Chemical speciation of metals from marine sediments: Assessment of potential pollution risk while dredging, a case study in southern Sweden

(cid:1) Metals were highly linked to the residual fraction showing a lower risk of pollution. (cid:1) While dredging, Pb and Zn could be potentially released with the intrusion of oxygen. (cid:1) There is a low risk of metal pollution while using dredged sediments on in-land uses.


Introduction
Metals have received high importance due to the environmental pollution caused by their toxicity, abundance, non-biodegradable characteristics, persistence and accumulation (Bastami et al., 2017).In marine environments, trace elements frequently affect aquatic ecosystems impacting animals, plants and benthic communities.Trace elements can also cause ecological problems while accumulating in the food chain, creating risks for humans and inland organisms (Thinh et al., 2018).
Lead, copper and nickel are highly studied metals due to their toxicity.Additionally, zinc and iron also receive significant attention due to their wide distribution in nature.More specifically, lead is a toxic metal with no nutritional value, which is associated with adverse effects on animals and human health.This element is abundant in nature and can accumulate in the food chain (Zhang et al., 2016).Leaded fuels, industrial discharges, paints and batteries are potential sources of this metal (Romieu et al., 1997).Copper is an essential element required by organisms, but, in high concentration, it can potentially be toxic (Mackie et al., 2012).This metal exists in nature, and its emissions to the environment are highly related to antifouling paints (Srinivasan and Swain, 2007) and corrosion from metal structures and electrical devices and cables (Barceloux, 1999).Chromium and nickel are originated from natural and anthropogenic sources and have known toxic effects on the environment and human health (Kang et al., 2017) (Duda-Chodak and Blaszczk, 2008).Zinc is an essential trace element for humans, plants and organisms, but in excessive concentrations, it becomes toxic.This metal is naturally present in soils and biomass, and it is mainly released to the environment by anthropogenic sources such as agricultural residues and industrial or solid waste (Plum et al., 2010).Iron is one of the most common elements on the Earth's crust (Fytianos and Lourantou, 2004) and is essential for organisms since it is a component of haemoglobin that is related to the transport of oxygen in bodies (Camaschella et al., 2020).The mobilisation of the metal is mainly linked to human activities such as removal of ores, combustion of fuel fossils and metallurgical production (Wang et al., 2007).
Sediments in the aquatic ecosystem act as a sink of pollutants by accumulating a major amount of metals, while a minor fraction remains in the water phase (Neyestani et al., 2016).The sediment column has a crucial role in the transportation, repository and release of metals into the environment (Kang et al., 2017).Contaminants are not all fixed in the crystal structure of the sediment, and its mobilisation to the water phase can occur under physicochemical changes of the surroundings (Prabakaran et al., 2019).Metals are partitioned into several chemical forms, where some are more labile during physical and chemical fluctuations.Variations of the aquatic system are caused by natural and human activities like dredging and use of engines, or under environmental changes such as fluctuation of salinity, pH, redox potential and ionic strength (Fathollahzadeh et al., 2014).Studying the bioavailability of metals in aquatic environments is critical to identify their potential environmental risks.The total concentration of metals is a poor indicator to specify mobilisation risks, whereas the speciation of metals contributes to quantifying the amount of metals that can be released from the different chemical fractions during variations in environmental conditions (Prabakaran et al., 2019).
One of the most used methods for metal speciation in sediments is the Tessier et al. (1979), which includes five geochemical fractions.The exchangeable fraction (F1) is the most rapidly bioavailable due to its weak bonds (Islam et al., 2015).Metal migration can occur easily after a shift in salinity, alteration of the adsorption equilibrium of the system or a change of ionic concentration in water.The second fraction is related to trace elements bind to carbonates (F2), and it is readily available and mobile under changes of pH.The third fraction (or reducible fraction) counts trace elements bind to iron and manganese oxides (F3).Mobilisation of metals can occur under changes of redox potential specifically under the reduction of available oxygen in the environment (Prabakaran et al., 2019).The fourth fraction (or oxidisable fraction) includes metals bind to organic material and sulphides (F4).This fraction is also sensitive to redox potential, and elements can be mobilised under oxidising conditions or by decomposition of organic matter (Chen et al., 2019).Lastly, the metals in the fifth or residual fraction (F5) are part of the mineralogical structure of the sediment, where elements are unlikely to be released due to the strong bonds to alumino-silicate minerals (Zhang et al., 2016).
Dredging of sediments is a worldwide activity necessary for the restoration of water bodies and maintenance of harbours (Radenovic et al., 2019).Common disturbances while dredging cause physical, chemical and ecological changes related to the notorious intrusion of oxygen, resuspension of solids and perturbation of sediments as well as possible variations of pH and temperature (Fathollahzadeh et al., 2014).Traditional dredging techniques involve mechanical excavation of sediments with buckets causing large ecological fluctuations (Patmont et al., 2018).The variation in the environmental conditions can possibly affect the aquatic ecosystem by promoting the mobilisation of metals to the surroundings.Studying the speciation of metals contributes to evaluate the risk of pollution while dredging (Xia et al., 2018).
Moreover, when sediments are already on the land, a disposal solution is required.Frequently, dredged sediments are disposed of in open oceans (Chen et al., 2019) or landfills (Mymrin et al., 2017).However, these methods are restricted by environmental and legal concerns.Disposal in open oceans is banned in several countries since it could pollute marine ecosystems (Cesar et al., 2014).Landfills are related to large area requirements and may pose a threat due to the production of risky by-products such as methane (greenhouse gas) and polluted leachate (Depountis et al., 2009).Using dredged sediments in beneficial uses could be a response to eliminate traditional disposal methods and to develop a new source of resources such as nutrients or raw material for construction or industrial processes (Baxter et al., 2004).However, when sediments are exposed to surrounding conditions, the environment is more oxidising, and the pH can easily drop.Leaching of metals could potentially occur, affecting the final use of sediments (Fathollahzadeh et al., 2015).Studying the speciation of metals contributes to evaluate the risk of mobilising metals from sediments to the new in-land environment (Xia et al., 2018).
The aim of this study is to evaluate the speciation of metals in sediments from Malmfj€ arden bay, Sweden to preliminary assess the potential ecological risk of releasing metals during dredging activities as well as input towards using the dredged sediments in beneficial uses.

Study site
Malmfj€ arden is a semi-enclosed bay located in Kalmar, southeastern Sweden (56⁰ 66 0 N, 16⁰ 36' E) and belongs to the Western Gotland Basin from the Baltic Sea (Fig. 1-b).The water body is situated in the city centre and is a crucial place for the municipality since it provides habitat for wildlife development and recreational spaces for the population.Domestic or industrial sewage discharges are absent in the bay, and the only water inlet to the water body is the emission of runoff collected from surrounding areas.The neighbouring sector of Malmfj€ arden includes residential and commercial zones and an old dumpsite.
Kalmar is continuously expanding, and its development plan includes building a large household complex in the south-western part of Malmfj€ arden bay.Since October 2019, the municipality started a minor pilot scale of a dredging project to reach a proper water level, which could recover the conditions of the bay and provides an improved environment for the new inhabitants.The full scale dredging project will start in autumn 2020 and finish in summer 2021.The complete future dredging area is shown in Fig. 1c.Previous studies suggested that in this zone, the sediments exhibit a high concentration of nutrients and medium-low concentrations of metals (Nilsson, 2013).The dredged sediments will be employed on beneficial uses to avoid landfilling of valuable resources.

Sample collection and processing
On June 2018, sediment samples were directly extracted from the bay since the dredging activities only started at the end of 2019 with minor operations.The sediment samples were taken from 4 different zones covering all the future dredging area.Each zone included 5e8 stations adding a total of 25 locations (see Fig. 1-C).Sediment cores of approximately 60 cm length and 10 cm of diameter were collected using a core manual sampler, and triplicates were taken in each station.Using a plastic spatula, each core was divided into top (0e20 cm) and bottom (21e60 cm) layers.All samples were stored at 4 C in pre-cleaned polythene bags.One composited sample was created for the speciation analysis.Subsamples of 50 g from all top and bottom samples (in total, 75 top and 75 bottom samples) were oven-dried at 40 C (until achieving a constant weight).The dry samples were ground using a pestle and mortar and then grinded through a 1 mm stainless-steel mesh.The composite sample was formed by manually mixing all ground subsamples.The procedure was adapted from several speciation studies such as Ianni et al. (2010) and Akcay et al. (2003).

Speciation method
A five-step speciation method developed by Tessier et al. (1979) was chosen to perform the speciation of metals (Pb, Cu, Cr, Ni, Zn and Fe).The experiment was carried out using 2 g from the composited sediment sample (the preparation procedure is illustrated in section 2.2.).The extraction procedure was performed as follows.
Step 1.The 2 g sediment sample was taken to a 250 ml wide-neck flask, and the first fraction was extracted at room temperature with 16 ml of 1 M magnesium chloride (Enola -Latvia) for 1 h on a magnetic shaker.
Step 2. The sediment left from the 1st step was carefully transferred to a 250 ml wide-neck flask, and the fraction was extracted with 16 ml of 1 M sodium acetate (Peaxnm -Russia) at room temperature for 16 h on a magnetic shaker.
Step 3. The sediment remaining from the 2nd step was taken to a 250 ml wide-neck flask, and the fraction was extracted with 40 ml of 0.04 M hydroxylamine hydrochloride (Enola -Latvia) for 16 h at 96 ± 1 C on a magnetic shaker.
Step 4. The sediment left from the 3rd step was transferred to a 250 ml wide-neck flask, and the fraction was extracted with a three-step procedure on a magnetic shaker.First, the sample was mixed with 6 ml of 0.02 M nitric acid (Enola -Latvia) for 2 h at 85 ± 2 C. Second, 6 ml of 30% hydrogen peroxide (Merck -Germany) was added and mixed for 3 h at 85 ± 2 C. Lastly, 10 ml of 3.2 M ammonium acetate (Sigma Aldrich e United States) was added and mixed for 0.5 h at 25 ± 1 C.
Step 5.The residual fraction was extracted through wet acidbased digestion with nitric acid and hydrogen peroxide.The original treatment with HFeHClO4 was exchanged due to internal laboratory restrictions.In the procedure, 100 ml of 65% (w/v) nitric acid (Enola -Latvia) along with 10 ml of 30% (w/v) hydrogen peroxide (Merck -Germany) was added.After 24 h, the solution was heated at 96 ± 1 C until half of the volume was evaporated.Nitric acid was added until complete sample mineralisation.
More details of the extraction procedure are shown in Table 1.At the end of each extraction step, the solution was centrifuged at 4500 RPM for 30 min (centrifuge WIFUG -United Kingdom).The obtained aliquot was collected separately from the solid phase by filtering using 0.45 mm filters (Frisenette -Denmark).Before analysis, samples were stored at 4 C. Inductively coupled plasmaoptical emission spectrometry (ICP-OES) (ThermoFisher Scientific -United States) was employed to analyse the metal concentrations from all extracted aliquots.The pre-analysis procedure was adapted from other speciation studies such as Bastami et al. (2017) and Medici et al. (2011).

Total metal concentrations in sediments
The total concentration of metals in the sediments was also analysed to evaluate the performance of the speciation procedure.A sample of 2 g from the composite sample was employed for the procedure.The metals were extracted through wet acid-based digestion with nitric acid and hydrogen peroxide.50 ml of 65% (w/v) nitric acid (Enola -Latvia) along with 5 ml of 30% (w/v) hydrogen peroxide (Merck -Germany) was added per gram of dry sediment.The pH during the digestion was adjusted to 2. After holding for 24 h, the solution was heated at 96 ± 1 C until half of the volume was evaporated.Nitric acid was added until complete sample mineralisation.The digestion procedure was adapted from Burlakovs et al. (2018), where laboratory restriction to implement the HFeHClO 4 digestion were also present.
After digestion, the solution was centrifuged at 4500 RPM for 30 min (centrifuge WIFUG -United Kingdom).The obtained aliquot was filtered using 0.45 mm filters (Frisenette -Denmark).The concentration of metals was analysed using an inductively coupled plasma -optical emission spectrometry (ICP-OES) (ThermoFisher Scientific e United States).

Quality control
Recovery rates were determined to compare the total concentration of each metal and the sum of concentration from the five fractions.Eq. ( 1) was employed to calculate the recovery rates, where C F1-5 represents the sum of concentrations from all fractions, and C tot indicates the total concentration calculated in the experimental procedure.Recovery rates between 100 ± 15% were classified as satisfactory, showing accuracy in the speciation procedure.
Besides calculating the recovery rates, other measures were taken to ensure reliable results.The speciation experiment was run in triplicates, and each extraction step included blank samples.Additionally, all glassware used during the experiment was previously soaked with 10% HCl solution for at least 12 h.

Risk indexes
The contamination factor (CF) and the risk assessment code (RAC) were chosen to measure the risk to release metals from the sediments to the surrounding environment.The CF was selected to compare the non-residual fraction over the residual portion, while the RAC was chosen to contrast the concentration of the most labile fractions (F1eF2) over the total concentration.The CF was calculated using the Eq. ( 2), where C F1-4 represents the sum of the concentration of the first four fractions, and C F5 denotes the concentration in the residual part.CF with values < 1 stands for no contamination; 1<CF < 3 shows moderate pollution; 3<CF < 6 denotes considerable contamination and CF > 6 states very high pollution (Håkanson, 1980).The RAC was calculated using Eq. ( 3), where C F1-2 represents the sum of the concentration of F1 and F2, and C T the total concentration calculated as the sum of concentrations from all the fractions.The criteria to interpret the risk assessment code is taken from Perin et al. (1985), and it is shown in Table 2.

Determination of organic matter content
Following the standard SS-EN 15169:2007, the organic matter content was calculated using 50 g of the composited sample, which was first extra dried at 105 ± 5 C until achieving a constant weight.The remaining material was taken to an oven for 1 h at 550 C. The organic content was calculated using the weight difference before and after heating at 550 C. The sample was analysed in triplicates.

Sediment characteristics
The results revealed that the organic content of the sediments had an average value of 12.9 ± 0.6%, highlighting a medium value according to SS-EN ISO 14688e2:2018.Table 3 shows the total concentration of metals.Lead had an average concentration of 37.2 mg kg À1 , copper 51.1 mg kg À1 , chromium 37.9 mg kg À1 , nickel 27.9 mg kg À1 , zinc 138.5 mg kg À1 and iron 23,855 mg kg À1 .Fathollahzadeh (2012) analysed the total concentration of metals in sediments from Malmfj€ arden, and most of the results are in agreement with the current work.The Fathollahzadeh (2012) study reported a concentration of Pb 74.6 mg kg À1 , Cr 40.7 mg kg À1 , Cu 53.2 mg kg À1 , Ni 16.7 mg kg À1 , Zn 150 mg kg À1 and Fe 29,000 mg kg À1 .Zinc, iron, copper and chromium had a variance under 15% comparing the previous and the current studies, while lead and nickel had larger differences in concentrations.Discrepancies between the results could be explained by the difference in the age of samples.The studies were separated eight years apart, and therefore the distribution of metals could change since the discharges might vary over time.

Recovery rates
The recovery rates are shown in Table 3.All the results were between 95 and 122%.Rates between 100 ± 15% were considered satisfactory.Spectral interferences with other components of the sediments are common for soils or sediment samples with a high level of iron (Pillay, 2020).Since the sample from the study present high levels of this elements, the spectral interference could be a reason to explain the deviation of the recovery rates away from the range.For the case of Cu, several samples had concentrations close to the detection limit of the equipment producing less accurate results (Akcay et al., 2003).

Metal speciation
Fig. 2 illustrates the distribution (in percentage) of metals in the five chemical fractions, while the risk factors values for all the studied elements are shown in Table 4.More details concerning each metal are in the following sections.
Among all metals (see Fig. 2), lead was one with a higher amount linked to the non-residual fractions, suggesting a probable anthropogenic origin.In the Malmfj€ arden case, the inlet of runoff could be related to the source of origin of this element due to the lack of sewage discharges.Additionally, the link of Pb to labile fractions can enhance the leaching potential due to changes in the redox conditions and fluctuations of pH.Considering all the mobile fractions, the contamination factor of 2.33 represented a moderate risk of pollution.The RAC of 14.7% indicated a medium risk of mobilisation of F1 and F2.
Dredging could induce substantial environmental changes in the aquatic ecosystem (such as the intrusion of oxygen and variations of pH).With fluctuations in the surroundings, the labile fractions of lead could potentially be released.Therefore, it is recommended to employ more environmental dredging methods, which could cause less resuspension of particles and variations in ecological conditions.

Copper
The residual fraction had the highest proportion of copper (54 ± 3%), followed by the organic matter-sulphides fraction (45 ± 3%).FeeMn oxides, carbonate and exchangeable fractions had a low amount of the metal (<1%).Results are explained by the high a RAC: Risk Assessment Code.

Table 3
Total concentration of metals along with the sum of concentrations in all fractions and recovery rates (mean ± SD; n ¼ 3).

Metal
Total concentration (mg kg affinity of the element to form compounds with organic matter (Chen et al., 2013).Similar distributions of sediments with a high amount of copper linked to the oxidisable (30e50%) and residual (20e50%) fractions were described by Tokalioglu et al. (2000), Fytianos and Lourantou (2004) and Lasheen and Ammar (2008).The distribution of copper suggests that the metal has anthropogenic and non-anthropogenic origins.Copper presented the highest amount linked to the organic matter-sulphide fraction (see Fig. 2).Due to strong binding to F4, copper can principally be released to the surroundings under oxidisable conditions or while degradation of organic matter.The metal also exhibited a high presence in the residual fraction, reducing its bioavailability.The RAC value of 1.1% represented a low risk of pollution of copper associated with F1 and F2.The contamination factor of 0.87 expressed no risk of contamination by this metal in the bay during dredging activities.

Chromium
The results exhibited that chromium was mainly linked to F5 (70 ± 1%) followed by F4 (22.8 ± 1%) and F3 (6 ± 1%).The metal had no considered amount related to F1 and F2 (<1%).The findings are explained by the fact that the element is prevalent on the Earth's crust (Baraud et al., 2017), and that organic matter has a high affinity for it (Radenovic et al., 2019).Results are in agreements with Morillo et al. (2004), who described marine sediments with a high amount of chromium linked to the residual fraction (80%) followed by the organic matter-sulphide portion (10%).
As observed in Fig. 2, chromium is the metal with less amount linked to the labile fractions, implying a considerable natural origin.The presence of this element in F4 may pose a threat of release under oxidising conditions or by the degradation of organic matter.Nevertheless, the low proportion of chromium in the labile fractions suggested a reduced risk of pollution.Findings are supported by the contamination factor of 0.42 and a RAC of 0.8%.These results suggested that dredging activities would be likely not connected to the release of chromium.
The distribution of nickel along all fractions suggested both anthropogenic and natural origins.This metal can potentially be leached out by changes in the redox potential, and either oxidisable or reducible conditions can affect its mobility.Nevertheless, the high amount of nickel linked to the residual fraction leads to less risk of contamination presented by the contamination factor of 1.05.The RAC of 4.9% indicated a low risk of pollution by mobilisation of the most labile fractions.Consequently, during dredging, it is not expected to generate contamination with nickel in the aquatic environment.

Zinc
In the sediments from Malmfj€ arden, zinc had one of the highest total concentration.The residual part (37 ± 4%) was the highest followed by the FeeMn oxide fraction (27 ± 4%).The metal was also bound to F2 (17 ± 1%) and F4 (14 ± 3%), while F1 had a low proportion (4 ± 0.5%).The results are explained by the fact that Fe and Mn are great carriers of zinc.Organic matter also exhibits a pronounced affinity, forming metal-organic complexes (Prabakaran et al., 2019), and carbonates can also bind to this element (Chen et al., 2019).Additionally, the presence of this metal in the  residual fraction is explained by its widespread in the Earth's crust (Plum et al., 2010).Results are in agreement with Chen et al. (2013), who reported zinc from sediments mainly linked to the carbonate (30%), FeeMn oxide (30%), organic matter -sulphide (10%) and residual (30%) fractions.Similar to lead, zinc is highly associated with the non-residual fractions suggesting an anthropogenic origin.In Malmfj€ arden, this element is probably associated with the runoff discharges since the bay lacks sewage inputs.Possible changes affecting the mobility of this metal are the decomposition of organic matter and fluctuations of pH and redox conditions.Medium risk of pollution of zinc to the surroundings was determined by the CF of 1.71.The RAC of 21.6% showed a medium risk of mobilisation of F1 and F2.Zinc can potentially be released into the aquatic ecosystem during the implementation of dredging activities.As a preventive measurement, it is recommended to employ more environmental dredging techniques, which are less associated with substantial variations on the bay conditions reducing the possibility to release this metal.

Iron
Iron was the metal with the highest total concentration.Nevertheless, most of its amount was linked to the residual fraction (58 ± 1%) followed by F4 (36 ± 4%) and F3 (7 ± 3%).F2 and F1 had no main linkage.Findings are explained by the broad presence of the metal in nature and its high affinity to bind to organic matter (Fytianos and Lourantou, 2004).Tokalioglu et al. (2000) reported similar results, where iron in sediments was mainly linked to the residual (60%) and oxidisable (30%) fractions.
Compared to other metals (see Fig. 2), iron had one of the highest association with the residual fraction, proposing a natural source of origin.The main risk of mobilisation of this element is connected to the decomposition of organic matter or changes to an oxidising environment.Nonetheless, the pollution risk of iron is low since most of its amount is found in the residual fraction.The contamination factor of 0.74 indicated no pollution with this metal, and the RAC of 0.1% showed no risk of mobilisation of the most labile fractions.During dredging activities, due to the low risk of pollution, it is not expected to generate contamination by iron into the surroundings.

Potential metal risk pollution while using dredged sediments in beneficial uses
In Sweden, SEPA (2009) provides the national maximum permissible concentrations of metals in soils/dredged sediments in order to be employed in different land uses.The regulation determines separate limits for less sensitive and more sensitive land uses.Fig. 3 compares the concentration of metals in each chemical fraction with the maximum permissible thresholds for more sensitive uses.The figure also illustrates the sum of the concentrations of the non-residual part for each metal.Iron is not included in the figure since the Swedish regulation absences a maximum permissible concentration for this element.
The results showed that, in each fraction, the concentrations of the metals did not overpass the maximum permissible limits of the sum of concentrations of the non-residual part.Even if metals from the non-residual fractions are potentially leached out due to changes in the environment, the released concentrations are always below the permissible limits.Hence, it is concluded that, for the case of Malmfj€ arden, using dredged sediments in beneficial uses is associated with a low risk of spreading of Pb, Cu, Cr, Ni and Zn.

Conclusions
The speciation of lead, copper, nickel, cadmium, zinc and iron in sediments from Malmfj€ arden bay, Sweden was carried out to assess the potential risk of pollution in the aquatic ecosystem during future dredging activities and while using the material in beneficial uses.All metals, besides Pb, had their largest amounts associated with the residual fraction.The organic matter-sulphide, FeeMn oxides and carbonate fractions also presented an important association with the studied metals.Fewer amounts of metals were linked to the exchangeable phase.The risk indexes (contamination factor and risk assessment code) showed a slight concern of pollution for Cr, Ni and Fe.The risk increased for Pb and Zn, where the CF was between 2 and 3 and the RAC between 10 and 30%, expressing a medium risk of pollution to the surroundings.During dredging, it is not expected to have contamination associated with Cr, Ni ad Fe.However, it is recommended to decrease the risk of pollution of Pb and Zn into the aquatic ecosystem.Potential solutions include employing more environmental extraction methods, which could be associated with reduced particle resuspension and less fluctuations at the bay.
For all metals, the sum of the non-residual concentrations was below the Swedish limits to determine pollution in soil/dredged sediments.The results suggested that, while using the dredged sediments in beneficial uses, there is a low risk of metal pollution with Pb, Cu, Cr, Ni and Zn.Further metal speciation and leaching tests from the future dredged sediments will contribute to determining the potential metal spread pollution while using the sediments at in-land uses.

Fig. 1 .
Fig. 1.Position of the study site: (a) Location of Sweden; (b) Location of Kalmar; (c) Malmfj€ arden bay and the distribution of sampling stations e dash-red area represents complete future dredging area.(For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2 .
Fig. 2. Percentage of trace elements in each fraction after sequential extraction.

Fig. 3 .
Fig. 3. Concentrations of metals (mg kg À1 DM) in each chemical fraction compared to the maximum permissible metal concentrations determined by the Swedish guideline (SEPA, 2009).Bold horizontal line represents the threshold for sensitive lands.Last column to the right represents the sum of concentration from the non-residual fractions.DM: Dry Matter, SD: Standard Deviation.

Table 1
Speciation-extraction procedure: Reagents and experimental conditions.

Table 2
Evaluation criteria for risk assessment code (RAC).

Table 4
Values of risk indexes for all the studied metals.
a CF: Contamination Factor.b RAC: Risk Assessment Code.