Summary:
The fine particulate deposition in the urban environment (road dust and sedimentary materials retained in the sewers adjacent to roads, following the flow of rainfall) reflects a wide range of anthropogenic activities and constitutes a useful tool for assessing the level of pollution as well as the distribution of potential pollutants they contain, such as increased concentration of potentially toxic elements (PTEs) and polycyclic aromatic hydrocarbons (PAHs). The urban drainage system plays a significant role in the retention and/or release of PTEs into water bodies. Therefore, the study of contemporary loose sediments within the urban environment of the capital of Greece, Athens, in the context of this dissertation, provides significant information for the first time regarding the concentration levels, mobility of PTEs, and their potential contribution to urban pollution. The research contribution may also be useful for assessing the long-term fate of pollutants in the urban environment.
Samples of urban sediment were collected and analyzed from different elevations within the Athens basin, based on the hydrographic network of the area. Initially, reconnaissance sampling was conducted at specific locations in the wider Athens area. Sites around the rainwater retention lake and around the area of Liosia and Acharnes in the northwest margin of the basin were considered representative of non-polluted sedimentary material and were used for comparison with sites in the Eleonas area, which is traversed by the historical bed of the Kifisos River and characterized by high human activity and heavy vehicular traffic on a daily basis. In the initial phase of the research, a total of 16 samples were collected and analyzed for physicochemical parameters such as magnetic susceptibility (χ) and frequency-dependent magnetic susceptibility (%xfd), total organic carbon (%TOC), total trace element concentrations, and their extractability percentage. Finally, the seasonal variation of extractable heavy metal concentrations was examined.
In the next phase of the research in Athens, 26 observation sites were designated from where periodic surface sediment samples were collected within the Athens basin based on the hydrographic network of the area. The samples from the first sampling period were analyzed for 33 elements after dissolution with aqua regia. The organic carbon content, pH, and grain size distribution were also determined, and measurements of magnetic susceptibility and mineralogical analysis by X-ray diffraction on powder samples were carried out to identify possible factors explaining the variability of element concentrations. The concentrations of PTEs in the analyzed sediments reached maximum values of 18 mg/kg for As, 2 mg/kg for Cd, 14 mg/kg for Co, 193 mg/kg for Cr, 640 mg/kg for Cu, 25600 mg/kg for Fe, 112 mg/kg for Ni, 3092 mg/kg for Pb, and 1469 mg/kg for Zn. The median values of the studied elements were estimated at 13 mg/kg for As, 1 mg/kg for Cd, 8 mg/kg Co, 98 mg/kg Cr, 215 mg/kg Cu, 17200 mg/kg Fe, 70 mg/kg Ni, 267 mg/kg Pb, and 598 mg/kg Zn, respectively. Cluster analysis of the results identified three element groups based on a similarity criterion above 43.59%. The first group contains elements of geogenic origin, including Co, Fe, Mn, and Ni. The parameters %TOC, χ, Cu, and Cr are grouped into a second cluster exhibiting a similarity level above 65%, while a third cluster groups the elements Pb, Zn, and Cd and is interpreted as anthropogenic. Finally, there is a separate group that includes pH, %CLAY, and As. Sequential extraction results correspond to this grouping as the anthropogenic elements tend to be released in the first two extraction steps. Cadmium and Zn are the only metals showing significant association with the exchangeable fraction, reaching 40%, suggesting that they are the most susceptible metals to mobilization during runoff. Moreover, Cu and Pb are largely associated with the oxidisable fraction. Regarding the seasonal variation in reactive elemental concentrations, significant difference (ANOVA, p < 0.05) during the following months was observed only for Cd and especially, for the period of December.
The doctoral research also examined the impact of sudden flooding events on the composition of runoff sediment using the catastrophic flood of Mandra, Attica, in November 2017 as a case study. Sudden floods are one of the most dangerous natural phenomena, with a wide range of significant short-term and long-term effects on human and environmental health. On November 15, 2017, a high-intensity storm caused a catastrophic flash flood in the town of Mandra, a western suburb of the metropolitan area of Athens, Greece. In this study, concentrations of trace elements in residual sediments and related soils were measured after dissolution with aqua regia, and the distribution of pollutants into geochemical fractions was evaluated using a sequential extraction protocol. Geochemical data were combined with measurements of physicochemical parameters and mineralogy to identify possible factors explaining the variability of trace element concentrations, while acidic leaching with dilute nitric acid was used to monitor changes in the chemically active fraction of trace elements during the year following the flood event. Concentrations of PTEs in the surface flood sediment reached values of 11mg/kg As, 1 mg/kg Cd, 24 mg/kg Co, 183 mg/kg Cr, 599mg/kg Cu, 1080 mg/kg Mn, 195 mg/kg Ni, 122 mg/kg Pb, and 945 mg/kg Zn. Multivariate statistical analysis of the results grouped the elements according to their natural or anthropogenic origin. Geogenic trace elements (As, Co, Cr, Mn, Ni) dominate in the material deposited by the floods. The cluster of anthropogenic elements (Cd, Cu, Pb, Zn) shows a significant correlation with %TOC and χ, while significant seasonal variation was observed for %TOC, Cd, and Mn content in sediments. Cadmium is the only metal showing significant association with the exchangeable fraction, reaching 40%, suggesting that it is the most susceptible metal to mobilization during runoff. A significant seasonal variation for %C, Cd and Mn has been observed during the months following the flash flood within an one year monitoring period. The results allow for a better understanding of the distribution of elements in surface cover during and after catastrophic events in urban areas and provide useful information regarding the long-term exposure of residents. Additionally, in the wider area of Mandra, additional sampling sites are cited where heaps of transported materials were deposited after the flood, and seasonal monitoring was conducted for χ, %xfd, %TOC, XRD, and partial dissolution with dilute HNO3.
Regarding PAHs, the present study examined their spatial and seasonal patterns, sources, and relative toxicity potential for human health at four sampling sites in Athens over the course of one year and at four sampling sites in the industrial area of the town of Mandra during one year after the occurrence of a sudden flood. Total PAH concentrations ranged from 451 to 4940 ng/g in Athens samples and from 509 to 6144 ng/g in Mandra samples, respectively. PAHs were dominated by relatively high molecular weight compounds, indicating that high-temperature combustion processes were the predominant source. Diagnostic ratios indicated that the primary source of PAHs in the samples was pyrogenic processes, such as oil combustion, gasoline vehicle exhaust, coal, and wood burning. Seasonal variation results showed that PAH concentrations did not follow a specific pattern, except for one sampling point in the industrial area of Mandra, where PAH concentrations remained consistent for two subsequent sampling periods, indicating the presence of a single point source of pollution. The potential toxicity of PAHs, calculated using the Toxicity Equivalent Quantity of benzo(a)pyrene (TEQBaP) index, had an estimated mean value of 71.1 ng/g in Athens samples and 135 ng/g in Mandra samples.
Finally, a comparative study of results with international data for similar materials was conducted, and measures are proposed for further monitoring and a broader strategy, as climate change and extreme weather events will intensify and increasingly affect the urban environment with both short-term and long-term consequences.
