Factors affecting arsenic content of unconsolidated sediments and its mobilization in the Ishikari Plain, Hokkaido, Japan

Arsenic (As) contamination of groundwater is often caused by As leaching from geological formations. This study focused on factors affecting As content in unconsolidated sediments by using three sediment cores in the Ishikari plain, Hokkaido, Japan. The geochemical properties of the sediments were characterized by the chemical and mineral compositions as well as leaching experiments and sequential extractions of As. The results showed that higher As content of 4–9 mg/kg was observed in peat layers with organic matter content > 10 wt%, and that higher As contents deeper than 8 m below the surface were also observed. The deeper sediments were likely to be formed in brackish water conditions due to lower weight ratios of organic carbon content to sulfur content (C/S < 10). The results of the leaching experiments showed that As was mobilized at slightly alkaline region. These indicate that the distribution of As depended on the organic content in the sediments and sedimentation condition, and that the mobility of As in the sediment was affected by pH.