Concentration–depth Profiles of Trace Nickel and Vanadium in Lake Mashu and the Possible Input of Anthropogenically Derived Nickel and Vanadium from the Atmosphere

The determination of trace nickel in water samples was conducted by isotope dilution microwave induced plasma mass spectrometry (ID-MIP-MS) to clarify the concentration-depth vertical profiles of nickel in Lake Mashu (maximum water depth : 212 in), Japan. This lake serves as the Baseline Station of the United Nations GEMS/Water (Global Environment Monitoring System/Water) Programme. The concentrations of nickel in the lake water samples decreased with depth from 0.3 m (surface) to 50 m (quarter of the maximum water depth) water depths. The nickel concentrations in rain- and snow-water samples retrieved at the lakefront and the wall of the caldera lake were higher than those in the surface lake water (0.3 m) samples. This phenomenon was also observed for the concentrations of vanadium in those water samples. The concentration ratio of vanadium to nickel of the snow sample is comparable in magnitude to the reported values in the particulate matter samples originating from fossil fuel combustion. Anomalously high concentrations of nickel in lake water at the thermocline depth were observed in September of 2007 and 2009. The sedimentation process was considered to be responsible for these phenomena since the density and the coefficient of viscosity increase drastically at the thermocline depth, and they would cause a decrease of the sedimentation velocity of the particulate matter containing nickel. The concentrations of nickel as well as vanadium in the water samples support the possibility of the input of anthropogenically derived particulate matter containing both nickel and vanadium, presumably originating from fossil-fuel combustion, into Lake Mashu.