Unveiling the potential mobility and geochemical speciation of geogenic arsenic in the deep subsurface soil of the Tokyo metropolitan area

Extensive excavations for urban and subterranean construction often lead to soil and groundwater contamination with geogenic arsenic (As), emphasizing the urgent need for effective management strategies, particularly considering the global excavation of millions of tons of soil annually. This study investigated the chemical speciation and solubility of geogenic As in soil samples collected at 25-cm intervals from boreholes extending up to 16 m deep within the alluvial Yurakucho Formation and the terrestrial Kanto Loam Formation in the Tokyo metropolitan area. Soils from the Yurakucho Formation exhibited significantly higher total As concentrations (10.5 ± 3.26 mg kg^-1) compared to those from the Kanto Loam Formation (5.58 ± 1.88 mg kg^-1), with notably elevated levels of water-soluble As throughout the profile. The XANES analysis revealed that As-bearing sulfide species, including As₂S₃ and FeAsS types, were the predominant forms in the Yurakucho Formation, while As(V) species were more prevalent in the Kanto Loam Formation. Micro-XAFS combined with micro-XRF analysis identified framboidal pyrite, characterized by micron-sized grains (∼10 µm), as the primary sink for As sequestration in the Yurakucho Formation, where As occurs mainly in sulfide-associated forms. These findings highlight the importance of characterizing geogenic As speciation to assess its leaching potential and associated environmental risks posed by As in excavated soils.