The cause of persistent chromium contamination derived from reduced Chromite Ore Processing Residue in urban street inlets in Komatsugawa, Edogawa-ku, Tokyo, Japan

Chromium (Cr) contamination persists in street inlets within a former chromite ore processing residue (COPR)-affected area in Tokyo, Japan. To clarify contamination sources and dispersion mechanisms, we analyzed Cr speciation and trace element compositions in street inlet water, sediments, and reddish-brown suspended solids, and interpreted the data using enrichment factors and hierarchical cluster analysis. The results indicate that Cr contamination at the site originates from two distinct sources: COPR and reduced COPR (rCOPR). Near street inlet 1, exceptionally high Cr(VI) concentrations in inlet water, together with the absence of pronounced Fe enrichment in sediments, suggest the presence of unreduced slag that continues to release Cr(VI). In contrast, street inlet 4 showed the highest total Cr in sediments and received suspended solids via an inflow pipe. In addition to Cr, these suspended solid contained markedly elevated iron (Fe) and arsenic (As), and their elemental associations were consistent with a COPR signature. The extremely high Fe concentration likely reflects Fe derived from the reducing agent used during reductive treatment, implying that the suspended solid represents a secondary products strongly influenced by rCOPR landfilled in Ojima Komatsugawa Park. The co-occurrence of high Fe and As appears to be critical for suspended solid formation, potentially through the adsorption of As and Cr oxyanions onto Fe (hydr)oxides and inhibition of Fe nucleation by As, which may promote the generation of highly mobile particles. Cr(III) clustered with As across all street inlets, suggesting coupled partitioning to the solid phase. Overall, two spreading pathways maintain site-wide Cr impacts: (i) Cr(VI) leaching from unreduced COPR and (ii) generation and transport of Fe–As-bearing suspended solids derived from rCOPR. Since suspended solids may be discharged to rivers during rainfall via the combined sewer system and similar rCOPR landfills exist at other nearby parks, region-wide monitoring and management are warranted.