elements in soils during enhanced rock weathering

Abstract

Terrestrial enhanced rock weathering (ERW) is a carbon dioxide removal technology that aims at accelerating one of the most powerful negative feedbacks on Earth's climate, the chemical weathering of silicates. To achieve this, ERW proposes to spread ground silicate rock on agricultural soils. According to many models, global application rates of 40 tonnes of ground basaltic rock per hectare and per year would be necessary to sequester a significant amount of CO ₂, representing up to 24% of the current net annual increase in atmospheric CO ₂. When assessing the viability of ERW as a global geo‐engineering strategy, a pivotal but overlooked question to address is whether ERW may lead to toxic trace element accumulation in soils at unauthorized and potentially harmful levels. This study evaluates the legal sustainability of ERW with regard to trace element contents in soils. We compare different trace element accumulation scenarios considering a range of rock sources, application rates and national regulatory limits. The results indicate that, at the suggested annual application rate of 40 tonnes per hectare, the first regulatory limits would be exceeded after 6 and 10 years for copper and nickel, respectively. This study argues in favour of close tailoring of ERW deployment to local conditions in order to tap into its climate mitigation potential while preserving long‐term soil uses.

Highlights

• Terrestrial enhanced rock weathering (ERW) is a carbon sequestration technology that consists in applying rock powder to soils.

• This study evaluates whether rock powder application can lead to trace metals accumulation in soils beyond regulatory limits.

• ERW can lead to the overrun of some environmental regulatory limits within less than a decade.

• Specific site and rock selection will be necessary for this method to preserve long‐term soil uses.