Steel slags are side products of iron and steel industry and are suitable for a variety of applications. In the present paper, the assessment of the hazard associated with steel slags used as harbor piers backfilling material was performed. The analyses were conducted on 64 samples (soil and groundwater) collected in 17 sites located in one of the piers of Taranto city (South Italy), a highly industrialized area, where steel slags have been used in the past as backfill material. The obtained data were compared to the national threshold levels and International Indexes (I_geo) for assessing anthropogenic impacts. The first comparative analysis revealed that only vanadium in soil and aluminum in groundwater, associated with high pH values in both matrixes (soil and groundwater) resulted of high concern. Nevertheless, I_geo values and the coupling of comparative analysis with multivariate techniques Principal component analysis and Hierarchical Cluster Analyses revealed that also Sb, Pb, Cr and Zn concentration have an extremely high hazard degree. All analyses confirm the impact of steel slags on contaminant leaching processes due to the strong alkaline nature of their leachates and the presence of strong relationship between metal mobility and other physical-chemical parameters related to the peculiar environmental context. These findings suggest that the use of such materials, albeit economically recommended, should be adopted under severe monitoring of all parameters that could promote the mobility of metals and other contaminants.