Radionuclides and stable elements in the sediments of the Yesa Reservoir, Central Spanish Pyrenees

Abstract

The sediments accumulated in the Yesa Reservoir (Central Spanish Pyrenees) have greatly decreased its water storage capacity and are a major threat to the sustainability of water resources in the region. This study examines the contents of radionuclides and stable elements in the reservoir sediments and relates their variations with the sediment composition and local sedimentary dynamics, particularly flood frequency and intensity, which are responsible for changes in the main supply and distribution of radionuclides in the basin. The sedimentary sequence accumulated in the Yesa Reservoir (471 Hm3), which supplies water to ca. 1,000,000 people and for irrigation, was examined in two 4-m long sediment cores (Y1, Y2) and one profile (Y3) retrieved at its central part. In the sediments, radionuclide activities of 238U, 226Ra, 232Th, 40K, 210Pb and 137Cs were measured using a hyperpure Ge coaxial detector. The stable elements Mg, Ca, Sr, Ba, Cr, Cu, Mn, Fe, Al, Zn, Ni, Co, Pb, Li, K and Na were analysed by ICP-OES. Complementary analyses to characterize the sediments included: XRD in the profile, grain size distribution by laser equipment and the contents of organic matter, carbonates and the residual fraction by loss on ignition. The variation in radionuclide activities is associated with grain size and sediment composition. The activity levels (becquerels per kilogram) ranged between 20 and 43 for 238U, 14 and 40 for 226Ra, 7 and 56 for 210Pb, 19 and 46 for Th232, 1 and 48 for 137Cs and 185 and 610 for 40K. Enriched activity levels are associated with clayey and silty layers, and depleted levels with sandy layers. The levels of radionuclides and trace elements were significantly lower in the cores than in the profile because of its higher silicate content and the influence of inflow of spring mineral-rich waters. The correlations among radionuclides, sediment components and stable elements provided evidence of a stronger influence of the dynamics of sediment supply by floods in the central areas closer to the main channel (cores) than in the littoral areas (profile). The radionuclide distributions were consistent with the history of the reservoir infilling and with the processes of transport and accumulation of sediments. Compared to the natural radionuclides, the artificial radionuclide 137Cs varied the most and showed distinctive patterns. The methods used allowed the identification of natural inputs into the system and its differentiation from fluvial transport and reservoir deposition. The results provide insights into the pathways and processes involved in the mobilization of radionuclides in the environment.