Toxicokinetics of a Pesticide Mixture in Earthworms Reveal Concentration-Dependent Bioaccumulation and Limited Interactions

Despite their fundamental role in sustaining terrestrial biodiversity and ecosystem functioning, soils are increasingly contaminated by pesticides, often occurring as complex mixtures. Understanding chemical uptake, distribution, and elimination in soil organisms is essential for reliable risk assessment. We investigated the toxicokinetics of a binary mixture of imidacloprid and epoxiconazole in the earthworm Aporrectodea caliginosa. Single-substance experiments were conducted to estimate uptake and elimination rates, and allowed the implementation of single substance toxicokinetic models. A mixture experiment tested five concentration ratios to assess potential interactions. Epoxiconazole, an azole fungicide, exhibited rapid uptake and elimination (k_u = 1.9 gsoil*gworm-1*d-1; k_e = 1.7 d-1), resulting in low bioaccumulation (BAF = 0.9), whereas imidacloprid, a neonicotinoid insecticide, accumulated slowly with low elimination (k_u = 1.6 gsoil*gworm-1* d-1; k_e = 0.12 d-1), producing high bioaccumulation (BAF = 20). The explicit inclusion of the 24-hour Petri dish depuration phase proved to be critical, especially for epoxiconazole, highlighting the influence of gut-clearance procedures on apparent kinetics. In accordance with only a small synergistic effect in the mixture, no toxicokinetic interactions were detected in the mixture; internal concentrations of each compound were consistent across ratios. Concentration-dependent patterns were observed, with indications of a saturation process at high exposure levels for imidacloprid, and an increased BAF at high concentrations for epoxiconazole. This finding questions the use of single, concentration-independent bioaccumulation factors for hazard assessment and highlights the value of integrated TKTD approaches.