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

Abstract

Purpose: 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. The study examines the contents of radionuclides and stable elements in the reservoir sediments and relates their variations with the sediment composition and sedimentary dynamics, particularly flood frequency and intensity, responsible for changes in the main supply and distribution of radionuclides in the basin. Materials and methods:. The sedimentary sequence accumulated in the The Yesa Reservoir (471 Hm3) that supplies water for 1,000,000 population and 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 (Bq kg-1) 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 (mg kg-1) were analysed by ICP-OES. Complementary analyses to characterize the sediments included: XRD in the profile, grain size distribution by laser equipment and contents of organic matter, carbonates and the residual fraction by loss on ignition. Results and discussion: Variation in radionuclide activities is associated with grain size and sediment composition. The activity levels (Bq kg–1) ranged between 20–43 for 238U, 14–40 for 226Ra, 7–56 for 210Pb, 19– 46 for Th232, 1–48 for 137Cs and 185–610 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 evidenced stronger influence of the dynamic of sediment supplies by floods in the central areas closer to the main channel (cores) than in the littoral areas (profile). Conclusions: The radionuclide distributions were consistent with the history of the reservoir infilling and with processes of transport and accumulation of sediments. Comparing with the natural radionuclides, the artificial 137Cs varied the most and showed distinctive patterns. The methods used allow identification of natural inputs into the system and its differentiation from the fluvial transport and reservoir deposition. The results provide insights into the pathways and processes involved in the mobilization of radionuclides in the environment.