Assessment of the Hydrophysical and Hydrochemical Characteristics of Lake Burabay (Akmola Region, North Kazakhstan)

: This paper shows the results of a study on the morphometric, hydrophysical, and hydro-chemical indicators of Lake Burabay, North Kazakhstan. The Burabay Protected Area, also known as Burabay National Park or Kokshetau National Park, is a protected natural area located in the north of Kazakhstan. It encompasses a diverse landscape characterized by lakes, forests, and unique rock formations. This analysis includes an eco-toxicological assessment of the hydrochemical composition of waters and benthal deposits by studying the content of metals. The degree of mineralization, ionic composition of water, hydrogen index, pollution index, and water quality class were also determined. Reductions in the area and depth of the lake were identiﬁed. The pollution index is 1.5–1.7, which belongs to class 3—moderately polluted. Relatively high concentrations of cadmium, nickel, copper, and arsenic were found in the lake sediments. Therefore, Lake Burabay and its surrounding ecosystem face certain environmental risks and potential water pollution. Although the increase in the number of tourists did not have a signiﬁcant impact on the water pollution index in Lake Burabay, some of the common threats that impact the area are induced by touristic development, agricultural practices and industrial pollution. Signiﬁcant efforts should be made to reduce these risks using the environmental indicators as a reference for control environmental quality.


Introduction
Global environmental problems include the pollution and desiccation of surface waters, which pose a threat to the environment [1,2]. Water scarcity hinders our ability to provide populations with water for drinking, recreation and economic purposes. This problem is both global and local, and it exists even in economically developed countries [3]. Among the post-Soviet states, Kazakhstan belongs to the lowest water-resource-rich republic. According to official data, water shortages in Kazakhstan by 2030 will reduce the amount of usable water to 40% of the total demand [4]. Due to water scarcity and pollution, the problem of the rational use, protection and restoration of lake resources is of particular importance. Lakes in specially protected natural areas are unique and widely used for recreational and balneological purposes. They are under the constant supervision of scientists [5]. Based on data analysis, recommendations have been developed for sustainable management and for the reduction in the negative impact of tourism on water and sediment pollution [6].
Performing environmental analysis using indicators in Kazakhstan is a necessary tool to ensure sustainability and ecological integrity. Kazakhstan is a vast and diverse country (MAB) on 15 June 2022, a decision was made to include NP Burabay in the UNESCO global network of biosphere reserves [22]. This makes it particularly necessary to monitor the ecosystems of this territory.
The annual increase in the recreational load may lead to reservoir pollution, making it necessary to conduct systematic environmental monitoring. The ecological state of NP Burabay lakes has been studied by several authors [23][24][25][26][27], but the data on the current ecological state of NP Burabay lakes are of a mosaic, fragmentary nature.
In this work, a comprehensive assessment of the dynamics of the hydrochemical composition of water and benthal deposits is given, focusing on the content of heavy metals. The morphometric indicators of Lake Burabay have also been studied using remote sensing techniques based on retrospective multichannel satellite images of the Earth by Landsat.

Materials and Methods
Lake Burabay is located in the Akmola Region of northern Kazakhstan near the village of Burabay, on the specially protected territory of NP Burabay ( Figure 1). The geographical coordinates of the lake are 53 • 04 30 N, 70 • 16 30 E, and its absolute altitude is 320.6 m above sea level [28]. preserve and restore unique forest and lake ecosystems, the "Burabay" State National Nature Park (hereinafter-NP Burabay) was established in 2000. At the 34th session of the International Coordinating Council of the UNESCO Program "Man and the Biosphere" (MAB) on 15 June 2022, a decision was made to include NP Burabay in the UNESCO global network of biosphere reserves [22]. This makes it particularly necessary to monitor the ecosystems of this territory. The annual increase in the recreational load may lead to reservoir pollution, making it necessary to conduct systematic environmental monitoring. The ecological state of NP Burabay lakes has been studied by several authors [7,, but the data on the current ecological state of NP Burabay lakes are of a mosaic, fragmentary nature.
In this work, a comprehensive assessment of the dynamics of the hydrochemical composition of water and benthal deposits is given, focusing on the content of heavy metals. The morphometric indicators of Lake Burabay have also been studied using remote sensing techniques based on retrospective multichannel satellite images of the Earth by Landsat.

Materials and Methods
Lake Burabay is located in the Akmola Region of northern Kazakhstan near the village of Burabay, on the specially protected territory of NP Burabay ( Figure 1). The geographical coordinates of the lake are 53°04′30″ N, 70°16′30″ E, and its absolute altitude is 320.6 m above sea level [2728]. The total area of the basin is 164 km 2 . The water is clear; the water surface of the lake is open; and, only along the western and northwestern coasts, there are thickets of reeds. The bottom of the lake is flat, with a slope to the north; sandy and rocky near the coast; and muddy in the middle. The lake has several small bays. In the northwestern bay there is a rocky island (Zhumbaktas), rising 20 m above the water level. On the shore of this bay lies the Okzhetpes mountain, about 300 m high. Figure 2 shows a space image of Lake Burabay received from the Yandex server via the SAS Planet program. The total area of the basin is 164 km 2 . The water is clear; the water surface of the lake is open; and, only along the western and northwestern coasts, there are thickets of reeds. The bottom of the lake is flat, with a slope to the north; sandy and rocky near the coast; and muddy in the middle. The lake has several small bays. In the northwestern bay there is a rocky island (Zhumbaktas), rising 20 m above the water level. On the shore of this bay lies the Okzhetpes mountain, about 300 m high. Figure 2 shows a space image of Lake Burabay received from the Yandex server via the SAS Planet program.
In this work, the Russian national GOST standards were used. The Russian Gosstandart (GOST) standards cover more than 20 industrial branches including the petroleum and chemical industry as well as mining and mineral sources, power and electrical equipment, and oil and gas products. The GOST organization, which was originally established during the Soviet Union era, was appointed to develop and put into practice state policy in the field of standardization. This institution was adopted by the Commonwealth of Independent States (CIS), and it is administrated by the Euro-Asian Council for Standardization, Metrology and Certification (EASC). In this work, the Russian national GOST standards were used. The Russian Gosstandart (GOST) standards cover more than 20 industrial branches including the petroleum and chemical industry as well as mining and mineral sources, power and electrical equipment, and oil and gas products. The GOST organization, which was originally established during the Soviet Union era, was appointed to develop and put into practice state policy in the field of standardization. This institution was adopted by the Commonwealth of Independent States (CIS), and it is administrated by the Euro-Asian Council for Standardization, Metrology and Certification (EASC).
Water samples were taken in accordance with GOST 31861-2012 [28]. Water samples were taken four times in a year: in winter (January), spring (April), summer (July) and autumn (October) during the 2018-2022 period. Water sampling was carried out at 3 points from a lake depth of 0.5 m: in the south, in the center and in the north of the lake. Benthal deposit sampling at 2 points-to the south and to the north of the places where water was sampled-were taken in summer, according to the international standard ISO 5667- 12-1995 [29] (Figure 3). Water samples were filtered before laboratory analysis. Hydrophysical parameters of water were determined: odor according to GOST 3351-74 [30] by the organoleptic method; color according to GOST 31868-2012 [31] by the visual method; turbidity according to GOST 3351-74 [30] by the photometric method. Chemical parameters were determined according to GOST 26449 [32]: pH of the medium by the electrometric method; carbon dioxide, potassium, sodium and nitrates by the potentiometric method; dissolved oxygen, dry residue mineralization, permanganate oxidizability, bicarbonates, carbonates and chlorides by the titration method; sulfates, calcium, magnesium and total hardness by the complexometric method; nitrites by the fluorometric method; heavy metal content by the flameless atomic absorption spectrometer on the MGA-915 spectrometer.
A morphometric parameters study of Lake Burabay was performed by remote sensing using retrospective multichannel satellite images from Landsat, based on STO GGI 52.08.40-2017 [33] and R 52.08.874-2018 [34]. Water samples were taken in accordance with GOST 31861-2012 [29]. Water samples were taken four times in a year: in winter (January), spring (April), summer (July) and autumn (October) during the 2018-2022 period. Water sampling was carried out at 3 points from a lake depth of 0.5 m: in the south, in the center and in the north of the lake. Benthal deposit sampling at 2 points-to the south and to the north of the places where water was sampled-were taken in summer, according to the international standard ISO 5667-12-1995 [30] (Figure 3). Water samples were filtered before laboratory analysis. Hydrophysical parameters of water were determined: odor according to GOST 3351-74 [31] by the organoleptic method; color according to GOST 31868-2012 [32] by the visual method; turbidity according to GOST 3351-74 [31] by the photometric method. Chemical parameters were determined according to GOST 26449 [33]: pH of the medium by the electrometric method; carbon dioxide, potassium, sodium and nitrates by the potentiometric method; dissolved oxygen, dry residue mineralization, permanganate oxidizability, bicarbonates, carbonates and chlorides by the titration method; sulfates, calcium, magnesium and total hardness by the complexometric method; nitrites by the fluorometric method; heavy metal content by the flameless atomic absorption spectrometer on the MGA-915 spectrometer.
A morphometric parameters study of Lake Burabay was performed by remote sensing using retrospective multichannel satellite images from Landsat, based on STO GGI 52.08.40-2017 [34] and R 52.08.874-2018 [35].
According to Kurlo's formulas, the Scholler diagram was prepared, and the water pollution index was calculated to determine complex indicators to assess water quality using Equation (1) [36]: where n is a strictly limited number of indicators (for surface waters, n = 6); C i is the concentration of the i-th pollutant in water; MPC i is the maximum allowable concentration of the i-th pollutant. According to Kurlo's formulas, the Scholler diagram was prepared, and the water pollution index was calculated to determine complex indicators to assess water quality using Equation (1) [35]: where n is a strictly limited number of indicators (for surface waters, n = 6); Ci is the concentration of the i-th pollutant in water; MPCi is the maximum allowable concentration of the i-th pollutant. The chemical concentration coefficient (Cc) of pollutants in benthal disposal was calculated using Equation (2) [36]: where C and C refer to pollutant concentration at studied and background sites, respectively.
To indicate the summary index (Z c ), the maximum permissible concentrations (MPC) of pollutants in the soil were used as background values according to Equation (3) [37]: To assess the technogenic impact on benthal deposits, we used the summary index (Zc) as calculated using Equation (4): The chemical concentration coefficient (Cc) of pollutants in benthal disposal was calculated using Equation (2) [37]: where C i and C φi refer to pollutant concentration at studied and background sites, respectively. To indicate the summary index (Z c ), the maximum permissible concentrations (MPC) of pollutants in the soil were used as background values according to Equation (3) [38]: To assess the technogenic impact on benthal deposits, we used the summary index (Z c ) as calculated using Equation (4): The calculation of the environmental value index of the lake was carried out using the ArcGIS program. Data processing was carried out by conventional methods using information systems. The studies were carried out in a licensed laboratory on certified equipment with an accuracy of ±0.001.

Morphometric Characteristics
The morphological features of lakes reflect the processes of the functioning of the lake ecosystem [39]. Mikhailov and Dobrovolsky [40] noted that, over the past 100-200 years, lakes have been drying up, and they found cyclical fluctuations in lake levels and precipitation occurring since 1850. Yapiyev et al. found that the total area of four large lakes of NP Burabay (Lake Burabay, Lake Bolshoe Chabachye, Lake Shchuchye and Lake Maloe Chabachye) decreased by 7% in the period from 1986 to 2016 [19]. The authors explain the decrease in the area and volume of the lakes of NP Burabay by a long-term deficit of the water balance, when evaporation from the lake surface exceeds the amount of precipitation. However, in recent years (2013-2016), precipitation has increased and stabilized the water level in the lakes of Burabay National Park.
According to Uryvaeva, in 1958 the area of the water surface was 10.5 km 2 . The maximum length is 4.6 km, and the maximum width is 3.2 km [41]. Our observations have shown that the area of the reservoir has decreased by 0.4-0.6 km 2 . The maximum depth of the lake decreased from 5.7 m in 1956 to 5.3 m in 2022; the average depth decreased from 3.4 to 3.0 m during the same years (Table 1). The main feeding of the lake is carried out by the Sary-Bulak River and three streams originating from the marshes. The streams and the Sary-Bulak River have a well-defined swampy floodplain, so they carry iron-rich water into the lake. The lake also receives water from underground springs. The bottom of Lake Burabay is flat and heavily silted in the southern part. The thickness of silt deposits in some places reaches 1-1.5 m. In the northern and eastern parts of the lake, the bottom is sandy.
The reduction in the surface area of the reservoir is associated with the silting of underground springs and the drying up of swamps due to global climate change. Also, until 2013, the water in Lake Burabay was used to supply the population with drinking water and household needs. In 2014, a water supply system was put into operation from the Sergeevskoye reservoir, known as the Yesil water reservoir, which drastically reduced the water intake from the lake.

Hydrophysical Characteristics
The hydrophysical properties of the water of Lake Burabay during the years of research (2018-2022) corresponded to acceptable standards.Smells were not detected. Under the conditions of a sharply continental climate, the water temperature in Lake Burabay fluctuated widely; amounted to 0 • C in January; and varied from 3.8 to 21.6 • C in April, from 12. In 2021-2022. Overall, there was a decrease in temperature and water color ( Table 2).

Hydrochemical Characteristics
The pH value and other hydrochemical indicators in most tectonic lakes of the Tibetan Plateau were shown to significantly exceed national surface water quality standards due to geographical conditions, climatic background and chemical characteristics of the water [42]. The values of the hydrochemical indicators and benthal deposites of Lake Burabay during the 2018-2022 are provided as Supplementary Materials (Tables S1-S10). Table 3 and Figure 4 show the hydrochemical indicators of Lake Burabay during the 2018-2022 period. The results of the chemical analysis showed that the pH value in the lake ranged from slightly alkaline, 7.5, to alkaline, 8.9, which generally does not exceed regulatory requirements. The exception was in 2022, when pH increased to 8.9, while the maximum permissible concentration is between 6.5 and 8.5. According to Kazhydromet, most of the freshwater lakes of the Kazakh uplands have alkaline and slightly alkaline reactions within their aquatic environments, which refers to an abiotic factor; that is, the alkalinity is of natural origin.
The concentration of bicarbonates in water averaged 142.7 mg/L. It increased from 117.4 mg/L in 2018 to 177.8 mg/L in 2022. This explains the increase in the alkalinity of the aquatic environment to 8.9 in 2022.
There was an increase in the total hardness of water from 1.95 ppm in 2018 to 4.15 ppm in 2022, which is associated with an increase in the concentration of bicarbonates in water; its average value was 2.6 ppm, which is 2.7 times lower than the MPC (MPC 7.0). The increase in the alkalinity of the aquatic environment is explained by the increase in the concentration of bicarbonates and the general hardness of the water. According to Kazhydromet, most of the freshwater lakes of the Kazakh uplands have alkaline and slightly alkaline reactions within their aquatic environments, which refers to an abiotic factor; that is, the alkalinity is of natural origin.
The concentration of bicarbonates in water averaged 142.7 mg/L. It increased from 117.4 mg/L in 2018 to 177.8 mg/L in 2022. This explains the increase in the alkalinity of the aquatic environment to 8.9 in 2022.
There was an increase in the total hardness of water from 1.95 ppm in 2018 to 4.15 ppm in 2022, which is associated with an increase in the concentration of bicarbonates in water; its average value was 2.6 ppm, which is 2.7 times lower than the MPC (MPC 7.0). The increase in the alkalinity of the aquatic environment is explained by the increase in the concentration of bicarbonates and the general hardness of the water.
According to the anionic composition, the salinity chemistry is sulfatehydrocarbonate-chloride. The average content of sulfates was 204 mg/L (MPC 500), the average content of hydrocarbonates was 143 mg/L (MPC 400), and the average content of chlorides was 14.5 mg/L (MPC 350). For the cationic composition, calcium-sodiummagnesium, the average content was, for each respective element, 35. The concentrations of total iron and ammonium nitrogen are not significant at 0.006 mg/L and 0.047 mg/L, respectively. The degree of water mineralization is also not high; it averaged 223 mg/L, with MPC values of 1000 mg/L. The content of anionic surfactants (surfactants) and synthetic surfactants (surfactants) is very low: 0.019 at an MPC of 0.5. The content of nitrates and nitrites also did not exceed the MPC and amounted to 1.14 mg/L and 0.03 mg/L, respectively. This does not pose an environmental hazard. Biochemical oxygen consumption fluctuated significantly over the years from 0.86 mg/L in 2021 to 3.28 mg/L in 2020 and averaged 1.9 mg/L, which is almost 2 times lower than the MPC (MPC 4.0).
Studies have shown a high content of soluble oxygen in water, 8.7 g/L, which is 2.2 times higher than the MPC (MPC 6 mg/L). This is a positive moment in terms of ensuring the active life of hydrobionts. It should be noted that the minimum content of soluble oxygen, 8.09 mg/L, was set in 2021, and the maximum, 9.16 mg/L, was set in 2020. These indicators are closely correlated with the minimum oxygen consumption in 2021 and the maximum consumption in 2020.
The concentrations of total iron and ammonium nitrogen are not significant at 0.006 mg/L and 0.047 mg/L, respectively. The degree of water mineralization is also not high; it averaged 223 mg/L, with MPC values of 1000 mg/L. The content of anionic surfactants (surfactants) and synthetic surfactants (surfactants) is very low: 0.019 at an MPC of 0.5. The content of nitrates and nitrites also did not exceed the MPC and amounted to 1.14 mg/L and 0.03 mg/L, respectively. This does not pose an environmental hazard.
Biochemical oxygen consumption averaged 1.9 mg/L, which is almost 2 times lower than the MPC. However, there was an increase in the total water hardness from 1.95 mg/L in 2018 to 4.15 mg/L in 2022. Its average value was 2.6 mg/L, which is 2.7 times lower than the MPC (MPC 7.0). A high content of soluble oxygen was found, 8.7 mg/L, which is 2.2 times higher than the MPC (MPC 4 mg/L). The concentration of bicarbonates in water averaged 142.7 mg/L. According to the anionic composition, the salinity chemistry is sulphate. The average content of sulfates was 204 mg/L at a maximum concentration of 500 mg/L, and the content of chlorides was only 14.5 mg/L at a maximum concentration of 350 mg/L.
For the cationic composition, calcium-sodium, the content of each respective element was 35.8 mg/L and 25.0 mg/L, which does not exceed the MPC value. The magnesium content averaged 8.5 mg/L, with an MPC of 20 mg/L. The content of nitrates and nitrites also did not exceed the MPC and amounted to 1.14 mg/L and 0.03 mg/L, respectively. The concentration of total iron and ammonium nitrogen is not significant at 0.006 mg/L and 0.047 mg/L, respectively. The degree of mineralization of the water is also not high; it averaged 223 mg/L, with an MPC value of 1000 mg/L. The content of AS and SS is very low: 0.019 at an MPC of 0.5.
The chemical composition of natural waters is inextricably linked to the composition and structure of the soil, which, in turn, was formed during the long evolution of the Earth's crust under the influence of climate.
The basis for systematization in existing classifications is the degree of mineralization, the predominant component or group, the ratio between different values of concentrations of various ions and increased amounts of certain specific elements of gas and salt regimes [43]. Water mineralization was determined by the total number of ions contained in the natural waters of the lake, including bicarbonates, carbonates, chlorides, sulfates, calcium, magnesium, sodium and potassium [44]. From less arid to more arid areas, the mineralization of lake water increases. The transformation of the elemental chemical composition of water (the content of anions and cations) occurs in the same direction: water from the carbonate class passes into sulfate and chloride and, from the calcium group, into magnesium and sodium. In the lakes of the forest zone, HCO 3 − and Ca 2+ ions predominate, whereas, in the lakes of the steppe zone, SO 4 2− , HCO 3 − , Na + and K + can be obtained [41]. According to the classification of Alekin [45], the water in Lake Burabay is fresh, and the degree of mineralization averaged 223 mg/L over 5 years. However, the degree of mineralization increased from 160 mg/L in 2018 to 321 mg/L in 2022. The anionic composition of the water was dominated by sulfate ions, the concentration of which increased in the 2018-2022 period from 178 to 252 mg/L. The content of hydrocarbonates averaged 143 mg/L over 5 years; their concentration increased from 117 to 178 mg/L over the years of the study. The chloride content in the water is insignificant. On average it was 14 mg/L, and the concentration also increased from 12 to 22 mg/L. Among the cations, calcium predominates (on average, 36 mg/L), followed by sodium (25 mg/L). The magnesium content is insignificant (8 mg/L). It should be noted that the concentration of these cations during the study years increased from 32 to 45 mg/L for calcium, from 23 to 57 mg/L for sodium and from 7 to 11 mg/L for magnesium ( Figure 4).

Content of Heavy Metals
The content of heavy metals (zinc, lead, cadmium, cobalt, copper and manganese) over the years of the study did not exceed the maximum permissible concentrations (Table 4). The calculations of the water pollution index in Lake Burabay showed that, for all elements, their indicators do not exceed the MPCs. According to the degree of pollution, the water, on average, belongs to class 3, with a pollution index of 1.5-1.7: the water is moderately polluted ( Table 5). The calculations showed a different correlation dependence of the indicators of the water pollution index over the years of the study: a direct close correlation was established between the years of the study with the pH value (r = 0.79); a very close correlation was established with the copper content (r = 0.91); a moderate association was established with BOD (r = 0.30); an inverse close correlation was established with the lead content (r = −0.77); an inverse medium relationship was established with the cobalt content (r = −0.60); and a moderate inverse relationship was established with the content of soluble oxygen in water (r = −0.37). According to the sum of concentrations of pollutants and the water pollution index, an inverse moderate correlation was established with the years of the study (r = −0.44).
The number of tourists visiting the territory of NP Burabay has been increasing year by year. So, if in 2018 there were 618,632 people, then in 2022 their number increased to 693,267 people, but in 2020 the number of tourists decreased to 481,714 people, which was associated with the pandemic. It should be noted that the increase in the number of tourists had a significant impact on the index of water pollution in Lake Burabay. The calculations showed an average correlation between the water pollution index in Lake Burabay and the number of tourists; the correlation coefficient was 0.63 ( Figure 5). It should be noted that the number of tourists has an indirect impact on the water pollution index. The coastal zone of Lake Burabay includes the village of Burabay, and there are many sanatoriums, recreation centers, entertainment centers and health centers. All of these sites have individual septic tanks, most of which do not have bottom waterproofing. For this reason, wastewater enters Lake Burabay via groundwater. The increase in the number of tourists is accompanied by an increase in the volume of wastewater. It should be noted that the number of tourists has an indirect impact on the water pollution index. The coastal zone of Lake Burabay includes the village of Burabay, and there are many sanatoriums, recreation centers, entertainment centers and health centers. All of these sites have individual septic tanks, most of which do not have bottom waterproofing. For this reason, wastewater enters Lake Burabay via groundwater. The increase in the number of tourists is accompanied by an increase in the volume of wastewater.
To eliminate this source of pollution, a sewerage system was built in the village of Burabay. Currently, the construction of treatment facilities is underway. The commissioning of this facility will reduce the level of water pollution in Lake Burabay and the dependence of water pollution on the number of tourists. Lake Burabay is located on the territory of the UNESCO Biosphere Reserve, so there are no industrial enterprises, which stops the lake from being polluted by industrial wastewater.

Benthal Deposits
The chemical composition of benthal deposits primarily depends on the geochemical conditions of the region, as well as on geochemical differentiation due to hydrodynamic physicochemical and biochemical processes in a certain landscape [46]. A unified methodology for assessing the quality of the benthal deposits of surface reservoirs has not been developed in the Republic of Kazakhstan yet. Therefore, as standards for the maximum permissible concentrations of pollutants in bottom sediments, we recorded their values in the soil.
The results of laboratory studies showed that the average cadmium content in benthal deposits for 2018-2022 was 0.83 mg/kg, which is 1.66 times higher than the MPC (MPC 0.5 mg/kg). It was also found that the maximum permissible norms of nickel, copper and arsenic content were exceeded by 5.7, 3.26 and 1.9 times, respectively. The content of other heavy metals was significantly lower than the maximum permissible concentrations: lead, 0.35 times; chromium, 0.77 times; manganese, 0.02 times. It should be noted that, in the bottom sediments, a decrease in the contents of cadmium, lead, copper and chromium was noted over the years (Table 6). The calculations showed that the manganese concentration coefficient increased from 0.01 in 2018 to 0.02 in 2022; a direct close correlation was established between its content and the years of study (r = 0.70). For arsenic, a direct moderate relationship was established (r = 0.39). On the positive side, the concentration coefficients of chromium, copper, nickel and cadmium were reduced. A close inverse relationship was established with the concentration of chromium (r = −0.80) and copper (r = −0.76), and an inverse average relationship was established with the concentrations of nickel (r = −0.54) and cadmium (r = −0.40). The total concentration of heavy metals had an inverse average correlation with the years of the study (r = −0.44).
The total concentration coefficient of heavy metals in the benthal deposits varied from 11.5 to 16.5 over the years and averaged 13.7. The level of pollution of the benthal deposits of Lake Burabay on the water pollution index showed systemic weakness, its value relative to the background ranging from 5.5 to 10.5 and averaging 7.7 [38].
There was a decreasing tendency in the level of pollution of the benthal deposits with heavy metals over the years: the total indicators of pollution of the benthal deposits with heavy metals amounted to 7.24 in 2018, to 10.52 in 2020 and to 5.49 in 2022 relative to the background ( Figure 6). The results show that the area of the water surface of Lake Burabay has decreased from 0.4 to 0.6 km 2 and that the average depth decreased from 3.4 to 3.0 m. The pH value in the lake ranged from slightly alkaline, 7.5, to alkaline, 8.9, which generally does not exceed regulatory requirements. The salinity chemistry is sulphate in anionic composition, and calcium-sodium in cationic composition. According to the degree of mineralization, the lake is fresh. The content of heavy metals (zinc, lead, cadmium, cobalt, copper and manganese) over the years of the study did not exceed the maximum permissible concentrations. According to the degree of pollution, the water belongs to class 3, moderately polluted, and the pollution index was 1.5-1.7. The increase in the number of tourists did not have a significant impact on the water pollution index in Lake Burabay.
In benthal deposits, the cadmium content averaged 0.83 mg/kg in 2018-2022, which is 1.66 times higher than the MPC. It was also found that the maximum permissible norms of nickel, copper and arsenic content were exceeded by 5.7, 3.26 and 1.9 times, respectively. The total concentration coefficient of polluting elements in bottom sediments averaged 13.7 and was weak, at 7.7, according to the scale of assessment of the level of pollution of the water system. An inverse moderate correlation was established between the level of pollution with toxic elements and the years of study: the correlation coefficient was 0.44.

Conclusions
The results of the study showed that Lake Burabay and its surrounding ecosystem face certain environmental risks and potential water pollution. Some of the common threats that impact the area include the following: 1. Touristic development: rapid urbanization and infrastructure development in the vicinity of the lake lead to increased pollution and habitat destruction. 2. Agricultural practices: intensive agricultural activities can result in runoff into the lake, leading to water pollution. 3. Industrial pollution: industrial activities in the region, such as manufacturing and In general, benthal deposits have a negative impact on water quality and the recreational value of Lake Burabay. Therefore, it is necessary to clean the lake of bottom sediments. According to Kazhydromet, the content of heavy metals is noted in recreational and other lakes.
The results show that the area of the water surface of Lake Burabay has decreased from 0.4 to 0.6 km 2 and that the average depth decreased from 3.4 to 3.0 m. The pH value in the lake ranged from slightly alkaline, 7.5, to alkaline, 8.9, which generally does not exceed regulatory requirements. The salinity chemistry is sulphate in anionic composition, and calcium-sodium in cationic composition. According to the degree of mineralization, the lake is fresh. The content of heavy metals (zinc, lead, cadmium, cobalt, copper and manganese) over the years of the study did not exceed the maximum permissible concentrations. According to the degree of pollution, the water belongs to class 3, moderately polluted, and the pollution index was 1.5-1.7. The increase in the number of tourists did not have a significant impact on the water pollution index in Lake Burabay.
In benthal deposits, the cadmium content averaged 0.83 mg/kg in 2018-2022, which is 1.66 times higher than the MPC. It was also found that the maximum permissible norms of nickel, copper and arsenic content were exceeded by 5.7, 3.26 and 1.9 times, respectively. The total concentration coefficient of polluting elements in bottom sediments averaged 13.7 and was weak, at 7.7, according to the scale of assessment of the level of pollution of the water system. An inverse moderate correlation was established between the level of pollution with toxic elements and the years of study: the correlation coefficient was 0.44.

Conclusions
The results of the study showed that Lake Burabay and its surrounding ecosystem face certain environmental risks and potential water pollution. Some of the common threats that impact the area include the following: 1. Touristic development: rapid urbanization and infrastructure development in the vicinity of the lake lead to increased pollution and habitat destruction. 2. Agricultural practices: intensive agricultural activities can result in runoff into the lake, leading to water pollution. 3. Industrial pollution: industrial activities in the region, such as manufacturing and mining, may generate pollutants that can enter the lake via runoff or direct discharge.
To effectively reduce environmental risks in the Lake Burabay protected area, a comprehensive approach utilizing environmental indicators is crucial. Firstly, regularly monitoring water quality indicators (such as pH levels, dissolved oxygen and nutrient concentrations) and the presence of pollutants can help identify potential threats and guide appropriate interventions. Additionally, tracking biodiversity indicators, including species richness and abundance, can provide insights into the health and resilience of the ecosystem. Implementing measures to preserve and restore critical habitats, such as wetlands and riparian zones, is essential for maintaining water quality and supporting diverse flora and fauna. Promoting sustainable land use practices in the surrounding areas, such as by minimizing the use of fertilizers and pesticides, controlling erosion and managing wastewater, can mitigate pollution inputs. The effective governance and enforcement of regulations are crucial for preventing unauthorized activities, such as illegal fishing or pollution discharge, within the protected areas. Finally, fostering public awareness and engagement via education and outreach programs can enhance the understanding of the importance of Lake Burabay, while promoting responsible behavior towards its conservation.