A study conducted by the UPV/EHU’s Animal Ecotoxicity and Biodiversity Group in collaboration with the University of Vigo has proposed an ecological threshold concentration of 9 metals for 10 taxa of aquatic macroinvertebrates from clean sites in the Nalón river basin (Asturias). This is the first step toward incorporating into river management plans quality criteria relating to the bioaccumulation of hazardous substances, as required by the EU.