Trace metal geochemical and Zn stable isotope data as tracers for anthropogenic metal contributions in a sediment core from Lake Biwa, Japan

Abstract

The catchment of Lake Biwa, the largest freshwater lake in Japan, has experienced rapid urbanization and industrialization in the second half of the 20th century; yet little is known about the historical trend of metal pollution. We analyzed the elemental concentrations and zinc stable isotope ratios (δ66Zn) in a sediment core from the Northern basin of the lake. The goals were 1) to reconstruct the historical trend of trace metals in the lake and 2) to understand whether the range of Zn concentrations was derived from natural or anthropogenic sources. The first increase in Cu and Zn concentrations from the late 1910s until approximately 1940 could be due to human-induced higher contributions from the surrounding mafic igneous rocks. A second increase in the Zn concentrations accompanied by a slight decrease in the δ66Zn values from the late 1950s coincided with the period of the industrialization and urbanization. The δ66Zn values fell in a binary mixing process between a lithogenic (∼+0.28‰) and an anthropogenic endmember (∼+0.14‰). Although industrial effluents were strictly regulated by law from the late 1960s, constant Zn concentrations throughout the 1980s indicated contributions from domestic wastewaters and diffusive sources such as runoff from paddy fields, roads, and industrial areas. Zinc, Ni, and Co were still enriched in recent surface sediment compared to the pre-industrial times. The metal fractions obtained by the BCR sequential extraction procedure showed that Pb was mainly hosted in the reducible fraction, and Cu and Zn were mainly hosted in the residual fraction. The increasing trace metal concentrations in the acid-labile and reducible fractions towards the surface indicate these fractions were the primary host for anthropogenic trace metals. We conclude it is unlikely that the natural condition of the lake sediment will be recovered owing to the continuing urbanization around Lake Biwa.