Analysis of the historical trend of heavy metal-pollution recorded in the pond sediment core at Oki Islands on the Japan Sea coast

Abstract

The historical trend of heavy-metal pollution recorded in core sediment collected from a small pond in Dougo of Oki Islands, Shimane Prefecture, was deciphered. The pollutant carried through the atmosphere seems to have accumulated in the pond sediment, because Oki Islands are separated from the source of industrial pollution. A core sample of 107 cm length was obtained, and sedimentation age of the deepest part was estimated to be 1880’s by using a ²¹⁰Pb method. The vertical distribution of the ¹³⁷Cs concentration in this core agreed well with the historical trend of its global fallout. The contamination of mercury for the sediment started in the 1920’s, and the anthropogenic flux at the water-sediment interface was 10∼12 ng cm⁻² yr⁻¹ in the 1960’s. The flux has become almost constant by the present. On the other hand, the contamination of lead started in the 1930’s, and the flux in the water-sediment interface has increased by the present. The anthropogenic flux of lead in the present sediment surface reaches 5000 ng cm⁻² yr⁻¹-1. The abnormal load of mercury for the sediment was recognized in the 1900’s. Further, the spiking load was also found for lead in the 1920’s. The cause of these anomalies in contaminations was uncertain if it was by human activity or by some natural phenomenon. The anthropogenic inventories of mercury and lead in this pond sediment were estimated to be 0.71 μg cm⁻² and 190 μg cm⁻², respectively. For the past 60 years, the average values of the anthropogenic fluxes from the atmosphere of mercury and lead in Dougo Island were also estimated to be 4.3 and 1100 ng cm⁻²yr⁻¹, respectively. From the result, concerning the environment of Oki Islands, the possibility of receiving the effect from the continent of China and the Korean Peninsula strongly than from the Kei-Hanshin industrial area in Western Japan was indicated.