High altitude soils are often shallow and thus fragile to erosional processes. Mountainous areas are therefore sensitive to humanly induced environmental changes (e.g. mining, clear cutting) and to climate change. Peatlands, a reoccurring feature in mountainous environments, acts as reservoirs of organic matter. Due to peats ability to retain trace metals these natural environments have accumulated contaminants since the beginning of the metallurgy and acts as filters for toxic elements (e.g. As, Hg, Pb). Based on a preliminary study we estimate that e.g. >600 ton of anthropogenic Pb is stored in organic soils on the northern slope of the Pyrenees. Similar conclusions can be drawn for other metals like Zn, Ag and Bi. The fate of these potentially harmful trace elements (PHTE) in relation to climate change or abrupt environmental changes (e.g. flood, forest clearance) is poorly understood. Once released from surrounding soils the catchment can be highly enriched in the bioavailable fraction of these PHTE:s causing a bioaccumulation in river biota. The objective of this project is twofold: First we will study the fate of trace elements in three various mountain catchments (Etangs de Bassies, Largentière and Bernadouze), and secondly we aim to infer the potential risk these elements pose on river biota (i.e. fish) and to the quality of drinking water. Our main research questions will be: Can the fate of PHTE:s, i.e. the release of Pb, Ag, Bi, Sb, Hg, As and U from bedrock, soil and peat into natural watersheds, be successfully traced and at what rate does this occur? How much of the released PHTE:s will be bioavailable and can an effect on the biota already be seen? To answer this we will combine analysis of radiocarbon, stable Pb-isotopes (anthropogenic Pb), and radionuclides (210Pb, 137Cs, 241Am and 7Be) to investigate the origin of these elements. In addition we will study the bioaccumulation within the catchments i.e. the transfer of metals from the water and organic matter to the river biota (e.g. fish) and assess the potential harmful exposure these elements pose to humans and society at large.