Linking contaminant distribution to hydrodynamic patterns in an urban estuary: The Douro estuary test case

An increase in anthropogenic activities in coastal regions can put at risk their flora and fauna and their ecosystem services. Therefore, it is important to evaluate possible impacts. In particular, we need to understand the links between contaminants concentrations and the hydrodynamic patterns of these highly productive regions to anticipate the effects of contaminants in the environment. Towards that aim there is the need to carry out regular campaigns to monitor the evolution of the coastal systems. In this work we analyse in-situ measurements of physico-chemical parameters, and look for possible relations between observed contaminants patterns and estuarine hydrodynamics. Data collected in the Douro estuary, one of the main estuarine regions of the Iberian western coast, revealed the presence of 5 hazardous and noxious substances (HNS), 14 polycyclic aromatic hydrocarbons (PAHs) and 6 trace metals in water and sediment samples. Water temperature and salinity analysis revealed a strong variability, which can affect the water solubility properties and the organisms' tolerance to certain toxins. A relationship between the salinity and the HNS and PAHs concentrations was found, caused by the existence of a salt-wedge that triggers the salting-out effect. Sinker contaminants (PAHs and trace metals) can be re-suspended both during low and high flow conditions associated with the salt-wedge and with strong river flows. Floater contaminants (HNS) are completely depended on the tide, which has the capacity to distribute them through the entire estuary, during low river flow regimes. However, strong river flows, with associated river plumes, can distribute both sinker and floater contaminants to the coastal region trapping them over the inner-shelf. The results clearly show that hydrodynamic patterns are a major driver for contaminants dispersion and pathways in coastal areas, inducing harmful effects to the flora and fauna and, consequently, to the ecosystem services of these regions.