Metal Accumulation Risks in Enhanced Rock Weathering: Risk Assessment Framework and Recommendations

Enhanced rock weathering (ERW) has emerged as a promising carbon dioxide removal (CDR) pathway that could mitigate climate change while improving agricultural soils. However, as the rocks used in ERW weather, metals posing an environmental safety risk can also be released. The nascency of ERW, complex dynamics of metal release and availability, and heterogeneity of global soil regulations make it difficult for practitioners to determine accumulation risks for many metals. To systematically evaluate these risks, we created the ERW Metal Accumulation Calculator (ERW-MAC), a conservative mass-balance model that combines global feedstock and soil datasets to estimate post-ERW deployment soil concentrations for 16 metals across a variety of baseline conditions, agricultural practices, and a range of basalt and peridotite application rates. Under deployment conditions for basalt feedstocks, modeled post-application soil metal concentrations remain below the majority of regulatory thresholds despite 8 metals displaying potential to become elevated above baseline concentrations. Under deployment conditions for peridotite feedstocks, Ni and Cr soil concentrations often surpass the median regulatory threshold. Finally, we show that the metal concentration per unit of potential CDR (metal intensity) varies by roughly an order of magnitude across the metals and feedstocks considered. Peridotite has 7-11x Ni and Cr intensity compared to basalt, whereas basalt has higher intensities, and greater variability in intensities, across a larger suite of metals (e.g., Cu, V, Co, and Mn). We conclude that given the current status of regulations and ERW deployments, soil metal accumulation risk assessment can be most effective when following a 3-part sequence: 1) characterizing feedstock composition and baseline soil conditions, 2) mass balance modeling of accumulation under maximally conservative assumptions, and 3) evaluating results against local regulations. All of these risk assessment activities should continue to be followed by in-field monitoring to ensure soil concentrations do not exceed regulatory requirements.