The chemical speciation of major inorganic elements and arsenic in the 1 m length sediment core obtained Lake Onuma, Hokkaido were examined by means of chemical extraction technique. The obtained core recorded the environmental history from the eruption of Hokkaido Komagatake volcano at 1929 to present. The arsenic contents in the sediment were low at 5-15 mg/kg. However, those at top 40cm layer of the sediment exceeded the background level of arsenic in the catchment area. This suggests that the arsenic is accumulated in the surface sediments of Lake Onuma. The arsenic in the sediment was mainly associated with iron/manganese oxide and iron sulfide. The contributions with the iron sulfide increased with depth, while those with iron/ manganese oxide decreased with depth. The systematic trends with depth indicate that the sedimented arsenic must be mobile via redox reactions in the sediments. The arsenic concentration in the lake water (0.4 μg/L) was almost twice higher than those from the inflow rivers. This may suggest that very small amount of arsenic in the surface sediments has been continuously released to the lake water.
