OHM territories are concerned by present and past pollutions. Especially potentially harmful trace elements or trace metals (here called PHTE) could have been released through industrial and mining activities in OHM watersheds. Previous OHM projects have shown that human activities have released different PHTE in the watersheds of the OHMs Haut-Vicdessos and Estarreja. For example Pb was dispersed in the environment since the Antiquity and especially during the Middle Ages in the High Vicdessos area. In Estarreja, anthropogenic arsenic and mercury are found in soils and sediments (Inácio et al.,2014).

Unlike in the OHM-Haut Vicdessos, in the other OHM territories, the presence of well-preserved peat and lake records is not clear but the presence of wetlands with organic sediment accumulation is testified. This is the case of the OHM Estarreja with riverbanks and lagoons present in the complex lagoon system draining into the Ria.

Wetlands such as the ones of Estarreja and Haut Vicdessos are intrinsically linked to hydrology, which creates particular physico-chemical conditions that make them different from well-drained terrestrial or fully aquatic deepwater systems. Moreover, the ecological consequences of environmental changes in the watershed are largely dependent on changes in the hydrological regime (such as sea level rise, decreased surface area and fluctuations of ground water levels), as well as on water quality. Therefore, knowledge of background fluxes and concentrations of PHTE are of primordial importance to understand present concentrations and transfers of anthropogenic PHTE. But these transfers of PHTE are also conditioned by more extreme events such as hydrological transfers like floods or droughts that will have a huge impact on the PHTE concentrations released from the soils to the watersheds. It is well known that wetlands exposed to environmental contamination act as sink and sources of PHTE. Soil particles may act as important transporters of environmental pollutants due to the chemical-physical processes that PHTE undergo during cycles of transport and temporary deposition. Thus, the soil-sediment-water system is evolving now as it was in the past. To understand the biogeochemical cycling of the PHTE is therefore crucial for purpose of environmental risk assessment, for retro-observation analysis or for prediction of natural hazards.

But these types of wetlands can provide important archives of past environmental, climate, hydrologic and vegetation changes, For instance, studying the trace elemental composition or isotopic signatures of trace elements within a fish otolith gives insight to the water bodies fish have previously occupied. Climate changes and environmental stresses can be assessed through an understanding of changes in tree growth. Generally, trees respond to changes in environmental variables by speeding up or slowing down growth, which in turn is generally, reflected a greater or lesser thickness in growth rings. Different species, however, respond to changes in environmental variables in different ways.

Given the importance of such an important type of terrestrial environment, we are developing in the High Vicdessos area an innovative approach combining different high resolution environmental archives of past pollution in the watershed: peat and lake records. Whereas peat records are used as archives of pure atmospheric signals, lake sediments record signals from both the atmosphere and the catchment. Combining both archives allow us to decipher the origin of the metals and also the potential lag between atmospheric deposition and transfer to the watershed.

Another major advantage is that this approach gives us insights also on preindustrial and, if old enough, on prehistoric levels of pollution.

There is thus a need to develop new integrative tools and improve the existent ones to understand recent biogeochemical transfers in the critical zone of the territories of the OHM. For that purpose, annual environmental archives like fish otoliths and tree rings can be used to understand recent transfers of PHTE in the environment.