3.1. Assessment of the contamination level of bottom sediments in the Uglovoy Bay relative to background concentrations and normative values
Average concentrations of heavy metals in bottom sediments of the study area are presented in Table 1. Background values for the Uglovoy Bay (a conditionally clean coastal water area of the Peter the Great Gulf) were taken from published data (Kalinchuk et al. 2010; Shulkin 2004).
As shown in Table 1, during the period from 2016 to 2021, the concentrations of heavy metals at almost all sediment sampling stations exceeded the background values, indicating a significant anthropogenic load on the Uglovoy Bay. The exceptions were several sampling stations in September 2016 (Zn and Pb content), in February 2017 at the exit from the bay near the low water bridge (Pb content), and in 2021 in the central part of the bay (Hg content). For the entire study period, the highest values exceeding the background levels were observed in February 2016. Between 2017 and 2021, the contamination of the bay relative to the background decreased significantly.
Table 1
Average concentrations of heavy metals (mg/kg dry weight) in bottom sediments of the Uglovoy Bay in 2016–2021.
Month/ Year | Fe | Mn | Cd | Cu | Ni | Со | Zn | As | Pb | Cr | Hg |
02/2016 | 30063 | 417 | 3.45 | 353.2 | 75.3 | 97.5 | 293 | 36.8 | 47.1 | 57.3 | 0.30 |
09/2016 | 53985 | 828 | 0.57 | 18.7 | 34.1 | 14.6 | 35 | 12.5 | 10.4 | 77.1 | 0.09 |
02/2017 | 46716 | 536 | 0.64 | 18.7 | 30.6 | 24.8 | - | - | 12.8 | - | 0.08 |
05/2019 | 40338 | 294 | 0.91 | 28.4 | 28.8 | 15.8 | - | - | 24.2 | - | 0.08 |
10/2021 | 38812 | 370 | 0.74 | 25.7 | 37.2 | 14.3 | 46 | 13.0 | 27.9 | 71.5 | 0.03 |
back ground | 15000 | 100 | 0.10 | 10.0 | 10.0 | 8.0 | 40 | 1.5 | 10.0 | 7.0 | 0.02 |
threshold concentrations | - | - | 0.80 | 36.0 | 35.0 | 20.0 | 140 | 29.0 | 85.0 | 100.0 | 0.30 |
TEL | - | - | 0.68 | 18.7 | 15.9 | - | 124 | 7.24 | 30.2 | 52.3 | 0.13 |
PEL | - | - | 4.21 | 108.0 | 42.8 | - | 271 | 41.6 | 112.0 | 160.0 | 0.70 |
Note: "-" – no data; background concentrations from (Kalinchuk et al. 2010; Shulkin 2004); TEL/PEL (Macdonald et al. 1996); concentrations exceeding threshold values are in bold face (Warmer and van Dokkum 2002). |
According to the normative documents, bottom sediments can be compared with international standards, for example, with threshold values in the Neue Niederlandische Liste (Warmer and van Dokkum 2002). The content of heavy metals in bottom sediments of the Uglovoy Bay compared to the threshold values is significantly lower than compared to the background level (Table 1). Maximum values exceeding the threshold values were observed in February 2016 for Cd, Cu, Ni, Co, Zn, As, and Hg. From September 2016 to 2021, the content of heavy metals in the sediments in the major part of the study water area did not exceed the threshold values. The exceptions were bottom sediments at several stations: in September 2016 for Ni (1.05–1.22 threshold), in 2017 for Cd (1 threshold), Ni (1.02 threshold), and Co (1.40–1.46 threshold), in 2019 for Cd (1.09–1.46 threshold), Ni (1.02 threshold), and Co (1.20 threshold), and in 2021 for Cd (1.20 threshold) and Ni (1.05–1.08 threshold).
It should be noted that the Neue Niederlandische Liste were developed based on the background concentrations of pollutants in the Netherlands and the adjoining water area. This is the only document regulating the assessment of marine sediments contamination in Europe. For the studied water area, however, it is more informative to assess the sediments contamination relative to the background concentrations in the Peter the Great Gulf.
To assess the ecological hazard of marine sediments, the obtained concentrations of pollutants can be compared with experimental data on the toxicity of these substances for marine organisms. Quality criteria TEL and PEL developed for marine surface sediments are well known. TEL/PEL are based on the statistical analysis of empirical data on the concentrations of pollutants that trigger various reactions in marine life (Vashchenko et al. 2010; Macdonald et al. 1996). TEL/PEL criteria are closer in their values than other similar criteria (Hübner et al. 2009). TEL is designed to assess the concentration of a chemical below which adverse effects are rare (i.e., threshold effect level). PEL is designed to estimate the concentration above which side effects occur frequently (i.e., probable effect level) (Macdonald et al. 1996).
According to the results of this study, in February of 2016, the content of pollutants in sediments was above TEL (Table 1). This fact suggests that at these concentrations of heavy metals, marine life occasionally suffer from toxic side effects. In terms of Cu, Ni, and Zn content, marine sediments exceeded PEL threshold (Table 1), which indicates that side effects occur frequently.
In September of 2016, Ni, As, and Cr content in marine sediments exceeded TEL. In 2017, Cd, Cu, and Ni content exceeded TEL. In 2019, Cd, Cu, and Ni content, and in 2021, Cd, Cu, Ni, As, and Cr content exceeded TEL. However, from September 2016 to 2021 concentrations of the analyzed polluting substances did not exceed PEL (Table 1). Therefore, there is a significant improvement of the environmental situation in the studied water area during this period.
3.2. Assessment of the degree of contamination of marine sediments in the Uglovoy Bay and their potential toxicity using various indices
The calculated values of the contamination factor (Cf) vary in a wide range from “low” to “very high”. The highest levels of bottom sediments contamination relative to the background (Cf > 6, “very high”) in decreasing order were obtained for Cu, Cd, As, Hg, Co, Cr, Ni, and Zn in February 2016; for Cr, As, and Mn in September 2016; for Cd in 2017 and 2019; and for Cr, As, and Cd in 2021 (Table 2).
According to the classification by Hakanson (Hakanson 1980), the degree of contamination (mCd) of marine sediments in the bay was “very high” in February 2016 and “high” in September 2016 and 2021. In 2017–2019, marine sediments in the Uglovoy Bay had the “moderate” degree of contamination (Table 2).
Table 2
Results of calculations of the contamination factor (Cf), the modified overall degree of contamination (mCd), the index of geoaccumulation (Igeo), and the sediment quality guideline quotient (SQG-Q) for the Uglovoy Bay for the period of 2016–2021.
Month/ Year | Cf | mCd | SQG-Q |
Fe | Mn | Cd | Cu | Ni | Со | Zn | As | Pb | Cr | Hg |
02/2016 | 2.0 | 4.2 | 34.5 | 35.3 | 7.5 | 12.2 | 7.3 | 24.6 | 4.7 | 8.2 | 15.0 | 14.1 | 1.13 |
09/2016 | 3.6 | 8.3 | 5.7 | 1.9 | 3.4 | 1.8 | 0.9 | 8.4 | 1.0 | 11.0 | 4.7 | 4.6 | 0.28 |
02/2017 | 3.1 | 5.4 | 6.4 | 1.9 | 3.1 | 3.1 | - | - | 1.3 | - | 4.0 | 3.5 | 0.25 |
05/2019 | 2.7 | 2.9 | 9.1 | 2.8 | 2.9 | 2.0 | - | - | 2.4 | - | 4.0 | 3.6 | 0.30 |
10/2021 | 2.6 | 3.7 | 7.4 | 2.6 | 3.7 | 1.8 | 1.1 | 8.7 | 2.8 | 10.2 | 1.4 | 4.2 | 0.31 |
Igeo |
02/2016 | 0.4 | 1.5 | 4.5 | 4.6 | 2.3 | 3.0 | 2.3 | 4.0 | 1.7 | 2.4 | 3.3 | 1–5 class |
09/2016 | 1.3 | 2.5 | 1.9 | 0.3 | 1.2 | 0.3 | -0.8 | 2.5 | -0.5 | 2.9 | 1.6 | 0–3 class |
02/2017 | 1.1 | 1.8 | 2.1 | 0.3 | 1.0 | 1.1 | - | - | -0.2 | - | 1.4 | 0–3 class |
05/2019 | 0.8 | 1.0 | 2.6 | 0.9 | 0.9 | 0.4 | - | - | 0.7 | - | 1.4 | 1–3 class |
10/2021 | 0.8 | 1.3 | 2.3 | 0.8 | 1.3 | 0.3 | -0.4 | 2.5 | 0.9 | 2.8 | -0.1 | 0–3 class |
Note: "-" – no data; in bold face: Cf > 6 (very high), 8 ≤ mCd < 16 (very high degree of contamination), SQG-Q > 1.0 – (highly toxic sediments), high probability of adverse biological effects and Igeo Class 5 (strongly to extremely contaminated sediments). |
Index of geoaccumulation (Igeo) in the Uglovoy Bay in February 2016 ranged from class 1 to 5 (from uncontaminated to strongly to extremely contaminated sediments). The maximum Igeo values were calculated for Cd and Cu, while the minimum values were for Fe (Table 2). In September of 2016, in 2017 and 2021, Igeo ranged from class 0 to 3, while in 2019 from class 1 to 3 (Table 2). Marine sediments were uncontaminated (class 0) with Zn (September 2016 and 2021), Pb (September 2016 and 2017), and Hg (2021) (Table 2).
The toxic effects of heavy metals on aquatic ecosystems range from the extinction of the living organisms to the minimum impact on the reproduction, growth and mortality of aquatic species (Hamed 2017). Heavy metals belong to the priority pollutants of marine ecosystems as they are known for their toxic effects on a variety of organisms. The list of priority pollutants of the marine environment in accordance with the Water Framework Directive of the European Union includes: Cd, Hg, Pb, Ni, as well as various types of hydrocarbon compounds (Warmer and van Dokkum 2002). In addition to direct exposure and toxic effects, Pb, Cd, and Cu bioaccumulate in all living organisms, including humans, animals and plants (Diaza et al. 2018). In addition, As and Cu have significant toxic properties.
The high content of heavy metals in the habitat of bivalve mollusks leads to their accumulation in biological tissues depending on their concentration. Marine flora shows the same response to high concentrations of heavy metals in the habitat (Kovekovdova et al. 2002). Studies for eel grass Zostera marina L. collected in the Uglovoy Bay indicate that the accumulation of Ni, Cu, Zn, Cd, and Mn exceed the concentrations of the range background (Savinok and Shishlova 2008; Chernova 2012). In addition, according to Chernova (2012), the reducing environment in bottom sediments contributes to the accumulation of mobile forms of Mn in the rhizomes of this plant.
Metals with physiological significance (Zn, Mn, and Fe) exhibit the greatest ability to accumulate in biological tissues, carapaces, and skeletons of aquatic organisms (Demina et al. 2009). On the other hand, in conditions of high turbidity, the filtering organisms that are widespread in the Uglovoy Bay are subject to significant accumulation of heavy metals.
To assess the potential toxic effect of accumulated heavy metals in the sedimentary layer of the bay, the sediment quality guideline-quotient (SQG-Q) was calculated (Table 2). The values of PEL were assumed as the main criteria. Concentrations of pollutants above this index most likely lead to negative biological consequences.
SQG-Q values calculated for February 2016 suggest highly toxic sediments with a high probability of negative biological effects. The cause of this is the high concentration of heavy metals in the sediments of the studied water area. Their content can be represented as a series in decreasing order relative to PEL: Cu > Ni > Zn > As > Cd > Pb > Hg > Cr.
From September 2016 to 2021, the sediment quality guideline-quotient (SQG-Q) values varied between 0.25 and 0.31 representing moderately toxic sediments with an average probability of adverse biological effects. Therefore, the probability of toxic effects decreased significantly.
Regardless of a significant improvement of the environmental situation in terms of heavy metals content in marine sediments during the study period, marine life in the Uglovoy Bay are still adversely affected by pollution. For example, according to Tishchenko et. al. (2021), underwater photography in 2019 and 2020 showed the absence of eel grass (Zostera marina L.) meadows in the Uglovoy Bay. A comparison of episodic studies of previous years (Rakov 2010; Savinok and Shishlova 2008; Chernova 2012) with the results presented by the authors (Tishchenko et al. 2021) indicates degradation of the bay ecosystem.
3.3. Analysis of the sources of contamination of marine sediments in the Uglovoy Bay
One of the main sources polluting the studied water area is the runoff of the Aerodromnaya, Peschanka, Ugolnaya and Saperka rivers which contain industrial and household effluents from the Artem and Vladivostok cities. Another source is the advection of water from the northern part of the Amur Bay influenced by the polluted runoff of the Razdolnaya River (Petukhov et al. 2019).
Contamination is also possible as a result of rain-wash from the industrial landscape and other sources. On the bay shore there are industrial buildings of the food industry, warehouses, railway and road infrastructure. Industrial activity in the coastal zone, as a rule, leads to pollution of the soil which largely determines the chemical composition of marine sediments in coastal waters.
In addition, bottom sediments in the Uglovoy Bay were greatly affected by the construction of the De-Friz-Sedanka low-water bridge between 2009 and 2012. A new four-lane highway now connects the De-Friz Peninsula and the Muravyov-Amursky Peninsula. Typically, hydrotechnical construction in the coastal zone is accompanied by a significant increase in turbidity and contamination with pollutants. The volume of suspended solids entering the sea can be compared with dumping and the annual runoff of large rivers (Mishukov et al. 2009). Further operation of the low-water bridge is also a source of heavy metals (Pb, Cu, Zn) entering the water area.
Before 2012, all untreated wastewater went into the sea. After the reconstruction of the treatment facilities in the Northern planning district in 2012, the volume of untreated wastewater decreased significantly. Despite this, pollutants were accumulated in bottom sediments for a long time. Together with their new inflow with river water and due to the operation of the low-water bridge, pollutants remained to have a negative impact on the water area of the Uglovoy Bay. As a result, according to data from February 2016, there was a significant contamination of marine sediments in the study area.
The main reason for the variation in the annual flow of rivers into the Uglovoy Bay is the monsoon climate. The annual arrival of tropical typhoons into the watershed of the Peter the Great Gulf is the main feature of the coastal climate in the Primorsky Krai. It is accompanied by a period of heavy rains, hurricane winds, and storms with the formation of surging waves, floods and high waters on the rivers (Astakhov et al. 2015). A typhoon in the shelf zone is accompanied by the removal of terrigenous sediments, as well as changes in the sedimentation regime. In September 2016, just before the sampling, Typhoon Lionrock hit southern Primorsky Krai. In 22 days, Vladivostok city received a total of 187 mm of rainfall that led to extreme river floods. Apparently, this caused removal of the surface layer of sediments from the shallow Uglovoy Bay and resulted in a significant decrease in the content of pollutants in the upper layer of marine sediments in the water area.
Figure 2 shows the spatial distribution of heavy metals in marine sediments of the Uglovoy Bay with the data for September 2016 as an example.
As shown in Fig. 2, sampling station 1 located in the strait of the Uglovoy Bay can be described as a sedimentation trap due to its hydrodynamic conditions. This fact determined the accumulation of heavy metals in marine sediments in this part of the water area. Clusters of oyster shells were found in the central part of the bay (station 18) during winter sampling in 2016. This area can also be considered as a predominant sedimentation zone due to vital activity of oysters inhabiting the bay.
The inflow of heavy metals into the bottom sediments of the bay near the low-water bridge (stations 4 and 5) is associated with the erosion of its dam. The results of our studies of the low-water bridge land fill in 2014 and 2016 support this conclusion. The average metal content in the samples of the bridge fill was: Fe 45800 mg/kg, Mn 2141.6 mg/kg, Cu 1323.8 mg/kg, Cr 943.6 mg/kg, Co 641.4 mg/kg, Zn 311.2 mg/kg, Pb 222.0 mg/kg, Ni 204.05 mg/kg, As 115.05 mg/kg, and Cd 6.5 mg/kg.
Sampling stations 7, 8, and 12 are affected by river and mainland runoff. Elevated concentrations of heavy metals here are most likely associated with the arrival of pollutants after heavy rains with rain-wash and river water.