Pollution and Ecological Risk Assessment of Potentially Toxic Elements in Road-Deposited Sediments Around the Active Smelting Industry of Korea

Potentially toxic elements (PTEs) were investigated in the different sizes of road deposited sediments (RDS) around the active smelting industry to understand their sources and to assess the pollution and ecological risk levels. The highest PTEs concentrations was shown near the raw materials import port and the smelting facilities. The ne particles of RDS showed extremely high PTEs concentrations. Zn has the highest mean concentration in the <63 mm particle size of RDS, followed by Pb>Cu>As>Cr>Ni>Cd>Hg. The PTEs concentrations of this study were the highest values compared to the soils around the smelter and the RDS in urban and industrial areas in the world. This indicates that these PTEs pollution in RDS were mainly attributed to the transportation of raw materials for the smelting industry. According to nemerow pollution index calculation, RDS at all sampling sites with particles of less than 250 mm was seriously polluted with PTEs. The ecological risk was also found to be very high in all RDS fractions and highly toxic elements such as Cd, Pb and Hg pose extremely risk. Given the total amounts PTEs in the road surface, it is necessary to apply RDS removal management plan to reduce the PTEs pollution. We studied the concentrations and loadings of PTEs in different particle sizes of RDS around the active smelting to gure out their pollution source and to assess the and potential ecological risk levels. PTE concentration in RDS increased with decreasing in particle size and the ne size (<63 mm) of RDS was heavily polluted with PTEs. Mean metal concentrations (mg/kg) in the ne size (<63 mm) were on the order of Zn (34,592) > Pb (13,561) > Cu (7071) > As (961) > Cr (596) > Ni (364) > Cd (225) > Hg (17). These concentrations of PTEs in this study were the highest values compared to the soils around the smelter and the RDS in urban cities in the world. Our results indicate that the PTEs in RDS might be affected by the truck spills during raw materials transportation and re-scattering of accumulated RDS because of high pollution levels not only around the smelting facility but also on the entire road surface. Road surface around the smelter have a signicant amount of RDS accumulated with mean of 21,678 mg/m 2 compared to urban areas. Cr, Ni, Cu, Zn, As, Cd, Pb and Hg were accumulated per unit area in amounts of 370, 169, 4984, 11802, 215, 47, 4177 and 4 mg/m 2 in the road surface of study area. The relative contributions of Zn, As, Cd, Pb and Hg in the fraction (<125 mm) that could transport to the surrounding environments via runoff and resuspension accounted for 39.6% (Zn), 57.9% (As), 63.8% (Cd), 52.3% (Pb) and 51.3% (Hg) of the total RDS.


Introduction
Road infrastructure and transportation are important component of urban area and have enabled the rapid development of industrialization and urbanization. Road deposited sediments (RDS) are highly contaminated with potentially toxic elements (PTEs) by various tra c and industry related sources such as vehicular exhaust and non-exhaust sources, atmospheric deposition and surrounding soil erosion and spill of industrial raw materials during transportation [1][2][3][4][5][6] . Thus, roads are often a prominent point-source and non-point source of dissolved and sediment-associated PTEs 6-10 . The particle size distribution of RDS is a very important factor as it determines the behavior and mobility of the particles and shows the highest concentration of PTEs in ne particles [11][12][13] . Environmental concern related to RDS is that RDS containing high concentrations of PTEs on the road surface adversely affects the surrounding environments as well as human health [14][15][16] .
The ne fraction of RDS are readily transported to the surrounding aquatic environments by stormwater runoff. Many studies reported that the ne particle (<44 mm 17 , <63mm 18 , <125 mm 6 ) largely contributed of total suspended solids (TSS) load in stormwater runoff from urban and industrial areas. The ner RDS are also re-suspended by strong winds and the high-speed movement of the vehicles, therefore, PTEs bound to ne particles of RDS and surrounding soils can enter the human body via inhalation, ingestion and dermal absorption [19][20][21] . Our previous study reported that 14.3-15.8 g/m 2 (<63 mm) and 3.2-4.2 g/m 2 (>1000 mm) of RDS in urban area accumulate on the road surface in Korea 22 . On road surface, RDS are deposited of 11.7 g/m 2 in urban area 17 and 174.6 g/m 2 in industrial area 6 . Industrial areas are characterized by a higher accumulation of road dust than urban areas. In Korea, the amounts and concentrations of PTES in ne particle of RDS were much higher in industrial area than in urban areas 5,[22][23] . Given the total length of road, a huge amount of PTEs would have been accumulated in road surface. Of course, PTEs pollution in industrial RDS is subjected to the complex in uence of tra c and industrial activities. Lanzerstorfer 13 reported that the PTEs concentrations in urban RDS can be used a useful indicator for environmental pollution. The potential sources of PTEs can be identi ed by evaluating the PTEs concentrations in RDS from different land use types and the elemental ratios of them 11,[24][25][26] . Although there are very few RDS studies in industrial areas compared to urban areas, the study of PTEs concentration in RDS from the industrial area will make it possible to differentiate between transport and industrial activities. The objectives of this study are to: (1) evaluate the PTEs pollution levels of different RDS sizes in industrial area where the smelting industry is active; (2) identify the pollution sources of PTEs; (3) assess the potential ecological posed by PTEs.

Materials And Methods
Sampling and PTEs analysis.
Total of 14 RDS samples were collected from Onsan Industrial complex including several smelting facilities of Korea (Fig. 1) during December 2013 following a dry weather periods of about 10 days. The RDS were collected in four and more sub-sampling foe each site using cordless vacuum cleaner (DC-35, Dyson Co., UK) with 0.5 m × 0.5 m space along the curb of road. After sampling, the samples were stored in a zipper bag and the vacuum cleaner was replaced or cleaned to avoid cross contamination. Each RDS sample were sieved individually using <63 mm, 63-125 mm, 125-250 mm, 250-500 mm, 500-1000 mm, >1000 mm 27 by using vibratory sieve shaker (Analysette 3 pro, Fritsch Co., Germany) with nylon sieves. Each fraction of RDS sample was weighted, pulverized (Pulverisette 6, Fritsch Co., Germany) and stored separately into pre-acid cleaned polyethylene bottle until metal analysis. About 0.1 g of each ground and homogenized RDS sample was weighted in Te on digestion vessel added with high purity (Ultra-100 grade, Kanto Chemical, Japan) of HNO 3 , HF and HClO 4 on a hot plate at 180 o C for 24 h for total digestion. After evaporation and redissolution with 2% HNO 3 , heavy metals of Cr, Ni, Cu, Zn, As, Cd and Pb were analyzed using inductively coupled plasma mass spectrometry (ICP-MS, iCAP-Q, Thermo Scienti c Co., Germany). Hg was determined using Hg analyzer (Hydra-C, Leeman Labs, USA) based on the USEPA 7473 method. The blanks and duplicate measurements were performed for quality control. Two types of certi ed reference materials for MESS-4 and PACS-3 (National Research Council, Canada) were used to check data accuracy. Recoveries ranged between 96.4% and 102.1% for MESS-4 and between 93.9% and 106.0% for PACS-3, respectively.
Pollution level assessment.
The geo-accumulation index (I geo ), proposed by Muller 28 , can be used to assess the pollution level of individual metal using the following equation: where C i and B i are the concentrations of RDS samples and the geochemical background values 29 , 1.5 is the background correction e cient. I geo value were classi ed into seven categories 28,30 .
The nemerow index (P N ) are widely used to make a comprehensive evaluation of the pollution levels of heavy metals in soils and sediments [31][32][33][34][35] and was calculated using the following equation: where PI represent a single pollution index of metal i, PI i =C i /S i . C i is the measured concentration of each metal i. The calculated results of P N using the geological background value can be overestimated the magnitude of metal pollution 36 . Therefore, the soil quality guideline values were used in this study to better re ect the comprehensive pollution level of heavy metals in Korea. S i is the soil pollution concern standard for road and factory site in Korea and its values (mg/kg) of Cr 6+ , Ni, Cu, Zn, As, Cd, Pb and Hg were 40, 500, 2000, 2000, 200, 60, 700 and 20, respectively 37 . In case of Cr, in Korea soil quality guideline, the concentration of Cr 6+ is recommended. Lazo 38 reported that the content of Cr 6+ accounts for more than 90% of total Cr in the contaminated area. Therefore, the application of total Cr concentration instead of Cr 6+ of this study did not signi cantly affect the results of pollution evaluation for eight metals using P N . This index divides pollution into ve grades 39 .
Potential ecological risk assessment.
Potential ecological risk index (PER), proposed by Hakanson 40 can be used to assess the risk of eight metals based on their toxicity response using the following equations: where C i and B i were the same as those in I geo calculation. Eir is the single factor ecological risk degree for PTEs. T i r is the toxic response factor for a single metal pollution (Hg=40, Cd=30, As=10, Cu=Ni=Pb=5, Cr=2, Zn=1) [40][41] . E i r were classi ed into ve classes 42 and the PER value were classi ed into four classes 40,43 . PASW statistics program (version 18) were used for the pearson's correlation analysis and hierarchical cluster analysis (HCA) to understand the relationship between metals and different size fractions of RDS.

Results And Discussion
PTEs contents in different size of RDS.
The minimum, maximum and mean values of the total RDS amount and Cr, Ni, Cu, Zn, As, Cd, Pb and Hg concentrations are shown in Table 1. The Cu, Zn, As, Cd, Pb and Hg concentrations signi cantly increased with decreasing in particle size of RDS (Fig. 2). Mean PTE concentrations in the ne particle size (<63 mm) of RDS was 5.0 (Cr) ~ 55.5 (Zn) times higher than those in the large particle size (>1000 mm). The mean concentrations of RDS (63 mm) was highest for Zn at 34,592 mg/kg, followed by Pb (13,561) > Cu (7,071) > As (961) > Cr (596) > Ni (364) > Cd (225) > Hg (17). The Cr and Ni concentrations in the ne particle size (<63 mm) showed highest values at S6 site, but the highest concentrations for Cu, Zn, As, Cd were observed in S4 and S5 sites which the smelting facilities exist.
The study area, Onsan industrial complex, has concentrated non-ferrous metal production industry of Korea.
There are many smelting facilities in operation that produces 1.2 Mt of nonferrous metals annually, including Cu, Zn, Cd and Pb. Garmash (1985) 44 found that nonferrous metal smelters are more contaminated with Zn, Pb and Cd in soils than iron smelters. There are a raw material import port and outdoor raw material storage for smelting industry on the north of S4 site. Raw materials are transported using a large truck. The highest PTEs concentrations were observed in all particle sizes of RDS around the smelting facilities, indicating that the anthropogenic source of these PTEs were attributed to the transportation of raw materials for the smelting industry. The high correlation between Cr and Ni was observed. RDS of this study is signi cantly correlated with among Cu, Zn, As, Cd and Pb. Hierarchical cluster analysis was also conducted to understand the relationship among the different size of RDS. The dendrogram of the different particle sizes of RDS show two cluster groups. Group 1 comprises two particle size fractions (<125 mm) with signi cant PTEs contamination. Group 2 corresponded to the particle size of < 125 mm with moderate PTEs contamination.
The  (Table 2).  (Fig. 3). Fig. 4 shows the spatial distribution of P N values in the different size of RDS. The high pollution degree of RDS (<125 mm) indicates that the ne particles of RDS are attached to the tires according to vehicle transport and spreads through the entire road surface.
Ecological risk assessmen in industrial RDS.
The results of single factor ecological risk degree (E i r ) are presented in Table 4. The highest mean E i r value is observed for Cd (75,044) in <63 mm of RDS and the lowest E i r value are observed for Cr (2.6) in >1000 mm of RDS. Similar to the PTEs concentrations, the single ecological risk was higher as the particle size of RDS decreased. The mean of single factor ecological risk degree (E i r ) values of Cr and Ni in all particle size was less than 40, which indicated that Cr and Ni concentrations of RDS correspond to the low ecological risk level.
The mean values of E i r of Cd were the highest among those of all PTEs for all sampling sites and ranged from 2,095 (>1000 mm) to 75,044 (<63 mm), indicating extremely potential risk levels (E i r >320). Hg has the second highest E i r values and exceed 320 in all particle sizes of RDS, showing extremely potential risk. For Cu and Pb, the mean of E i r values was also obtained extremely potential risk except for the large RDS size >1000 mm. Generally, the E i r values were ranked in the following order: Cd>Hg>Pb>Cu>As>Zn>Ni>Cr. The mean of PER values, the comprehensive ecological risk of eight PTEs, ranged from 4,434 (>1000 mm) to 96,435 (<63 mm) and the ne particle was 21.7 times higher that large particle. The PER values exceeded 600, indicating very high ecological risk for all studied sites and particle size of RDS except for >1000 mm at S11 site (Fig. 3).
PTEs loads in RDS on the road surface around the active smelting industry.
The mean of RDS amount in road surface were 104 for >1000 mm, 200 for 500-1000 mm, 373 for 250-500 mm, 272 for 125-250 mm, 138 for 63-125 mm, 85 g/m 2 for <63 mm, respectively ( Table 1). The amount of RDS with particle size of 250-500 mm was the most abundant in this study. We also calculated the PTEs loads and the contribution of each particle size fraction using GSF loading (Fig. 5). A signi cant amount of PTEs (21,872 mg/m 2 ) has accumulated on the road surface in industrial area. The each PTEs load in industrial RDS was much higher than in urban RDS 17  Conclusions RDS is highly polluted by various pollutants, especially PTEs, and has received much attention as one of important pollution sources in the terrestrial, coastal and atmospheric environments as well as human health problems. We studied the concentrations and loadings of PTEs in different particle sizes of RDS around the active smelting industry to gure out their pollution source and to assess the pollution and potential ecological risk levels. PTE concentration in RDS increased with decreasing in particle size and the ne size (<63 mm) of RDS was heavily polluted with PTEs. Mean metal concentrations (mg/kg) in the ne size (<63 mm) were on the order of Zn (34,592) > Pb (13,561) > Cu (7071)  Given the amount of PTEs deposited in the road surface, it is necessary to apply RDS removal management plan to reduce the PTEs pollution.

Declarations Data availability
All Data for this study are available from the corresponding author on request.

Competing interests (mandatory)
The authors declare that they have no con ict of interest.

Figure 1
Locations of sampling sites for road-deposited sediments from Onsan industrial complex including the smelter of Korea. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.     Spatial distribution of nemerow index (PN) values in the different sizes of road-deposited sediments. Blue star symbol means a smelting facility. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.