Arsenic in soil is subjected to different chemical and biological transformations, such as red-ox or methylation processes, giving rise to different chemical forms such as As(III), As(V) and organic chemical forms with different toxicity towards humans and the environment. A detailed knowledge of the As fate in water-soil-plant systems (assessment of speciation, bioavailability and ecotoxicity) is of ultimate importance in order to avoid the spreading of contamination in other environmental compartments and along the food chain. The research is carried out in collaboration with other Research Units (RUs) for the characterization of soils contaminated by mining activities (Scarlino) an industrial activities (Valle Anzasca). The research activity is studying the arsenic distribution and speciation, together with other elements, by micro X-ray Fluorescence (μ-XRF) and scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM-EDX). In particular the mobilization processes of As in the soil-plant system under different red-ox conditions, which notoriously could promote the formation of different As species with different toxicities, is studied. The risk derived from the presence of arsenic in the contaminated soil is assessed by measuring the actual bioavailability and ecotoxicity in the epigeic earthworms. In particular the bioavailability and the distribution and speciation of arsenic accumulated in the body of earthworms is studied. Different biomarkers are applied for the study of chronic toxicity such as the Neutral Red Retention Time (NRRT) and the Comet Assay, metallothioneins, phenoloxidase, superoxide dismutase (SOD), catalase, malondialheide assays and reactive oxygen species (ROS).