The fate and contamination of trace metals in soils exposed to a railroad used by Diesel Multiple Units: Assessment of the railroad contribution with multi-tool source tracking.

Soil samples from cut slopes from lightly loaded railway lines used by Diesel Multiple Units for 5 years in Gdansk (Poland) were collected and analyzed for trace metals (TMs): Zn, Pb, Cd, Ni, Cr, Cu, and Fe. The main aim was to assess soil enrichment, contamination status, and distribution of TMs relative to the distance from the railway track. Extensive source tracking analyses were performed using cluster analysis (CA) and the Pb isotope ratios approach (206Pb, 207Pb, and 208Pb). Soil samples were affected by Cr, Cu, Pb, and Zn (max values in mg/kg d.w.: 31.1, 145, 80.5, and 115, respectively). The Enrichment Factor showed moderate (Cr, Zn, Pb) to very severe (Cu) enrichment. CA allowed TMs to be divided into two general groups: a) containing Zn, Pb, Cd with slight interaction with Cu; and b) containing Fe and Ni with slight interaction with Cr. Correlation analyses indicated Cr as an outlying TM delivered from a separate source associated with the specificity of the construction of railroad 248, where alloys containing Cr were used to counteract increasing wear-and-tear of the rails. Pb isotopic ratios in the ranges of 1.16-1.20 (206Pb/207Pb) and 2.05-2.10 (208Pb/206Pb) corresponded to anthropogenic supplementation (coal combustion, road vehicles, and railroad transport) of Pb and Pb-correlated TMs (Zn, Cd, and partly Cu). Despite the research focus on the impact of the railroad contribution, a comparison with other forms of transport indicated that road transport appeared to have a higher contributing factor to TM pollution at the investigated site. This general conclusion again emphasizes the lower environmental footprint exhibited by railway transport in comparison to road transport.