Coastal environments are often environmentally burdened by port and traffic infrastructure, large industrial areas, urban settlements, and tourism activities. Thus, the flux of various anthropogenic contaminants, especially heavy metals, are discharged and accumulated into marine coastal ecosystems [1
]. Heavy metals are among the most persistent pollutants in the aquatic ecosystem, and they are not nullified from water by self-purification [6
]. Sediments are a significant repository for heavy metals present in the upper water column and, in addition, they act as a secondary source. Due to a series of physical and biochemical processes (e.g., diagenetic, bioturbation, and resuspension processes, currents and waves, dredging, and shipping activities) the remobilisation of heavy metals occurs at the sediment water interface, affecting overlying water, aquatic organisms, and threatening human health [7
The appearance and loading of heavy metals in vulnerable marine environments originates from both anthropogenic and natural processes, e.g., geological background [10
]. A precise designation of anthropogenically/geogenically derived heavy metals is of importance in the evaluation and prevention of the pollution burden, and in planning the appropriate remedial action. Further on, the mobility potential and bioavailability of heavy metals in the surrounding environments is heavily dependent on their specific chemical forms and type of bindings.
The geochemical characteristics of sediment from the Gulf of Trieste (North Adriatic Sea, Italy) have been studied in detail by several groups [13
] over a significant period of time. Few studies, however, have been conducted on the adjacent Adriatic sediments of the Koper Bay, especially from the Port of Koper [22
]. Thus, precisely defined anthropogenic/geogenic character, heavy metal mobility potential, the application of sediment quality guidelines (SQG), and eligible statistical analyses in Koper Bay sediments, was undertaken for the first time.
Koper/Capodistria is a small coastal city located in Slovenia´s Mediterranean region. Due to its geographic location, it is influenced by various anthropogenic contamination sources from an international port—the Port of Koper, an industrial zone with a chemical factory, agriculture, and viticulture developed in the hinterland of the city. The port is one of the biggest and most important ports in the Northern Adriatic Sea and, thus, represents a major contaminant source to sediments found in the Koper Bay. Additionally, in the framework of the Global Environmental Facility (GEF) project, sediments from the Koper Bay have already been marked as a contamination “hot spot”. Due to massive cargo and intense port activities, we have also included, in our study estimating heavy metals’ anthropogenic impact, Mo, Au, and Sb. Mo can be introduced into the environment through its presence in steel, catalysts, dyes, lubricants; (2) raw Au gold represents an urgent material for jewellers; and (3) Sb is type-metal alloy (with Pb to prevent corrosion); we can find it also in electrical applications, semiconductors, flameproof pigments, and glass [25
In this context, the main objectives of the present study were as follows: (1) to determine detailed mineralogical and elemental content in the surface sediments of the Port of Koper area; (2) to estimate the degree and distribution of heavy metal contamination in the sediments using contamination indices, sediment quality guidelines (SQGs) and various methods of statistical analysis; and (3) to define the possible natural and/or anthropogenic sources and pathways of heavy metals, as deduced from a sequential extraction procedure.
3. Results and Discussion
The surface sediment in this study varies widely from clay to sand, with an overall dominant silt composition (Figure 4
). There was no spatial difference noted and, thus, the samples are defined as fine sandy medium silt. This composition character was also found in the open area of the Koper Bay [22
Total C content in the samples studied varies from 3.40% to 4.43% (Figure 5
), and total S concentrations are quite low (0.28–1.08%) (Figure 6
), which is in accordance with Faganeli et al. [22
The mineral composition of sediment from the Port of Koper (Figure 7
a) is comprised of high quantities of quartz, calcite, muscovite/illite, followed by albite, dolomite, chlorite, halite, and pyrite (Table 2
). The above mineral association reflects the geological lithology of the Koper hinterland (e.g., carbonate and flysch) [24
]. Minerals halite and pyrite form “in situ” as authigenic minerals. The XRD analysis (Figure 7
b) showed trace quantities of kaolinite and interstratified smectite/illite layers present.
The concentrations of SiO2
, MgO, CaO, Na2
, MnO, and Cr2
, detected in the sediment samples, are summarised in Table 3
. The abundance of the major oxides in the sediment samples is very similar, especially for Na2
, MnO, and Cr2
. These data correspond to results found in surface sediments from the open Adriatic Sea by Dolenec et al. [44
]. As expected, major element values also correspond to the sediment mineralogy.
Concentration ranges for heavy metals in the sediment samples, with reference values, are included in Table 4
. There are no significant differences in the metal concentrations between samples and no spatial distribution pattern is apparent for any of the metals along the individual sampling transects.
The Pearson correlation matrix results (Table 5
) assigned a highly significant association between As and Sb, with a SAKFMP component and a positive correlation between Co, Cu, Ni, and Pb, with a SAKFMP component. This relation clearly points out the geogenic origin of As and Sb, and the slightly anthropogenic relation of Co, Cu, Ni, and Pb to sulphides, e.g., pyrite. Also, there was a significant relation noted between As and Sb, and between Co, Cu, Ni, and Pb, showing their common origin and geogenic/anthropogenic pathway. There were no other significant correlations recorded, especially for clay, silt, Mo, Sn, Zn, and TOT/C component. We attribute this to the fact that all values/data cannot present a very strong correlation, due to their similarity.
Sediment EF values around or lower than 1.0 indicate that the element in question originates predominantly from crustal material and/or weathering processes [45
], whereas EF values greater than 1.0 reflect some level of anthropogenic contamination of the metal [46
]. More recently, six contamination categories have been interpreted as suggested by Chen et al. [47
EF < 3 minor enrichment (anthropogenic impact)
EF = 3–5 moderate enrichment
EF = 5–10 moderately severe enrichment
EF = 10–25 severe enrichment
EF = 25–50 very severe enrichment
EF > 50 extremely severe enrichment
Enrichment factor calculations revealed that As, Cd, Cu, Mo, Pb, Sb, and Zn exhibit the lowest EF values among the heavy metals (Figure 8
); these six elements are thus only minorly enriched in the sediments from the Port of Koper. The calculated EF value for Ni signified a moderately severe enrichment. The highest overall EF values were determined in samples from locations 1A–1G (Cu, Pb, Ni, Au, and Sb) and 3A–3C (Mo, Zn, As, and Cd). Ni originates from geogenic sources, i.e., the Eocene flysch basin from inland [24
]. An average estimated amount of Ni in fine-grained clastic rocks is 60 mg kg−1
]. Eocene flysch basin is composed of carbonates and clastic rocks, especially siltstones and mudstones [24
Additionally, the obtained results were compared to consensus-based sediment quality guidelines (SQGs), defined by [43
] (Table 3
). A comparison of sediment heavy metal concentrations with the consensus-based TEC and PEC values revealed that only mean concentrations of Ni are higher than both the TEC and PEC special values. As a result, the biota of the Koper Bay may currently be in danger of being contaminated with this element.
The EF and SQGs results suggest that ship traffic, the variety of cargo and the activity of the nearby chemical factory contribute to the anthropogenic loading of Ni in the Koper Bay area. However, the results of EF values and comparison with SQGs represent preliminary information regarding the levels of anthropogenic pressure on surface sediments from the Port of Koper.
PCA (Figure 9
) accounted for 62.5% of data variance in the first two ordination axes, revealing highly significant positive correlations in samples from location 1 between Sb, Pb, Cd, Cu, and Ni present in clay fractions. These heavy metals are generally incorporated into kaolinite lattice and fine carbonate mud with low content of organic matter. Samples from locations 2 and 3 showed elevated concentrations of Zn, As, pyrite, and dolomite. As is closely related to pyrite mineral. Samples 3a, 3b, and 3c displayed elevated values of Mo and Sn due to increased TBT (tributyl tin) sources. Quartz, plagioclase, chlorite, and muscovite/illite originated from flysch geological background. Fe, K, and Al were found in the silt fraction, which is prevailing in location 3.
An important and final insight defining heavy metal pathways (bioavailability) and binding forms was decoded with the results of a sequential extraction procedure. The relative proportion in percent (%) obtained for each metal in different extracted fractions is illustrated in Figure 10
. As displayed, similar proportions of each element were partitioned in respective fractions in all samples.
In the sequential leaching experiment, the labile/residual fractions considered were as follows: water-soluble fraction (1), exchangeable fraction (2), oxidisable fraction (3), reducible fraction (4), and residual fraction (5). Invariably, the data showed that the proportion of heavy metals fractionated in the residual fraction clearly dominated over those in the non-residual fractions.
Further, relatively minor trace amounts of As, Cd, Cu, Ni, Pb, Sb, and Zn (range from 0.01 to 4%) in the sediments were measured in the water-soluble fraction (1). However, Mo contents in this fraction were significantly higher (up to 24%, Figure 6
). Mo becomes very mobile in neutral to alkaline conditions [50
], and Mo adsorption on solid components is promoted with decreasing pH values [25
]. It is deduced that the high proportion of Mo (Figure 10
) in the water-soluble fraction of the samples indicates a greater potential ecological risk compared to other heavy metals.
Similar proportions of all elements studied were detected in the exchangeable fraction (2) (ranging from 0.2 to 5.8%, Figure 10
), and there was no significant difference noted between the samples [52
The oxidisable fraction (3) of As, Mo, Ni, and Zn counts approximately for 1%–5%, followed by 6%–10% for Cd, Pb, and Sb. Up to 10.1% of Cu was measured in the oxidisable phase (Figure 10
), and additionally, other studies [52
] have also reported an association between Cu and the oxidisable fraction, occurring as an organically complex metal species. Cu shows a high affinity with humic substances, which are a fraction of the natural organic matter chemically active in complexing such metals [57
For the reducible fraction (4), Pb was the most prominent element (up to 50%). Pb can form stable complexes with Fe and Mn dioxide [58
] and, accordingly, the highest proportion of Pb was found in the reducible phase. The reducible Pb predominating in the non-residual fractions has been reported by many studies [52
]. The highest non-residual proportions were also determined for As, Cd, Ni, and Zn (Figure 10
). As is often linked to amorphous Fe and Mn oxides [54
], while Ni and Zn are usually linked to organic matter and sulphides in the oxidisable phase [52
]. This could be connected with lower TOC values [22
] and the prevailing sediment mineralogy of the Koper Bay (silt and clay prevail).
Cd speciation, as a typical anthropogenic element, is defined as highly variable, and it mostly enters into the aquatic environment through the discharge of industrial effluents [61
]. On the other hand, a large proportion of Cd deposited in sediment can be remobilised as a result of the mineralisation of organic matter under oxic conditions [61
The residual phase (5) comprises metals strongly bound within aluminosilicate minerals (crystalline lattices) [62
] and they are, therefore, unlikely to be easily released in the aqueous phase under natural conditions.
More than 50% of As, Cd, Cu, Mo, Ni, Sb, and Zn (Figure 10
) were measured in the residual phase, indicating a strong association within crystal lattices and, for this reason, they represent a geogenic character from a geological background with (1) no observable ecological risk to the surrounding ecosystems and (2) low pollution levels in sediments.
Metal speciation in sediments is of critical importance, indicating their potential toxicity and mobility to the surrounding ecosystems [63
]. The “Risk Assessment Code” (RAC) has been used to assess the potential mobility and hazard of heavy metals based on the percentage of heavy metals in the water-soluble (1) and exchangeable (2) fraction. These fractions are considered to have weakly bounded metals that equilibrate with the aqueous phase and, thus, become rapidly bioavailable to the surrounding ecosystems [53
According to calculated RAC values (%) (fraction (1) + fraction (2)), there are the following risks: <1%, no risk for the aquatic system; 1–10%, low risk; 11–30%, medium risk; 31–50%, high risk; and >50%, very high risk [64
]. The RAC values are listed in Table 6
. The results showed that the RAC values for Pb posed no risk; for As, Cd, Cu, Ni, Sb, and Zn, they exhibited a low risk, and Mo was defined as a medium ecological risk.
Although the mean concentrations of Ni are higher than the TEC and PEC special values, the calculated RAC values implied its low ecological risk. We can conclude that Ni origin in sediments studied is generally geogenic (flysch deposits from geological background, [24
]. Medium ecological risk values for Mo indicated that Mo could be treated as anthropogenically imported, in particular, due to various cargo ship activities, the application of artificial fertilisers in agriculture and untreated domestic wastewater from the nearest coastal cities [25
]. However, total Mo concentrations measured in the sediment samples are very low (mean values 1.1 mg kg−1
), and its extraction values in water-soluble (1) and exchangeable (2) fraction are measured in parts per billion. We could not evaluate the abovementioned fact as a result of anthropogenic impact, though, we point out that Mo mobility potential, in comparison with other heavy metals, is very high.
Additionally, the Port of Koper has been an extremely anthropogenically burdened area for a long period of time and, generally, there is no aggressive anthropogenic heavy metals enrichment detected in the studied sediments.
An abundance of As, Cd, Cu, Mo, Ni, Pb, Sb, and Zn, and its environmental impact, were determined for surface sediment samples from the Port of Koper (Koper Bay, Adriatic Sea, Republic of Slovenia). The mineral association, quartz, calcite, illite/muscovite, albite, dolomite, and chlorite reflects the lithology of the source hinterland area.
Total concentrations of heavy metals exhibited a relatively slight spatial variation in the surface sediments of the Koper Bay, indicating mainly nonpoint source inputs. The environmental indices (EF) results showed that samples were marginally enriched with Cd, Cu, Mo, Pb, Sb, and Zn, moderately to severely enriched with Ni. A comparison of sediment heavy metal concentrations with the consensus-based TEC and PEC values pointed out that the mean concentrations of Ni are higher than both special values.
From the sequential chemical extraction experiment results, one can deduce that the majority of heavy metals (As, Cd, Cu, Mo, Ni, Sb, and Zn) can be considered immobile because of their high percentages (more than 50%) measured in the residual phase, suggesting that they are strongly bound to the crystal lattice of minerals. These metals have minimal anthropogenic derivative, and would likely be immobile from sediments (for bioavailability) due to environmental changes in pH and/or Eh.
By contrast, the highest proportion of Pb was found in the reducible phase and Mo was, relatively speaking, the heavy metal most easily extractable in the water-soluble fraction, which indicates adverse vulnerability to the resident biota.
The heavy metals’ potential risk to the surrounding environments as defined by “Risk Assessment Code” values exhibited generally no or low risk for all heavy metals, except for Mo. However, total Mo concentrations in the sediments and in water-soluble and exchangeable fractions are below alarming levels.