Assessment of heavy metal pollution in surface sediments of the Montenegrin coast: a 10-year review

Abstract

Purpose

Heavy metals are among the most common environmental pollutants, which can be introduced into coastal areas from natural and anthropogenic sources, and thereby possibly impact marine organisms and human population. Therefore, the aim of this study was to evaluate the pollution level of Montenegrin coastal sediments by determining the concentrations of 10 metals and metalloids (Fe, Mn, Zn, Cu, Ni, Pb, Cr, Cd, As, and Hg) during one whole decade.

Materials and methods

Sediment samples were collected from 11 sites along the Montenegrin coast during the 2005–2016 exposure to different levels and sources of anthropogenic impact. The extent of pollution was estimated by determining total element concentrations in the sediment. Mineralized samples were analyzed for Cu, Ni, Fe, Mn, Cr, As, Pb, Zn, Cd, and Hg. Pollution status was evaluated using the contamination factor, pollution load index, and geo-accumulation index, as well as statistical methods, such as Pearson correlation coefficient (r) and cluster analysis (CA).

Results and discussion

This study showed that concentrations of individual metals at some locations were extremely high. The metal concentrations (in mg kg−1) ranged as follows: Fe 1995–45,498; Mn 135–1139; Zn 10–1596; Cu 3.8–2719; Ni 2.94–267; Pb 0.1–755; Cr 2.5–369; Cd 0.1–5.4; As 0.1–39.1; and Hg 0.01–14.2. The calculated concentration factor and pollution load index indicates enrichment by either natural processes or anthropogenic influences. The geo-accumulation index value (Igeo) showed that one location was strongly or extremely polluted (3.78 < Igeo ≤ 6.15) with Hg in all investigated years, while extreme Igeo values for four bioactive elements, Pb, Cd, Cu, and Zn, were found in only a few single samples.

Conclusions

On the basis of the obtained values, it can be concluded that generally higher metal contents were distributed in Boka Kotorska Bay sites, although some extreme values were also recorded at the locations outside of the Bay. Geo-accumulation index and pollution load index showed that the metal levels were high enough to pose risk to the ecosystem.

Introduction

Heavy metals are among the most common environmental pollutants and their elevated concentrations may indicate the presence of anthropogenic sources of pollution. Heavy metals from lithogenic and anthropogenic sources continuously enter the marine environment. Their concentration and distribution are affected by sedimentary structure, mineralogical composition, hydrodynamic transports, industrial discharges, effluents, and shipping activities (Gopinath et al. 2010; Nobi et al. 2010; Vallejuelo et al. 2010). As its basic component, the sediment is considered a suitable medium to identify sources of heavy metal pollution in the aquatic environment. Sediments act as both carriers and sinks for contaminants, reflecting the history of pollution, and providing a record of pollutant inputs into aquatic ecosystems (Casas et al. 2003; Mwamburi 2003; Singh et al. 2005; Adeyemo et al. 2008). The presence of toxic metals at trace levels and essential metals at elevated concentrations can cause various toxic effects in aquatic organisms, as well as in people who are exposed to these contaminants (Fong et al. 2008; Salem et al. 2014). Since the sediments reflect the long-term quality of marine systems, the knowledge about their chemical characterization and possible interactions between pollutants is vital for assessing the quality of the entire ecosystem. This is also very important for the understanding of natural processes and human influence on these processes (Qishlaqi and Moore 2007; Adeyemo et al. 2008).

The Mediterranean Sea is an area with a large variety in geochemical composition of sediment: metal concentrations depend on the specific characteristics of the area and inputs from the surrounding coastal environment. The anthropogenic impact is strong in estuarine and coastal areas. Therefore, the investigation of metal contents in the southeastern Adriatic Sea sediments has been mostly focused on the coastal sediment (Rivaro et al. 2004; Stanković et al. 2014). Montenegrin coast (Southern Adriatic Sea) is also under a great impact of anthropogenic factors and the activities on the shore. For that reason, the aim of this study was to determine the concentrations of iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), nickel (Ni), lead (Pb), chromium (Cr), cadmium (Cd), arsenic (As), and mercury (Hg) in coastal sediments of Montenegro and to assess the pollution level using the geo-accumulation index, contamination factor and pollution load index, and Pearson correlation coefficient and cluster analysis.

Materials and methods

Sampling and sample preparation

Fifty-five sediment samples were collected from 11 sites along the Montenegrin coast: Port of Kotor (S1), Dobrota (S2), Orahovac (S3), Port of Risan (S4), Porto Montenegro (S5), Shipyard of Bijela (S6), Herceg Novi (S7), Žanjice (S8), Budva (S9), Port of Bar (S10), and Ada Bojana (S11) (Fig. 1). The sediment samples were taken in the period between 2005 and 2016.

Fig. 1
figure1

Map of the investigated area with the possible sources of pollution (Montenegrin coast). Eight locations (S1–S8) are in Boka Kotorska Bay, and three locations are outside of the Bay (S9–S11) Figures 1 and 3 contains poor quality and small text inside the artwork.The improved version of Figures 1 and 3 was uploaded. Figure 1 was rearranged according to the Instructions for Authors using Foxit Reader and exported in .pdf file format in order to preserve its quality.

The sites Port of Kotor, Dobrota, and Orahovac are located near the city of Kotor and its harbor. Other inner shore locations of the Bay, Port of Risan, Porto Montenegro, Shipyard of Bijela, and Herceg Novi are tourist locations with small marinas. The location Žanjice is in the vicinity of Žanjice beach, which is situated at the entrance to the Bay from the open sea. Budva is an urban and tourist town situated in the center of the Montenegrin coastline. Port of Bar is one of the largest ports on the eastern side of the Adriatic, with trading, passenger traffic, and industrial function. These activities, especially reloading dry bulk cargo, grains, and liquid cargo such as crude oil and oil products, can potentially endanger the environment. Ada Bojana is one of the most popular tourist places on the Montenegrin coast. What all 11 sampling sites had in common was long-standing exposure to different levels and sources of anthropogenic impact, such as marine traffic, trade and passenger ports, ship repair, industrial, domestic and agricultural waste, and sewage. The lowest anthropogenic influence was noticed at locations Žanjice and Ada Bojana, leisure places with restaurants and cottages, but also under stronger influence of the open sea.

The upper 5 cm of the surface sediment samples, collected by a petite ponar grab (Wildco—dimensions 40 × 40 cm), was placed in polypropylene boxes and stored in a cold place (4 °C) until transferring to the laboratory. In order to prepare the sediment samples for analysis, after the homogenization, they were frozen and then freeze-dried at − 40 °C for 48 h (CHRIST, Alpha 2-4 LD plus, Germany). Following the freeze-drying procedure, the samples were sieved and the fraction less than 63 μm was used for trace metal analysis. Before the analysis, the prepared sediment samples (0.2 g) were digested in a closed vessel microwave digestion system using 5 ml of HNO3 (> 68%, Fisher Chemical), 2 ml HF (47–51%, Carlo Erba), and 2 ml of H2O2 (> 30%, Fisher Chemical). The microwave digestion was performed in two steps: first with these reagents, and the second, after adding 10 ml of 4% (w/v) H3BO3 (99.97%, trace metals basis, Sigma-Aldrich). The digested samples were diluted (25 ml) using Milli-Q water and for each batch of analysis, two blank digests were prepared in the same way. This is an advanced, but routine technique (CCME 2001), with improved detection limits and the accuracy, primarily because of very low possibility of external contamination, as well as because of shorter time of sample preparation and smaller quantities of acids (Valeria et al. 2003).

In 2005 and 2006, the mineralized samples were analyzed for Cu, Ni, Fe, Mn, Cr, Pb, and Zn content in nine collected samples by flame (F-AAS, Perkin–Elmer, AAnalyst 200) and graphite furnace technique (GFAAS, Perkin–Elmer, 4100ZL) of an atomic absorption spectrometer, while later on, the same elements were measured in 46 samples by an inductively coupled plasma optical emission spectrometer (ICP-OES, Spectro Arcos). Also, in 2005 and 2006, Hg levels were determined by a cold vapor atomic absorption spectrometer (CVAAS, Perkin–Elmer, AAnalyst 200), and in the following years, a direct mercury analyzer (Milestone, DMA-80) was used. The concentrations of As were determined following a CVAAS procedure, using a Perkin–Elmer Hydride System coupled to an atomic absorption spectrometer (AAS), while Cd contents in the sediment samples were determined by a graphite furnace atomic absorption spectrometer, first Perkin–Elmer, 4100ZL (in 2005 and 2006) and then Agilent Technologies, GTA 120 Graphite Tube Atomizer. The obtained results of the investigated elements in sediment were expressed in mg kg−1 of sample dry weight (dw). The accuracy of the analytical procedure was checked using the certified reference material, IAEA 158 (Marine sediment), which was also digested and analyzed along with the samples. The recovery rates for heavy metals in the standard reference material ranged between 85 and 110%. In order to determine the precision of the analytical processes, the samples were prepared and analyzed in triplicate.

Statistical analysis

The correlation coefficients between the average levels of the investigated elements in the surface sediments were calculated by Pearson correlation analysis in Statistica software (Data Analysis Software System, v.10.0, StatSoft, Inc., Tulsa, OK, USA). Cluster analysis (CA) was performed to classify the samples of bottom sediments by grouping all the investigated elements in a dendrogram, which can show the similarities and/or dissimilarities of the bottom sediments in terms of their chemical composition.

Assessment of heavy metal contamination

Contamination factor

The contamination factor and degree of contamination were used to determine the contamination status of the sediment in the present study. Contamination factor (Eq. (1)) was calculated according to Tomlinson et al. (1980),

$$ CF= Cmetal/ Cbackground $$
(1)

Dolenec et al. (1998) indicated that the distribution of major, minor, and trace elements in the Central and Southern Adriatic is dominantly related to the catchment geology, structural type of the sediment, and the prevalent currents, pointing the inflowing current from the Southern Adriatic as one of the two main sources of trace metal accumulation in the Central Adriatic. These two basins have the similar granulometric composition (Ilijanić et al. 2014) of the seabed and show the general trend of decreasing heavy metals concentrations towards the open sea (Dolenec et al. 1998; Rivaro et al. 2004). Considering the accumulation time and vicinity of investigated sites to the coastal anthropogenic sources, as well as the previous studies (Žvab Rožič et al. 2012), in this study, the average trace element contents in surficial sediments from the Southern Adriatic (Dolenec et al. 1998) were considered as the background concentrations for the majority of elements. Only for Cd background concentration, an average surficial sediment concentration from the Central Adriatic was used (Žvab Rožič et al. 2012). CF has four classification categories: CF < 1: low contamination factor; 1 ≤ CF < 3: moderate contamination factor; 3 ≤ CF < 6: considerable contamination factor; CF ≥ 6: very high contamination factor (Žvab Rožič et al. 2012).

Pollution load index

The Pollution Load Index (PLI) that refers to heavy metal concentrations was also used (Tomlinson et al. 1980). The Pollution Load Index (PLI) is obtained as concentration factors (CF) and gives a summative indication of the overall level of heavy metal toxicity in the particular sample (Barakat et al. 2012). The PLI of the specific location was calculated from the n-CFs that were obtained for all the metals using the Eq. (2):

$$ \mathrm{PLI}={\left({\mathrm{CF}}_1\times {\mathrm{CF}}_2\times {\mathrm{CF}}_3\times \cdots \times \mathrm{CF}n\right)}^{1/n} $$
(2)

PLI value higher than 1 suggests pollution existence, while lower than 1 indicates no pollution load.

Geo-accumulation index

A common criterion to evaluate the heavy metal pollution in sediments is the geo-accumulation index. Classification by geo-accumulation index allows mapping of a study area and discriminating different sub-areas according to their degree of contamination. Geo-accumulation index (Eq. (3)) proposed by Müller (1981) is used to determine metal contamination in sediments, by comparing current concentrations with preindustrial levels (Buccolieri et al. 2006) and can be calculated using the following formula:

$$ \mathrm{Igeo}={\mathrm{Log}}_2\ \left[\left(\mathrm{C}n\right)/1.5\ \left(\mathrm{B}n\right)\right] $$
(3)

where Cn is the measured concentration of the examined metal “n” in the sediment and Bn is the geochemical background concentration of the metal “n” (average surficial sediment contents—Southern and Central Adriatic) (Dolenec et al. 1998; Žvab Rožič et al. 2012). Müller (1981) distinguished seven classes of geo-accumulation index (Igeo) values (Müller 1981): 0 unpolluted, 0 to 1 unpolluted to moderately polluted, 1 to 2 moderately polluted, 2 to 3 moderately to strongly polluted, 3 to 4 strongly polluted, 4 to 5 strongly to extremely polluted, and > 5 extremely polluted.

Results and discussion

The concentrations of trace metals (Fe, Mn, Zn, Cu, Ni, Pb, Cr, Cd, As, and Hg) investigated in sediment samples collected during the period from 2005 to 2016 along the Montenegrin coast are summarized in Table 1 and Table S1 (Electronic Supplementary Material). Considering all the investigated metals and sampling sites of this study, the obtained mean values decreased in the following order: Fe > Mn > Zn > Cu > Cr > Ni > Pb > As>Hg > Cd. Based on the obtained values, we cannot single out the location with the highest content of the examined elements, but some locations were distinguished by a significantly higher concentration of certain elements compared to other locations within the investigated period, which is presented in Fig. 2. Comparison of trace metal concentrations in surface sediments from the Montenegrin coast with values used to evaluate natural levels of these elements (Dolenec et al. 1998; Žvab Rožič et al. 2012) showed that concentrations of certain trace metals in sediments of the Montenegrin coast were significantly higher than those background levels. For example, at the location S3, concentrations of Fe in 2014 and Mn in 2016 had the maximum recorded values of 45.5 × 103 mg kg−1 and 1139 mg kg−1, respectively (Fig. 2). High values of metals, compared with the natural levels, especially Fe (Dolenec et al. 1998) at this site, can be explained by wastewaters and leaching from the mainland, which is typical of the area of Boka Kotorska Bay (Joksimović et al. 2011). Regarding the values obtained for these elements during the investigated period, it can be noted that at some locations, such as S9 and S10, and S7 in particular, Fe contents increased in the period 2014–2016 in comparison with 2005 and 2006, as well as Mn contents at S9. However, Mn content decreased at the location S11 in 2016, compared with its content in 2005 and 2006. Also, at the locations S2 and S3 a continuous decrease of Fe concentration was noted from 2014 to 2016, as well as decrease of Mn content at S2 in the same period (Fig. 2). The highest concentrations of Cr were found in sediment samples at the location S6 in the summer of 2010 (369 mg kg−1) and at S2 in the summer of 2011 (354 mg kg−1). Besides these extreme concentrations and also high Cr contents at S6 in 2011 and 2016, at different single locations, the concentrations of Cr were similar during the investigated years. This was particularly the case in 2010 and 2011, when Cr contents at all investigated sites were quite similar, while during the period of 2014–2016, the contents were generally higher, although there were some new study sites (Fig. 2). Thus, the increased Cr concentrations, in comparison with the natural levels, were also measured at the site S3 during 2014–2016 (Dolenec et al. 1998). The highest concentration of Cu (2719 mg kg−1) was found at the site S7 in the summer of 2010. At the site S6, a high Cu concentration was also found in both seasons of 2010 and in the summer of 2011, while in the fall of 2011 and therefrom, the concentration decreased. The concentrations of Zn, Pb, and Cd were the highest at the station S10 during 2011, with the values 1597, 756, and 5.43 mg kg−1, respectively. The maximum concentrations of these elements during 2011 were several times higher than the natural levels, but their concentration decreased significantly during 2014, 2015, and 2016 (Fig. 2). Elevated levels of trace metals in the aquatic systems are mainly caused by the effluents from the urban areas. Different factors, such as municipal runoff and domestic and industrial effluents and also the atmospheric deposition, contribute to high Cd, as well as Cu levels (Khan et al. 2017). The concentration of As was the highest at S5 (39.1 mg kg−1) during 2014, while the maximum value for Hg was recorded at the same location during 2016 (14.2 mg kg−1). As contents were also high at S6, as well as at S10 and S2 in 2011, but the concentrations decreased during the period 2014–2016. Regarding Hg temporal trends, beside the extreme values at S5, other concentrations were quite similar, with only noticeable decrease of Hg contents at S1 in the period from 2014 onwards in comparison with 2010 (Fig. 2). High concentrations of As, but also Zn and Cu (in comparison with the natural levels), are often the consequence of the anthropogenic pollution. Also, due to the human activities, Hg contents are enriched by at least a factor of 3 in the biosphere, and Hg can easily reach the offshore regions through wet and dry atmospheric deposition (Mason et al. 2012). During the study period, the highest Ni concentrations were recorded at the location S11 and the maximum concentration was found in sediment sample in 2006 (267 mg kg−1) (Fig. 2). In summary, for the majority of the elements, increase in concentrations was noticed over the years, i.e., from 2010 onwards, in comparison with the results from 2005 and 2006. However, it should be noted that the number of sampling sites increased in the inner Bay from 2010, which may cause the apparent increase in concentrations. Average concentrations of Fe, As, Cu, Cd, Hg, Pb, and Zn in surface sediments from the Montenegrin coast were higher than those in surface sediments from the Southern Adriatic Sea (Dolenec et al. 1998; Ilijanić et al. 2014; Komar et al. 2015), while concentrations of Mn, Cr and Ni were similar or even lower than the average concentrations in surface sediments of Southern and Central Adriatic (Dolenec et al. 1998; Žvab Rožič et al. 2012). However, except for the extreme maximum concentrations of some elements, specific for one or two samples, our results were in the same range as the results obtained for the different parts of the Adriatic (Kljaković-Gašpić et al. 2009; Acquavita et al. 2010; Joksimović et al. 2011; Tanaskovski et al. 2014) and the Mediterranean, i.e., West Mediterranean, Barcelona, Spain (Guevara-Riba et al. 2004); Izmit Bay, the Marmara Sea, Turkey (Pekey 2006); the Tyrrhenian Sea, Southern Italy (Sprovieri et al. 2007); Saros and Gökova Gulfs, and Aliağa Bay, the Eastern Aegean Sea, Turkey (Uluturhan 2010; Neşer et al. 2012).

Table 1 The concentrations of trace metals in surface sediment for all sites (mg kg−1 dw)
Fig. 2
figure2

Change of metal concentrations in surface sediments during the investigated period

The results also show that the mean heavy metal concentration in the sediment of the 11 stations followed the order: S3 > S6 > S2 > S11 > S4 > S1 > S5 > S7 > S10 > S9 > S8. In general, higher metal contents were distributed in Boka Kotorska Bay sites. This is quite different from the results obtained in previous research conducted along the whole Montenegrin coast over the period of 2 years, i.e., the fall of 2005 and the fall of 2006 (Joksimović et al. 2019), when two locations with the highest metal contents were both outside of the Bay and one of them was Ada Bojana (S11 in this study). Unlike the previous study, this study includes results obtained during one whole decade, which is more relevant in terms of contamination assessment for the area. The Montenegrin coastal area still receives a heavy influx of sewage and effluents from industry, ports, and shipping area, as well as domestic and agricultural wastes, all of which contain various hazardous chemicals. Although some of the sampling sites changed their purpose over time and luxury resorts replaced some heavy industry and shipyard spots, the repercussions of that pollution are still visible. Additionally, tourism and recreational activities in the coastal area further pollute the Montenegrin coastal waters (Joksimović et al. 2011).

The calculated concentration factor values were greater than 1 for the majority of the investigated elements. This indicates enrichment by either natural processes or anthropogenic influences (Pekey 2006). The CFs for the 10 metals and metalloids in different samples show a great variety ranging from 0.001 and 0.009 (the lowest values for As and Pb) to the extreme values for Zn, Pb, Cd, Cu and Hg. The highest CF values, indicated the most extreme contamination (Tomlinson et al. 1980), were found at the location S5 in 2016 for Hg (107), at S10 in 2011 for Cd (77.6), Pb (68.7) and Zn (21.0), and at the location S7 in summer of 2010 for Cu (78.4). However, the increasing order of CFs, in general, is as follows: Mn < Ni < Fe < Cr < As<Zn < Cu < Hg < Pb < Cd. High values of the CFs (> 1) for all the measured metals, except for Fe, Mn, Ni, and Cr, can be the consequence of the mechanic activities such as indiscriminate disposal of metal-containing compounds, including used engine oil (Ololade 2014). Nevertheless, average CF values for all the locations for the whole investigated period showed that this area was highly contaminated only with Cd and Pb (average CF ≥ 6), although considerable contamination factor was also measured for Zn, Cu, and Hg (3 ≤ CF < 6).

The pollution load index values are presented in Fig. 3a. While computing the pollution load index (PLI) of sediments of the studied region, the average trace metal contents for the surficial sediments from the South and Central Adriatic were used as the background values (Dolenec et al. 1998; Žvab Rožič et al. 2012). As a convenient measure of geochemical trends, PLI is used for comparison among areas. The PLI values, which in this study ranged from 0.09 to 5.16 (Fig. 3a), confirmed that coastal Montenegrin bottom sediments are generally polluted with metals. Locations with the highest PLI values in surface sediments were locations S6 and S10, while locations S8, S9, and S11 had PLI values lower than 1 in all investigated years. Low PLI values at these locations implied no appreciable input from anthropogenic sources.

Fig. 3
figure3

Box plot for the pollution load index (a) and geo-accumulation index (b) of surface sediments

The geoaccumulation index results indicated that the surface sediments were not contaminated with Fe and Mn. According to the results for Ni, Cr, and As, the entire investigated area was also practically unpolluted, except location S11 for Ni, as well as S2 and S6 for Cr, which generally were unpolluted to moderately polluted. Regarding the Igeo values for As, only locations S5 and S6 were classified as unpolluted to moderately polluted or moderately polluted (Fig. 3b). In summary, Igeo values suggested that the investigated sites were mainly grouped as unpolluted or unpolluted to moderately polluted. Extreme values were found only for Zn, Cu, Pb, Cd, and Hg for different single samples at some locations. According to this, only the location S10 was characterized as strongly polluted with Zn in the summer and fall of 2011 (with the calculated Igeo values of 3.42 and 3.81, respectively), as well as extremely polluted with Pb (5.52 and 5.44) and Cd (5.69 and 5.49) in the same period, respectively. The location S7 was also characterized as extremely polluted with Cu in one single sample (5.71) in the summer of 2010 (Fig. 3b). Regarding sediment Hg contamination, only 1 out of 11 locations was strongly or extremely polluted, with the Igeo values between 3.78 and 6.15 for all the analyzed samples (location S5), whereas the rest (10 locations) were unpolluted to moderately to strongly polluted. However, Igeo values for Pb, Cd, Cu, and Zn classified the entire investigated area in the range from unpolluted to strongly polluted, where the majority of samples were unpolluted or unpolluted to moderately polluted with Cu and Zn. The contents of Cu, Zn, Pb, Cd, and Hg have been shown to be increased as a result of anthropogenic activities in the area (Mucha et al. 2003; Çevik et al. 2009). In addition, industrial activities, trading, and traffic in ports, the use of anti-fouling paints for ships and boats, aquaculture, and nautical tourism, together with insufficient wastewater treatments, pose a threat to the marine life in the study area (Neşer et al. 2012).

To investigate the common characteristics of metals in the sediments from the Montenegrin coast, correlation analyses between metals were calculated. Correlations between Fe with Mn, Zn, and Ni, as well as Cu with Zn and Cd were significant at the p < 0.05 probability level, as shown in Table 2. Very significant correlation was also recorded between Pb and Zn (0.94). These correlations between metal concentrations suggested either a common or a similar geochemical behavior or origin.

Table 2 Pearson’s correlation matrix between metals in the sediments of Montenegrin coast

Cluster analysis (CA) of the metal data was performed to explore the grouping of trace metals in bottom sediments at Montenegrin coast. It is shown as a dendrogram in Fig. 4, where the metals were grouped into three main clusters. Cluster 1 consisted of Fe–Mn, cluster 2 consisted of Hg–Cd, while cluster 3 was composed of As–Cu–Pb–Zn–Ni–Cr. The metals in each cluster had similar characteristic features and possibly similar inputs. Strong similarity between Fe and Mn contents showed that relationship of these metals come from the same origin and indicated naturally high concentrations of these elements in the surface sediments. However, strong similarity of Cu, Zn, and Cr contents, which are naturally present in relatively high concentrations, with As, Pb, and Ni indicated strong anthropogenic input. This is probably due to the domestic and industrial effluents, from which metals are mainly precipitated as soluble oxides, but the source of these metals might also include a fuel combustion during vehicle exhaust or metals smelting (Khan et al. 2017). Also, high concentrations of As, Pb, Cu, Zn, and possibly Cr and Ni, can be found in marine service areas, such as primarily S5, S6, and S10. Elevated contents of these elements are often the consequence of the procedure that removes old paint layers from the boat hulls (Obhođaš and Valković 2010). Elements with the lowest contents (Hg and Cd) were parts of the same separated cluster. Since the highest contents of these elements were noticed at the location S5 for Hg and S10 for Cd, their content was probably the consequence of the harbor activities, municipal and industrial effluents, and the pollution from the past industrial activity in the case of the location S5, where the military shipyard is now replaced with the luxury marina (Hortellani et al. 2005; UNEP 2013; Khan et al. 2017).

Fig. 4
figure4

Metals cluster analysis dendrogram for sediment samples

The study points out that although there were slight variations in the results of the two indices, the combination of the Igeo and PLI gave us a comprehensive understanding of heavy metal risks in the bottom sediments of the Montenegrin coast. The high correlation between Fe and Mn suggests the presence of Fe/Mn compounds (Zabetoglou et al. 2002; NAVFAC 2003). The environmental occurrence of Cd can be attributed to its considerable usage in electroplating and the manufacture of plastic stabilizers and batteries (Eisler 2007; Li et al. 2017). Elements such as Pb and As can be attributed to the atmospheric deposition (marine traffic, vehicle emissions, and oil combustion), as well as the usage of anti-fouling paints (Liu et al. 2016; Obhođaš and Valković 2010), while Hg content in the sediment could be related to the coastal discharge (Liu et al. 2016).

Conclusions

Comparing the results obtained for 10 selected heavy metals (Cu, Ni, Fe, Mn, Cr, As, Pb, Zn, Cd, and Hg) in 55 bottom sediment samples, collected from 11 locations along the Montenegrin coast during the period 2005–2016, it can be concluded that the anthropogenic impact and geographical location are probably the main factors contributing to the differences observed for these samples. The metal contents in sediment samples in 2010 and 2011 were generally higher than in other years of research. Also, on average, the concentrations of trace metals in the sediment of Boka Kotorska Bay were higher in comparison with some locations at the open area of the Montenegrin coast, although the extreme values were also recorded at the locations outside of the Bay. High concentrations of trace metals at any site depend on feasibility and availability of respective pollution sources. PLI showed that the investigated area can be generally classified as polluted, while Igeo determined the area as mainly unpolluted or unpolluted to moderately polluted. From PLI, the extreme value was recorded at locations S6 and S10 (5.16 and 5.06, respectively), while the values at other locations were lower. The extreme Igeo values for Cu, Pb, Cd, and Hg (extremely polluted) were found in only one (for Cu at location S7) or two (Pb and Cd at location S10, and Hg at location S5) single samples out of 55 samples. Cluster analysis grouped all the investigated elements into three major clusters. Higher concentrations of metals were found near urban areas, harbors, and marinas, revealing that their concentrations had been strongly affected by anthropogenic influences.

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Funding

This study has been supported by the Environment Protection Agency of Montenegro and Regional TC project (RER 7009).

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Joksimović, D., Perošević, A., Castelli, A. et al. Assessment of heavy metal pollution in surface sediments of the Montenegrin coast: a 10-year review. J Soils Sediments 20, 2598–2607 (2020). https://doi.org/10.1007/s11368-019-02480-7

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Keywords

  • Montenegrin coast
  • Pollution indexes
  • Sediment
  • Trace metals