Cadmium, manganese, nickel and lead contents in surface sediments of the lower Ulla River and its estuary (northwest Spain)

Abstract

The authors determined the total contents of cadmium, manganese, nickel and lead in 16 surface sediment samples along the last 7 km of the Ulla River and in its estuary. Prior to chemical analysis, the grain-size fractions below 500, 63 and 2 µm were separated. The metal concentrations were analysed in the fraction below 63 µm, and in selected samples from the clay fraction (< 2 µm). The soil samples were digested with a HF + HNO3 + HCl mixture and the total concentrations of Cd, Mn, Ni and Pb were analysed with the AAS method. The highest contents of metals were found in the riverine sediments, near the town of Padron, and in the estuary. The enrichment of metals in the sediments was mainly related to the proximity of the industrial sources, and to the fine particles transported and deposited in the sediments. The Ni and Mn contents found are considered very high, at some points reaching values of up to 357 mg kg‐1 and 2 110 mg kg‐1, respectively. Lead and Cd were mainly found within the range considered as natural background levels in the sediments of the Galician rias, with concentrations of 25-68 mg kg‐1 for Pb and 0.01-0.92 mg kg‐1 for Cd. The lower part of the Ulla River and its estuary present: l) moderate to high contamination levels of Mn and Ni; 2) moderate Pb pollution levels; and 3) no Cd contamination. The highest concentrations of metals were obtained in the river, near the wastewater discharges, and in the estuary, associated with fine particles.