Trace element concentrations in road deposited sediments of Athens, Greece: A comparison with baseline soil data

Urban areas are typical examples of disturbed natural environments where human development has significantly altered the geochemical background of trace elements in surface soil and sediment. Road dusts and gully sediments are reflective of a wide range of anthropogenic activities in cities and are a useful resource for evaluating the level and distribution of trace metal contaminants in the surface environment. The evaluation of contamination in these sinks provides useful information of how the drainage system of the cities contributes to urban pollution.

A total of 26 urban road deposited sediment samples were collected from different altitudes within the Athens basin based on the hydrographic network of the area. The samples were analyzed for 33 elements following an aqua regia dissolution. Sample organic carbon content, pH and grain size distribution have been determined and magnetic susceptibility measurements and mineralogical analysis by powder X-ray diffraction were also performed in order to identify possible factors explaining the variability of elemental concentrations. Also, sixteen samples were analyzed for polycyclic aromatic hydrocarbons (PAHs) in order to detect their sources in the Athens urban environment.

Aqua regia concentrations in the analyzed sediments reached maximum values of 18 mg/kg for As, 2 mg/kg for Cd, 14 mg/kg Co, 193 mg/kg Cr, 640 mg/kg Cu, 25600 mg/kg Fe, 112 mg/kg Ni, 3092 mg/kg Pb and 1469 mg/kg Zn. The median values of the studied elements were estimated to be 13 mg/kg for As, 1 mg/kg for Cd, 8 mg/kg Co, 98 mg/kg Cr, 215 mg/kg Cu, 17154 mg/kg Fe, 70 mg/kg Ni, 267 mg/kg Pb and 598 mg/kg Zn, respectively. With the exception of Co and As, both maximum and median values were found to be much higher than those in Athens soils from a previous study. Cluster analysis on the results identified two major groups of elements based on an over 43.59% criterion of similarity. The first cluster contains elements of geogenic origin including Co, Fe, Mn and Ni. The parameters of % organic carbon, magnetic susceptibility, Cu and Cr are grouped together in a second cluster showing a similarity level over 65% while a third cluster groups together Pb, Zn and Cd and is interpreted as anthropogenic.

In a previous systematic baseline study of Athens, it was found that the major factor controlling variability of the chemical composition of surface soil was the bedrock chemistry, resulting in a significant enrichment in concentrations of Ni, Cr, Co and possibly As. Anthropogenic influences were also significant in soil, controlling a spectrum of elements that are typical of human activities, i.e. Pb, Zn, Cu, Cd, Sb, and Sn. The clustering of elements in the present study indicates that although the geogenic origin of some elements is retained in road sediments, a greater number of elements indicate anthropogenic influence in their distribution. Briefly, it was documented that road deposited sediments reflect the characteristics of the anthropogenic activities taking place, and that traffic- related activities are the primary sources of contaminants.