Heavy metals in Lake Yanaka, Japan, an artificial retarding basin established for remediation of heavy metals contamination in the Watarase River

Abstract

BackgroundThe Watarase Retarding Basin (WRB) involving Lake Yanaka was established for the sedimentation of the heavy metals contaminating the Watarase River in 1989. The Ashio Copper Mine poisoning (ACMP) incident that occurred from the late 19th century to the 1970s caused serious heavy metal contaminations along the Watarase River. Since stopping the mining activity in 1973, the heavy metal concentration in the water of the Watarase River significantly decreased. MethodsTo see the status of the heavy metal contaminations in Lake Yanaka, water, sediments, and Japanese river snails (marutanishi) were analyzed by using ICP-MS. ResultsThe concentration of As, Cd, Cu, Pb, and Zn in the waters exhibited a good correlation (r>0.96) with the concentration of those in the sediments. The concentration of As, Cd, and Pb in the water was under the Standards for Protecting Human Health in Japan (<10 µg/L, respectively). Whereas heavy metal concentrations in sediments of Lake Yanaka were not decreased and remained in the same range as those of the Watarase River reported in 1984. The concentrations of As and Zn in the sediments of Lake Yanaka were beyond the regulation standards (<15 mg/kg for As and <120 mg/kg for Zn). Heavy metals were highly concentrated in the shells, intestinal organs, and muscles of marutanishi snails. The bioconcentration of the heavy metals comparatively reflected the concentration in the sediments rather than those in the waters. In particular, the concentration of the heavy metals in the shells exhibited a good correlation (r>0.98) with those in the sediments. Heavy metals were more concentrated in the internal organs than shells and muscles. ConclusionSince the institution of Lake Yanaka in 1989, the water and sediments of Lake Yanaka were newly brought from the Watarase River. Although heavy metal concentrations in the sediment did not change, those in the water decreased. The function for precipitating heavy metals of WRB seems to work and Lake Yanaka works as a reservoir of over-flowed water and sediment, which include heavy metals, of WRB. This study reminded us it would take a long term to remediate the environment from heavy metal pollution.