Distribution of Sb and Sn in soil near railway tracks
Tin and antimony are naturally present in the soil in certain amounts. According to Geochemical Atlas of Europe (Salminen et al. 2005), the median for Sb and Sn in European topsoil is 0.6 mg/kg and 3 mg/kg, respectively. In the north-eastern part of Croatia, soils contain from 0.83 to 1.46 mg/kg of Sb and from 4.00 to 7.00 mg/kg of Sn (Salminen et al. 2005). The obtained data indicate not only a great variability of Sb and Sn contents, but also an exceptional enrichment of the soil with Sb and Sn at certain sites. Indeed, the Sb and Sn contents in the studied soils range from the minimum values close to the medians reported for European soils to the maximum values of 52.0 mg/kg for Sb and 97.6 mg/kg for Sn, with RSDs of 139% and 117%, respectively. At the sites where the highest values were recorded, the mass fractions of Sb and Sn in the soil are 87 and 33 times higher, respectively, then the median for European soils; 36 and 14 times higher, respectively, then the highest value reported for north-eastern part of Croatia; and 31 and 24 times higher, respectively, then the background value for the studied area; clearly indicating the extreme contamination of the studied soils with these elements.
The literature clearly confirms rail transport as a source of soil pollution with numerous contaminants (Barcan et al. 1998; Brooks 2004; Bukowiecki et al. 2007; Burkhardt et al. 2008; Gehrig et al. 2007; Ikarashi et al. 2005; Jarvis et al. 2006; Kohler et al. 2000; Liu et al. 2009; Lorenzo et al. 2006; Malawska and Wiłkomirski 1999; Schweinsberg et al. 1999; Wierzbicka et al. 2015; Wiłkomirski et al. 2012; Zhang et al. 2012). However, data for Sb and Sn in soils near railways are extremely sparse and, to our knowledge, include only three studies conducted in Poland (Table 2). Dzierżanowski and Gawroński (2012) investigated Sn and Sb concentrations between railway stations in the suburbs of Warsaw, in the vicinity of two parallel railway tracks near stony railway embankments. Measurements were performed in situ with XRF instruments, and due to the high detection limits (less than 18 mg/kg and 218 mg/kg for Sn and Sb, respectively), Sn and Sb were not detected. Wiłkomirski et al. (2013) carried out investigations in the area of four railway stations in north-eastern Poland (Białystok, Sokółka, Hajnówka and Kuźnica) in different functional parts of the stations (the siding, the cleaning bay for rolling stock, the classification yard, the receiving yard and the hold yard). Sb was not determined and the maximum value of Sn was 23 mg/kg and was measured in the platform area. Staszewski et al. (2015) conducted a study in northern Poland, at the Iława Główna railway junction. Soil samples were collected between the rails and outside the rails to the end of the railway sleepers. For Sn, concentrations were 2–5 times higher compared to control samples, with the highest value of 33 mg/kg in the railway platform area, while Sb was not considered in the investigation.
A comparison of data from this study and the cited literature shows the highest recorded levels of Sn in soils near railways in Croatia, while for Sb the lack of data precludes further comparison.
Table 2
Published research on Sn and Sb along the railway
Literature
source
|
Country
|
Locality
|
Functional part
of railway
|
Distance from
the tracks
|
Year of soil
samples
collection
|
Sn
mg/kg
|
Sb
mg/kg
|
Dzierżanowski
and Gawroński (2012)
|
Poland
|
suburbs
of Warsaw
|
between stations
|
0–1 m from the
track embankments
|
2011
|
below
detection
limit
|
below
detection
limit
|
Wiłkomirski
et al. (2013)
|
Poland
|
Białystok, Sokółka,
Hajnówka and Kuźnica
|
railway stations:
in different
functional parts
|
not specified
|
2012
|
3–23
|
not
investigated
|
Staszewski
et al. (2015)
|
Poland
|
Iława Główna
|
railway
junction
|
< 2 m
|
2008
|
4–33
|
not
investigated
|
Properties that affect the content of Sn and Sb in the soil
The levels of HMs, including Sb and Sn, in soil are influenced by both natural and anthropogenic factors; whereby natural ones include physical, chemical, biological, and climatological factors; while anthropogenic refer to various types of pollution and degradation.
Influence of soil properties
In order to identify the role of different natural factors (grain size distribution, texture, humus content, pH) affecting the content of Sb and Sn in studied soils, their relationship was examined by the principal component analysis (PCA). The eingenvalues of the first four principal components (PCs) were larger than 1, indicating their significance and explaining 75.4% of the total variability among 11 variables. Thereby, the first component (PC1) contributed 34.5%, while the second, the third and the fourth corresponded to 17.6%, 13.2% and 10.3%, respectively, of the total variance of the data set. Results of PCA are presented on the PCA loading plots (Fig. 2a and b), illustrating the orientation of the variables with respect to principal components. On PC1 vs PC2 plot (Fig. 2a), the highest negative PC1 loadings exhibited fine and coarse silt (-0.84 and 0.80, respectively), while coarse sand content and pHKCl displayed positive PC1 loadings (0.93 and 0.81, respectively). The humus, Sb and Sn content exhibited the greatest negative effect on PC2 (from − 0.82 to − 0.71), whereas none of the studied parameters showed positive values (> 0.5) with respect to this component. On PC3 vs PC4 plot (Fig. 2b), the highest negative PC3 loadings exhibited texture (-0.66), whereas fine sand displayed positive PC3 loading (0.51). The pH and ΔpH, exhibited the greatest positive effect on PC4 (0.70 and 0.71, respectively), whereas none of the studied parameters showed negative ( < − 0.5) values with respect to this component.
According to Fig. 2a, the variation of the data is mainly influenced by the soil texture, especially by the ratio of finer (coarse silt, fine silt and clay) and larger fractions (coarse sand) in the soil itself. While the influence of the finest fraction of the soil as a metal carrier is well known (Rieuwerts et al. 1998), the results show the significant influence of the largest fraction on the distribution of Sb and Sn. To some extent, this is to be expected, as the soil texture near railway embankments is not entirely of natural origin, i.e. formed by decomposition of the parent substrate. Indeed, on railway embankments there are gravel stones and with increasing distance from the tracks such material is replaced by natural soil. The next factor affecting the content of Sb and Sn in the studied soils is the humus content (Fig. 2a). This is consistent with previous findings suggesting higher adsorption of metals in soils with higher organic matter content and cation exchange capacity (Rieuwerts et al. 1998; Young 2013). As can be seen in Fig. 2b, the following factors influencing the variance of the data are the texture (class) of the soil and the soil pH.
Influence of distance from railway
One of the factors affecting the mass fractions of Sn and Sb in the topsoil is the distance of sampling sites from the railway as a linear source of pollution. Different authors have studied gradients at different distances from the tracks, e.g. Baltrenas et al. (2009) at 1, 2, 3, 5, 10, 15, 25, 50 and 75 m; Bobryk (2015) and Bobryk et al. (2016) at 0, 25, 50, 100 and 250 m; Chen et al. (2014a, b) at 1, 5, 15, 20 and 50 m; Liu et al. (2009) at 2, 10, 25, 50, 100 and 150 m; Ma et al. (2009) at 0, 10, 20, 30, 50, 100, 200, 300, 500 m from the track edge; Malawska and Wiłkomirski (2000) at 0, 15, 30, 50 and 100 m; Mazur et al. (2013) at 2, 10, 20 and 30 m from the railway line; Meng et al. (2018) at 5, 10, 25, 50, 100 and 150 m; Radziemska et al. (2016) at 1, 10, 20 and 30 m from the railway line; Samarska and Zelenko (2018b) at 5, 10, 15, 20, 30, 50, 100 m; Šeda et al. (2017) at 1.5, 3, 10 and 25 m; Stojic et al. (2017) at 0.03–4.19 km from the railway; Vaiškūnaitė and Jasiūnienė (2020) at 1, 5, 10, 15 and 25 m from the sleepers; Zhang et al. (2012) at 2, 5, 10, 20, 30, 50, 60, 70, 80, 100, 150 and 200 m from the railway; Zhang et al. (2013) at 2, 5, 10, 20, 30, 50, 60, 70, 80, 100, and 150 m from the embankment bottom. In the majority of the reported studies and for most of the investigated chemical elements, the general conclusion is that their mass fractions decrease with increasing distance from the railway tracks. The results of this study are consistent with such observations. Indeed, the measured Sb and Sn soil contents showed a decrease with increasing distance from the rails, although this decrease was not linear (Fig. 3a and b). By dividing the samples by distance into the following categories, 1 m, 1–2 m, 2–3 m, 3–5 m and 5–10 m (Fig. 4a and b), a statistically significant difference (p < 0.05) between the above groups was found only for Sb. Moreover, for both elements, an initial decrease in content was followed by an increase (at a distance of 2 to 3 m) and a further decrease.
A similar observation was made by other authors as well. According to Ma et al. (2009), the highest contents for Ni, Cr and Cu were measured at about 10–30 m distance. Mazur et al. (2013) recorded the highest Ni contents at a distance of 20 m, and for Cr and Co at a distance of 10 or 20 m, depending on the study site. Meng et al. (2018) found that Cd showed an increasing trend from 5 to 50 m. A possible explanation for this distribution pattern is related to the exhaust gasses, which can migrate with the airflow and deposit off-track (Ma et al. 2009). Based on the obtained results, different authors define safety distances for agricultural production. Šeda et al. (2017) stated that the safety distance is about 10 m, Vaiškūnaitė and Jasiūnienė (2020) defined the safety distance to the railway stations as 15 m, while Baltrenas et al. (2009) stated that it should be 15 m or more. In agreement with the study by Liu et al. (2009), in China, areas within a 15 m distance along railways are regulated as safety zones and the cultivation of crops is prohibited in this area. However, the authors emphasize that the influence of Mn, Zn, and Cd can extend beyond 15 meters, so crops at these distances can still carry a potential risk of pollution from these HMs. Meng et al. (2018) stated that in China, crops and cropland should not be planted within 30 m of the railway line, and they suggested an even greater distance because the influence of HMs on soils can spread even further.
Influence of site functionality
Functional parts of railway infrastructure have a significant influence on soil Sn and Sb content according to the results of this study. As shown in Fig. 5, for both elements, the lowest mean mass fractions were measured along lines between stations, higher ones at stops, even higher ones at stations, and the highest ones at large junction stations. Indeed, the most contaminated sites are located near large junction stations such as Varaždin, Zagreb Main Station and Zaprešić.
However, unexpectedly high levels of Sn (61.4 mg/kg) and Sb (43.2 mg/kg) were found in the soil at Žeinci stop. A possible explanation is that a large amount of cargo or another substance (e.g. fuel) containing these two elements was spilled at this site in the past. It is also interesting to note that the maximum concentration of Sb (52.0 mg/kg) was found at the Konjščina station, which could be the result of mining activities in the past, from 1885 to 1962. Lignite (coal) was mined near Konjščina, which is known to contain antimony; and the coal was loaded and unloaded near the soil sampling point.
Statistically, a significant difference (p < 0.05) was found between the sites in terms of functionality for Sb, while this was not the case for Sn.
The dependence of HM concentrations and functional parts of railway infrastructure was systematically studied only by a group of researchers from Poland. Malawska and Wiłkomirski (2000) in their study distinguished the following functional parts of junctions: loading ramp, platform area, vehicle cleaning facility, the siding, and parts along the tracks outside the stations. They found the highest contamination with HMs at the loading ramp and the lowest along the tracks. The same authors, but in a different area, found the highest contamination at the cleaning bay and in the siding area (Malawska and Wiłkomirski, 2001). In the work of Wiłkomirski et al. (2011), the highest HM metal levels were found at the siding and in the platform area. Mętrak et al. (2015) found no typical pollution profiles for the differently used areas of railway, while Staszewski et al. (2015) established the highest soil contamination in the platform area.
Influence of site topography
Although scarcely studied, topography has a very important influence on the distribution of pollution in the environment. Liu et al. (2009) pointed out that the topography profile influences the metal level and distribution pattern along the railway. The elevated concentrations of HMs varied from 5 m to 100 m away from the tracks depending on the topography and elements.
In this study, the highest Sb and Sn soil contents were found in areas located at the same level as the railway line, while they were lower in locations either above or below the railway level (Fig. 6a and b), confirming the influence of topography on their spatial distribution. Moreover, a statistically significant difference (p < 0.05) between sites in terms of topography was again found only for Sb. Chen et al. (2018) made the same observations and attributed them to leaching and erosion. As one of the possible solutions to prevent the spreading of pollutants from railways Vo et al. (2015) suggest stormwater management.
Influence of railway age
Chen et al. (2014a) investigated the effect of railway operating time on contamination levels. Their results indicate an increase in contaminant levels (Cd and Pb) with the duration of railway operating time. Wiłkomirski et al. (2011), in comparative studies at the same sites conducted in 1995–2008 (after 13 years), indicated a significant increase in PAH levels in the area of the railway junction during this period, although the same was not observed for HM concentrations. In fact, elevated HM concentrations were observed only in some studied locations.
According to statistical analysis, the age of the track, i.e. the time of pollutant accumulation, has a statistically significant influence on Sb concentrations, although both Sb and Sn show slightly higher values in soils sampled near older railway tracks (Fig. 7a and b).
Anthropogenic sources of soil pollution with Sn and Sb
For pollutants such as Sn and Sb, very few studies were conducted along the tracks, although the rail network is quite extensive worldwide and is a well-known source of pollution. In general, studies of HMs along railways, compared to urban and industrial areas, are relatively rare as they are considered trivial (Chen et al. 2014a). Wiłkomirski et al. (2012) pointed out that there are surprisingly few reports in the scientific literature describing environmental problems caused by rail transport, while Popp and Boyle (2017) emphasized that there is a conspicuous lack of research related to railways and their impact on wildlife. This is especially true since the study of HMs in dust (which in dry and wet deposition ends up in the soil) in various urban functional areas in the city of Shijiazhuang (North China) revealed that the area around the North Railway Station is among the most polluted (Wan et al. 2016). Similarly, Chillrud et al. (2005) studied steel dust in the form of particulate matter (PM) in the New York City subway and found that the levels of Fe, Mn, and Cr were more than 100 times higher than outdoors. Other elements that were above the outdoor mean included Ag, Al, As, Cu, Sn, and Sb. Possible sources of these elements were suggested to be contact points of electrical relays and switches, composite metal breaker pads, and trace impurities in some types of steel. A computational estimate of significant pollutant sources and levels from the Swiss Federal Railways (SBB) network in 2003 indicated that particulate matter was the dominant form of pollution (Burkhardt et al. 2008). The main substances introduced by regular railway operations include metals (about 2270 t/year), hydrocarbons (1357 t/year) and herbicides (3.9 t/year). Among these, the main emission of PM is generated by abrasion of brakes (73%), rails (21%), wheels (5%) and overhead lines (1%). In this context, the abrasion of iron sintered brakes, which may contain 0.09% Sn and 0.01% Sb among other metals, is considered as a possible source of contamination of the surrounding soil with these two elements. In addition, Zn-Sn and Cd-Sn alloys have also been reported as commonly used coatings in hydraulic brakes (Adriano 2001). Furthermore, Clemente (2013) suggested that the use of Sb as a fire retardant in brake linings emits significant amounts from vehicles into the environment. Furthermore, Sn is released into the environment by burning of fossil fuels (coal and oil) and exists in the atmosphere as gases and fumes and attached to dust particles (Cima 2018).
All the above can be considered as additional sources of Sb and Sn in soils along railway lines, which in the present study conditioned the mass fractions of Sb higher than the upper limit for regional soils (Salminen et al. 2005) for a total of 51 samples and in 16 sites by a factor of 5 and higher. A similar situation was observed for Sn, where the content above the upper limit for regional soils (Salminen et al. 2005) was measured at 39 sites, while this factor was higher than 5 at only three sites.
In general, rail transport is considered a diverse source of HMs introduced by various activities, including freight transport, friction in systems; wheel-brake blocks, wheel-rail, pantograph contact wire, bearings; use of herbicides; coal heating of cars; exhaust fumes from locomotive engines; migration from wooden and reinforced concrete sleepers, from rubble and ballast cut-off materials; garbage discarded from trains and on platforms, etc. (Samarska and Zelenko 2018a). In line with that, Staszewski et al. (2015) pointed out that the levels of metals in soil near railways are higher than the corresponding values found along traffic roads and in city centres. Similar results were obtained by research in the city of Varaždin in northwestern Croatia (unpublished data), where the highest concentrations of HMs in topsoils were found in the area of the railway station. According to the above, railway stations in the urban areas, and very often located in the centres of cities, are points of very high emissions of pollutants, which should not be neglected.
Future perspectives
Currently, neither Sn nor Sb are usually included in the existing regulations, including the existing Croatian maximum permissible metal levels in soil (OG 09/14). According to the Croatian Agricultural Soil Protection Policy (OG 20/18, 115/18), the maximum permissible concentrations of HMs are defined, but tin and antimony are not included on this list. According to the European Chemicals Agency (ECHA 2020a, b), antimony is recognized as a toxic element, while tin is not considered harmful to humans and the environment.
However, both elements are in certain chemical forms toxic to humans and living organisms (Cima 2018; Cooper and Harrison 2009). Recent extensive research in Europe conducted by Eze et al. (2020) has shown that railway transport, particularly long-term exposure to noise and air pollution, affects human health and causes DNA methylation associated with inflammation and immune responses, and thus accelerates biological aging and causes various diseases (respiratory, cardiovascular, and others). In addition to the negative impacts of rail traffic on human health, negative impacts on ecosystems and wildlife have also been confirmed (Barrientos et al. 2019). The fact that synergistic effect of even low concentrations, below permissible values, of several pollutants together can have a toxic effect on organisms of different trophic levels (Wierzbicka et al. 2015), should also be considered.
It is, thus, crucially important to determine the presence of tin and antimony in soil along roads and railways, as well as chemical forms of Sn and Sb in order to properly assess their potential toxicity.