3.1. Water Geochemistry
The natural water of the Kola Region is characterized by a low total dissolved solids (TDS) content of 20–30 mg/L and the following pattern of the main ions: HCO3−
]. In the lakes located close to the Barents Sea, due to the marine aerosols from the Barents Sea, the main ions are distributed as follows: Cl−
]. The median pH of the water is 6.5, which lies on the border between the slightly acidic and neutral values.
Over Murmansk’s more than a century-long history, the water chemistry in the city’s lakes has undergone significant changes. The pH value of the lake water in 2018–2019 ranged from 6.6 to 9.34. One of the highest values (almost at the border between the alkaline and strongly alkaline values) was recorded in Lake Ledovoe, which receives water from the Varnichny Stream, running through the densely populated city core. The lake is located alongside the city’s main street—Kola Avenue (Figure 1
). High water pH values also were recorded in Lake Yuzhnoe—ranging from 8.02 to 9.11 with an average of 8.42, which is considered alkaline and slightly alkaline. The water of the other lakes had neutral pH values (Table 2
). Lake Ledovoe had both the highest pH and the highest TDS value—up to 735 mg/L. Lake Yuzhnoe came second in terms of both pH and TDS values, with TDS ranging from 140 to 371 mg/L with an average of 294 mg/L. TDS values in these lakes were dozens of times higher than the values (20 mg/L on average, which can be considered the background level) recorded in the lakes of the northeastern Murmansk region in the Barents Sea water catchment [21
]. The lowest TDS values were recorded in Lake Okunevoe and Lake Semenovskoe (63 and 73 mg/L on average, respectively), but these were also above the background level. According to the classification [20
], all natural waters by chemical composition are divided in concordance with the prevailing anion (equivalents) into three classes: hydrocarbonate (and carbonate) (HCO3−
), sulfate (SO42−
), and chloride (Cl−
). Each class according to the prevailing cation is divided into three groups: calcium (Ca2+
), magnesium (Mg2+
), and sodium (Na+
). According to the classification proposed in [23
], the lake water in the city of Murmansk is mainly chloride and sodium. The exceptions are Lake Okunevoe and Lake Srednee, where the lake water is typical for most lakes in the Murmansk region, which have a distribution pattern of the main ions in the calcium group and hydrocarbon class (Table 2
). A ternary diagram is constructed for the basic cations and anions, which shows the two groups of lakes by the predominant ions—chloride–sodium and hydrocarbonate–calcium (Figure 3
). The studied lakes have supply characteristics in different seasons of the year. In winter, when the lakes are covered with ice, groundwater is the main source of water supply, and in summer, atmospheric waters are the main source. Therefore, the mineralization of water in lakes in winter is more than in summer. In this article, the seasonal features of the chemical composition of the water of the lakes are not considered in detail; another article will be devoted to this.
The highest content of the nitrogen group (ammonium ion and total nitrogen) was recorded in the water of Lake Ledovoe, which is associated with the influx of wastewater from the city containing high concentrations of nitrogen compounds and with the decomposition of petroleum products, which the sediments of the lake is rich in (Table 2
). Oil and petroleum products are known to contain nitrogen in the amount of tenths of a percent [24
]. The content of NH4+
in Lake Ledovoe was equal to the content of the main cation K+
in terms of equivalent concentration (248 µeq/L). The highest content of NH4+
in Lake Ledovoe (6390 µgN/L in the bottom layers at a depth of 13 m) was more than 300 times higher than the average content of this ion (16 µgN/L) in the lakes of the northeastern Murmansk region in the Barents Sea catchment [21
]. The high levels of NH4+
in the water of Lake Ledovoe are indicative of severe pollution [25
]. In the other lakes, the level of ammonium ion was much lower, but still higher than the average level in the lakes of the Barents Sea catchment. The content of nitrate ion in the water of Lake Ledovoe was three orders of magnitude lower (Table 2
) than that of ammonium ion, which indicates the presence of a reducing environment and decomposition of plant residues and organic substances. A similar pattern (ammonium ion higher than nitrate ion) was also observed in Lake Yuzhnoe and Lake Semenovskoe, which indicates the development of a reducing environment in the bottom water layers of the lakes. During water sampling in the rather deep Murmansk’s urban lakes (Table 1
), summer and winter thermal stratification occurred in July and April, respectively, when the denser (colder in summer and relatively warmer in winter) waters are located in the deep layers of the lake water column. The thermal and density stratification does not allow mixing of the lake water column; therefore, a reducing environment with high NH4+
concentrations is formed in deep water layers as a result of the decomposition of high loads of organic matter. The smell of hydrogen sulfide emanating during the sediment sampling confirms the conclusion about the reducing environment in the deep layers of the water column. Massive development of cyanoprokaryotes was noted in Lake Ledovoe in the summer, which causes an increase in the water pH value. The pH value in the surface water layer in July 2019 was 9.34, and 7.18 in the bottom layer.
High content of phosphorus compounds (phosphate ion and total phosphorus in filtered and unfiltered water) was also observed in the water of Lake Ledovoe (Table 2
), which is associated with the influx of urban wastewater containing phosphorus compounds. PO43−
in the bottom layers at a depth of 13 m in this lake was up to 65 µgP/L, and total phosphorus was up to 1677 µgP/L, which is one to two orders of magnitude higher than the average (2 and 9 µgP/L, respectively) in the lakes of the northeastern Murmansk region in the Barents Sea catchment [21
]. Slightly higher values of these indicators were also recorded in Lake Yuzhnoye and Lake Semenovskoe (Table 2
). High phosphorus concentrations (especially PO43−
), as well as high NH4+
concentrations, suggests an anoxic hypolimnion in the water of Lake Ledovoe.
The organic matter content in the water of the studied urban lakes (Table 2
) was less than the average in the lakes of the northeastern Murmansk region in the Barents Sea catchment [21
] 62 Pt°, 9.5 mg/L, and 7.7 mgC/L for Color, CODMn
, and TOC, respectively, except for Lake Okunevoe and Lake Ledovoe, where the average value of TOC in the water was not less than the background value. This is probably due to the fact that the studied lakes receive surface runoff from urban areas, to a lesser extent covered by a soil and plant layer compared to natural areas, which reduces the inflow of allochthonous organic matter. This explanation is supported by the observation that the highest organic matter levels, comparable with the above average indicators of the lakes in the Barents Sea catchment, were found in Lake Okunevoe, the least exposed to pollution. Concerning its Color and TOC, Lake Okunevoe has surrounding forest and is similar to the polyhumic forest lakes of the boreal forest regions of Finland, on which much has been published [26
In the water of Lake Ledovoe, the highest concentrations of heavy metals Ni, Cu Zn, Co, Mo, and W were found (Table 3
). Judging by the main ions, biogenic elements, and heavy metals, this lake is most exposed to anthropogenic impacts and is the most polluted of all the investigated lakes. In the water of Lake Okunevoe, one of the relatively clean lakes among the studied ones, the highest concentrations of Cd and Pb were found. With rare exceptions, the levels of all heavy metals in the water of urban lakes are higher than the average content in the lakes of the Barents Sea catchment [22
], and especially in the lakes of Northern Fennoscandia [28
The highest V level was found in the water of Lake Srednee and Lake Okunevoe (Table 3
), the closest to the Murmansk thermal power plant, which is fed with fuel oil containing high concentrations of this element. The highest levels of Cd and Pb in Lake Okunevoe can also be associated with the emissions of the Murmansk thermal power plant.
3.2. Sediments Geochemistry
Sediments in all the studied lakes in Murmansk, with the exception of Lake Yuzhnoe, according to the organic matter content (based on LOI), are classified as organosilicate deposits [29
] The sediments of Lake Yuzhnoe are classified as mineral deposits. The highest organic matter levels were recorded in the sediment of Lake Okunevoe (45%) and Lake Semenovskoe (40%). In the sediments of Lake Srednee, the organic matter content was 26% of the total mass, while in Lake Ledovoe and Lake Yuzhnoe, it was 15% and 10%, respectively.
The water pollution level in the lakes of Murmansk, as determined based on a combination of indicators, is also reflected in the condition of the sediments of these water bodies. The total content of almost all heavy metals in the sediments of the lakes of Murmansk exceeds the background level, which is interpreted as the content of the same elements in the preindustrial layers of the sediments of small lakes in the Republic of Karelia, Russia [17
], located to the south of the Murmansk region (Figure 4
). Only the background level of Mn is higher or at the same level as in the investigated urban lakes, which may be due to the natural accumulation of this metal in sediments. Concentrations of other heavy metals significantly exceed the background level.
The highest accumulated levels of heavy metals were found in the sediments from Lake Ledovoe, where the background level was exceeded by a factor of 23 for W, 20 for Pb, 17 for Sb, 12 for Cr, 9 for Co, 7 for Ni, 6 for V, 5 for Zn, Cu, and Cd, and 2 for Mo. In the sediments of this lake the pollution load index is 7.6, which corresponds to an extremely heavy pollution level. This is largely due to the presence in the immediate vicinity of the lake of two gas stations, several car repair shops, car dealerships, a parking lot, a granite workshop, a public transit stop, and several garage buildings.
In the sediments of Lake Semenovskoe, located in a recreational area of Murmansk and surrounded by busy streets, the background level was exceeded by a factor of 13 for Pb, 11 for Sb, 10 for V, 8 for Co, 7 for Ni, 6 for Cr, 5 for Cd and Zn, 4 for Cu and W, and 2 for Mo. In the sediments of this lake the pollution load index is 6.0, which corresponds to an extremely heavy pollution level.
In the sediments of Lake Srednee, the background level was exceeded by a factor of 14 for Sb, 10 for Co, 7 for V and Ni, 6 for Pb, 5 for Cd and Cr, 3 for W and Zn, and 2 for Mo and Cu. In the sediments of this lake, the pollution load index is 5.1, which corresponds to an extremely heavy pollution level. Lake Srednee is located near a busy road and thermal power plant, which can explain the high pollution level in the water body.
In the sediments of Lake Yuzhnoe, the background level was exceeded by a factor of 8 for Cr, 5 for Co, 4 for Pb, Ni, Sb, and V, 3 for Cd and Cu, and 2 for Zn. In the sediments of this lake, the pollution load index is 3.1, which corresponds to an extremely heavy pollution level. Local sources of lake pollution can be the gas station, several car repair shops, and the many warehouse buildings located near the lake. On the other hand, the lake is located on the outskirts of Murmansk and therefore is exposed to a lower pollution load compared to lakes Ledovoe, Semenovskoe, and Srednee.
In Lake Okunevoe, located in the center of the city, the background level was exceeded by a factor of 5 for Pb and Sb, 4 for Co, 3 for Ni, Zn, Cr, V, W, and 2 for Cd and Cu. In the sediments of this lake, the pollution load index is 2.6, which corresponds to a heavy pollution level. This is the lowest value of the index among all the lakes studied. This is primarily due to the fact that the lake is surrounded by forest, located away from busy roads, and there is only a camp site, medical facilities, and several garage buildings on its shores.
As indicated in the state report on the state of the environment in the Murmansk region, emissions from Murmansk thermal power plant (TPP) are the main source of air pollution across the region and in the city of Murmansk. In 2017, the power plant emitted 14.1 thousand tons of pollutants and 15.31 thousand tons in 2018. Until 1964, the power plant was coal fired, but in the 1960s the gradual transition of the Murmansk TPP to fuel oil began. Fuel oil is still the main type of fuel used by the Murmansk TPP. Emissions from any fuel oil-based thermal power plant are known to have elevated levels of Fe, V, Ni, Cr, and other elements [30
]. This can explain the elevated levels of V, Ni, and Cr in the water and sediments of the urban lakes in Murmansk, especially in lakes Semenovskoe, Srednee, Ledovoe, Okunevoe, located within the direct impact zone of the TPP emissions in the city’s urban core.
On the charts of the vertical distribution of heavy metals in the core sediments of Lake Semenovskoe, it is shown that the concentrations of almost all elements in the upper layers of the sediments are higher than in the lower layers (Figure 5
). A particularly sharp increase in metals was noted at a depth of 16 cm. It is here that a smooth increase in the concentrations of V and Ni in the studied sediments begins, which can be taken as the beginning of the operation of a thermal power plant using fuel oil in the 1960s. Given that the company still uses this type of fuel, constantly increasing capacity due to the needs of the city and urban factories, the concentrations of V and Ni are still increasing. Thus, the average sedimentation rate in the lake during the past 55 years is about 2.9 mm per year. It is higher than the average sedimentation rate in the small lakes of the Murmansk region, which varies from 0.03 to 1.25 mm per year. Similar behavior of these metals is noted in other lakes of the city of Murmansk. In addition, similar patterns were previously revealed in the study of sediments of a small urban lake in the Republic of Karelia (Russia), which is also subject to the influence of TPP emissions [31
Significant Pb pollution is observed in almost all lakes, which is due to the use of tetraethyl lead as a gas additive worldwide between the 1930s and 1990s [32
]. In the recent sediments, the vertical distribution of Pb records the history of anthropogenic impacts on the aquatic environment and on the catchment area of the water body [34
]. The vertical distribution of Pb in the core sediments of Lake Semenovskoe confirms the conclusions about the accumulation of lead in this period (Figure 5
). The decrease in the concentration of this metal in the uppermost layers of sediments of Lake Semenovskoe reveals a decrease in its release into the environment after the abandonment of leaded gasoline in Russia in 2002.
In addition, Pb can also enter the environment with industrial emissions, and its migration range is hundreds and thousands of kilometers [7
]. Thus, elevated Pb concentrations, as compared to background (natural) levels, are found in lake sediments in the relatively clean areas of the Murmansk region [6
], Russian Karelia [19
], and Finland [9
]. As noted in numerous studies of the behavior of heavy metals in the environment, emissions from the combustion of fossil coal served as the main source of heavy metals at the beginning of the industrial era development of society [39
]. Moreover, the trends in the behavior of these elements in the environment coincide, starting from the end of the 19th century to the present. In addition, metals entered and are entering the atmosphere as a result of activities of enterprises for the extraction and processing of metal ores and the incineration of household waste [43
]. This may explain the increased accumulation of Sb, Cd, and Zn and other metals in the sediments of the urban lakes in Murmansk.
It should also be noted that one of the key sources of pollution in Murmansk is coal dust originating from the commercial port of Murmansk. Many heavy metals (Sb, Cr, Ni, Co, W, Mo) may be contained in coal and in coal dust as impurities, which become environmental pollutants as a result of atmospheric transport. A further detailed study of the vertical distribution of these metals in sediment core of the urban lakes in Murmansk, together with sediment dating, will make it possible to more accurately detect all sources of pollution of the studied lake ecosystems.
Calculation of the geoaccumulation index (Igeo
) reveal uncontaminated and moderate pollution levels for Mo and Mn. For Cu, Zn, and Cd, the index shows moderately and heavily contaminated levels for all the lakes. The highest level of pollution is revealed for Pb, Sb, W, V, and Ni. The most polluted lakes of Murmansk are Ledovoe, Semenovskoe, and Srednee, where the index detects heavily and extremely elevated levels of pollution by Pb, Sb, and V. Analysis of the core sediments from Lake Semenovskoe, the most heavily and extremely contaminated level of the sediments is traced from upper layers to a depth of 18 cm (Figure 6
Increased accumulation of heavy metals in the sediments can initiate secondary pollution of the aquatic environment and the involvement of hazardous chemical elements in biological cycles. For this reason, analyzing the bioavailable forms of metals is an important part of environmental and geochemical studies of urban lakes.
Data on the content of the mobile forms of heavy metals (the sum of water soluble and exchange fractions) in the sediments of the urban lakes in Murmansk demonstrate that the most mobile and bioavailable elements in the lakes are Mn, Co, Zn, and Cd (up to 40% of the total content). However, while in Lake Okunevoe and Lake Srednee there is a gradual increase in content of mobile Co and Zn forms in the more recent sediment, the concentration of mobile forms in the top layers of the core samples (0–5 cm and 5–10 cm) from Lake Semenovskoe decreases significantly to 2–3% (Figure 7
). On the other hand, in the sediment from Lake Semenovskoe, the mobility of Cd increases with depth. In Lake Okunevoe there is also a noticeable increase in content of mobile forms of Cd and Ni in the more recent layers of sediment.
V, Cr, Cu, W, Mo, Pb, and Sb are the most strongly associated with residual phases and unbioavailable elements in all studied lakes along the entire depth of the sediment core (not more than 1–3% of the total content). Probably, these elements are bound with organic matter, iron oxides, or finely disseminated mineral particles, which retain pollutants in the sediments of rivers and lakes [2
]. Further study of the sediments in the lakes of the city of Murmansk is necessary to give a conclusion concerning this aspect.