No abstract

Abstract. We have quantified the caesium-137 in surface waters and sediments in both the Portuguese and the Spanish sections of the Tagus River over six years. This river has its source in Spain, and of its 1100 km length about 230 km are in Portugal, 43 km serving as a natural frontier between the two countries. In the two regions under study the water flow is completely regulated by a chain of dams. There are three nuclear power plants, along the Spanish section of the river that might be sources of radioactive contamination. The Almaraz Nuclear Power Plant (ANPP) is the radioactive release source closest to Portugal. A concentration gradient of caesium was observed in the water, with the maximum values occurring at the sampling sites near the ANPP. However, there was no dependence of the caesium concentrations measured in the sediments on the proximity to the source. Instead, the values were related to the sediment type and its particle size. Distribution coefficients were evaluated in order to compare the radionuclide concentrations in water and sediments. In the calculation we took into account the time the caesium takes to move between these two media.