Heavy metals speciation analysis was carried out on sediment samples accumulated within soakaways in an old stormwater infiltration facility in Tokyo, Japan and on a soil core sample collected near the facility. Heavy metals content in soakaways sediments were much elevated compared to nearby surface soil with the content for Zn, Pb and Cd reaching about 5 to 10 times the content in surface soil. Speciation results revealed that significant amount of the accumulated heavy metals were present in potential mobile fractions, posing threat of release to underlying soil with changing environmental conditions. Detail analyses of soil characteristics indicated significant heterogeneity with depth, especially between the surface soil and underlying soil at site. Decrease in potential adsorption sites with depth was observed in case of underlying soil. Reduced adsorption capacity for heavy metals was evidenced for underlying soil when compared with surface soil. Furthermore, less capability of the soil organic matter to bind heavy metals was evidenced through speciation analyses, which raises concern over the long-term pollution retention potential of the underlying soil receiving infiltrated runoff.