Addressing Geographic Variability in the Comparative Toxicity Potential of Copper and Nickel in Soils

Comparative toxicity potentials (CTP), in life cycle impact assessment also known as characterization factors (CF), of copper (Cu) and nickel (Ni) were calculated for a global set of 760 soils. An accessibility factor (ACF) that takes into account the role of the reactive, solid-phase metal pool in the soil was introduced into the definition of CTP. Geographic differences in fate, accessibility, bioavailability, and terrestrial toxicity were assessed by combining the USEtox characterization model, empirical regression models, and terrestrial biotic ligand models. The median CTPs for Cu and Ni with 95% geographic variability intervals are 1.4 × 103 (1.7 × 102 to 2.0 × 104) and 1.7 × 103 (2.1 × 102 to 1.1 × 104) m3/kg·day, respectively. The geographic variability of 3.5 orders of magnitude in the CTP of Cu is mainly associated with the variability in soil organic carbon and pH. They largely influence the fate and bioavailability of Cu in soils. In contrast, the geographic variability of 3 orders of magnitude in the CTP of Ni can mainly be explained by differences in pore water concentration of magnesium (Mg2+). Mg2+ competes with Ni2+ for binding to biotic ligands, influencing the toxicity. Our findings stress the importance of dealing with geographic variability in the calculation of CTPs for terrestrial ecotoxicity of metals.