Status quo analysis of the Aidar-Arnasay Lake System

. The Aidar-Arnasay lake system, situated within a natural depression in the southern expanse of the Kyzylkum desert, southwest of the Shardara reservoir, constitutes a unique hydrological entity. Comprising drainless lakes sustained by inflows from the collector-drainage network and the Shardara reservoir, this system has, over the span of 50 years, accumulated an astonishing 2.5 times more water than all 53 reservoirs combined in the province. However, recent studies conducted to assess the water quality in the Aidar-Arnasay lake system have revealed alarming concentrations of pollutants. A comprehensive analysis of the water samples has unveiled significant exceedances across nearly all measured indicators. Notably, the general mineralization of water stands out as a critical concern, surpassing the permissible norm by more than 25 times. This excessive mineralization poses a severe threat to the biological diversity within the system, impacting the overall ecological health of the region. The repercussions of these heightened pollutant levels extend beyond mere water quality concerns, adversely affecting the delicate balance of the ecosystem. Urgent intervention and remediation efforts are imperative to mitigate the ecological consequences and preserve the integrity of the Aidar-Arnasay lake system, ensuring its sustainability for future generations.


Introduction
The Aidar-Arnasay lake system (AALS) is a system of closed lakes (Aidarkul, Tuzkan and East Arnasay lakes) in the Republic of Uzbekistan, with a total area of 4000 km 2 .EPAS is located at 260 kilometers from Tashkent, in the middle reaches of the Syrdarya River, south of the Shardara reservoir (Republic of Kazakhstan), on the territory of the Jizzakh and Navoi viloyats of the Republic of Uzbekistan and occupies a natural depression in southern Kyzylkum.The bottom of the huge AALS was occupied by dried salt marshes and blinders, and before the development of the Hungry Steppe (Mirzacho'l) began, only the basin of Lake Tuzkan, fed by the Kly River, was annually filled with water.In 1969, when the peak of the flood on the Syrdarya River was cut off, more than 21 km 3 was discharged from the Shardara reservoir into the Arnasay lakes water.At the same time, a restructuring of the hydrographic network of the East Arnasay lakes took place, the filling of the basin of Lake Aidarkul, which, after the breakthrough of the cofferdam, connected with Lake Tuzkan, forming a single lake system under the general name -Aidar-Arnasay.Over the years, very favorable conditions have been created in the new hydroecosystem for the rapid development of flora and fauna.Currently, the AALS has accumulated more water than all reservoirs in the Central Asian province.The components of the incoming part of the water balance of the AALS are water coming from the Shardara reservoir, collector-drainage waters discharged into lake depressions, precipitation falling on the water surface of lakes and groundwater directly entering the AALS [1].From the moment of the formation of the AALS until now, the area of the water surface, the volume of water, the amount of incoming water from the Shardara reservoir and the collectordrainage network, the natural loss of water and, of course, the mineralization of water have changed.The system, in fact, is a receiver of drainage water from the irrigation networks of the Syrdarya and Jizzakh provinces (a storage lake for collector-drainage and waste waters).The location of the reservoir in an arid zone, a decrease in the discharge of fresh water from the Shardara reservoir and an increase in collector-drainage inflow, led to an increase in the mineralization of lake water and its pollution.Mineralization of the waters of the lake system increases every year, and a decrease and even absence of water discharge from the Shardara reservoir in 2000-2022 had a negative impact on the ecological state of the AALS [2][3][4].For the last fifty years, the Aidar-Arnasay lake system has been a reservoir not only of fishery importance, but also an important nesting, migration and wintering place for many birds, including rare and endangered species listed in the International Red Book and the Red Book of Uzbekistan.This is a place of natural reproduction of dozens of species of fish, nesting and habitat of a large number of birds, such as mallard, red-headed duck, red-headed and rednosed duck, greylag goose, pelican, sandpiper, heron, swan, cormorant and many others.The reed and tugai thickets are home to pheasants, wild boars, jackals, badgers, reed and steppe cats and other animals.A wide range of plants are found on the lakes -from moisture-loving plants growing on the edge to desert saline associations.In 2008, the AALS was included in the Ramsar list of wetlands of international importance, which, according to environmental experts, will attract the attention of the world community to the problem of preserving and improving the ecological state of this unique biological system [5][6][7].The water, hydro chemical and hydrobiological regimes of the system are not stable, many elements of the regime change very rapidly, which makes it extremely difficult to track their dynamics even with the availability of cartographic and analytical materials, therefore many modern publications contain incomplete or outdated information.In such conditions, a perfect system for monitoring the ecological state of water bodies using a stationary observation network, complex expeditionary research using modern equipment and observation methods, including remote sensing methods, satellite information, etc. are needed.The article reveals some of the research conducted on the study, analysis and assessment of the dynamics of changes in water resources in the AALS.

Materials and methods
And the research was carried out in June 2023.The Aidar -Arnasay lake system (40°56′13″ N 66°03′18″E) was chosen as the object of study (Figure 1).Water samples were taken at 8 points at the entrance to the AALS from collectors -Kli, Okbulak, Central Mirzachul collector (Golden Bridge), Pogranichny, from Lake Tuzkan -near the villages of Navruz, Chimkurgan, Uchkuloch, from Lake Aidarkul near the village of Kyzyl-Kum.The Aidar-Arnasay lake system (AALS) is a system of closed lakes in Uzbekistan, which includes lakes Aidarkul, Tuzkan and East Arnasay lakes.It is located in a saline depression in the extreme east of the Kyzylkum desert, southwest of the Shardara reservoir.The total area of the lakes is 4000 km 2 .As a result of the discharge of the collector-drainage runoff of the Golodnaya Steppe, as well as part of the flood waters of the Shardara reservoir, the water flooded the Prituzkan Lowland, raised the level in Lake Tuzkan, and filled the basin with the Aidar salt marshes.In 1969, during a catastrophic flood in the Syrdarya River basin, the Arnasay depression was used as a battery.The total discharge from the Shardara reservoir amounted to 21.8 km 3 water.The Arnasay lake system emerged.The level of Lake Tuzkan rose by 10 m.As a result of the release of water into the Aidar salt marsh, the water level in the Upper Arnasay lakes decreased by 2 -3 m.According to N.E.Gorelkin and A.M. Nikitin, a lake system 155 km long was formed, with a maximum width of 33 km, and a volume of up to 20 km 3 and a water surface area of 2300 km 2 .In the Aidar-Arnasay lake system, an intra-annual regime of changing phases of winter-spring filling, summer decline and autumn-winter equilibrium are observed.Aidarkul -a large closed lake in the north-eastern part of Uzbekistan, is an artificial reservoir in the Arnasay system of lakes.In 2005, there were 44.3 km 3 in Aidarkul water.Today the area of the lake is about 3000 km 2 .Its length is almost 250 km, and its width is up to 15 km.The mineralization of water in Aidarkul is only 2 grams per liter (0.2%).Many species of fish were introduced into the lake, including carp, pike perch, bream, catfish, asp, sabrefish, and snakehead, which serve as the basis for fishing.Tuzkan -a drainless brackish lake, part of the Arnasay system of lakes, the second largest lake in the system after Aidarkul.Unlike other reservoirs of Arnasay, which were formed entirely from waste waters, Lake Tuzkan has an ancient natural origin, although human activity has left its mark.Lake Tuzkan is located in the Farish fog of the Jizzakh province, 56 kilometers northeast of the city of Jizzakh.It is located in the extreme eastern part of the Kyzylkum desert.In the AEA, Tuzkan occupies the southernmost position, connecting to Aidarkul in the northwest.The lake extends in the direction from northwest to southeast and has an approximately triangular shape.Modern length of the lake 35 km, width is about 25 km, taking into account the bays on the east coast up to 35 km.The Upper Arnasay reservoir has an area of 140 km 2 with relatively low water mineralization up to 1.5 g/dm 3 .Lake Tuzkan does not have a permanent outline.About 15-20% of the water surface is overgrown with vegetation.The coastline in the northeast is highly indented, forming many narrow and long shallow bays.Some of them are laced into separate reservoirs.Expanses of water are often surrounded by salt marsh.This part of the lake is also replete with islets.Adjacent to it is a desert plain, in places overgrown with saxaul, and there are reed thickets.To the south, the coastline flattens out.At the southern end, the brackish Kly River (name Sanzara in the lower reaches), Akbulaksky and Central Golodno-Steppe collectors flow into Tuzkan.Not far from a large reservoir there are salt lakes Togai and Tuzchikudukkul.The nearby area is swampy.The southwestern coast meets the low Pistalitau ridge (a spur of the Nurata ridge).From the west, sands approach Tuzkan and there are hills [8].To the east of the AALS there are gray-oasis soils, significantly modified by irrigated agriculture: newly irrigated gray-meadow, medium-loamy, moderately and highly saline.Directly next to Lake Tuzkan there are light gray soils, loamy and sandy loam, solonchak and saline with patches of salt marshes.To the south and southeast of the Upper Arnasay reservoir there are light gray loamy soils, in places sandy loam, moderately and highly saline [8].In place of shallow waters drying out as water levels fall and separating from the lake.In Tuzkan, the bays, saline soils and salt marshes are formed, from which salts are blown out and transferred to all these saline lands.Soils in the drying areas of reservoirs in arid zones are composed of sedimentary materials (weakly overgrown loess and sandy soils, bottom silts, effusive salt marshes, often blinders), and often on the surface they have a plump crust of salts, then in a layer of 1.5 cm they contain 45... 60% [8].The climate of the study area is sharply continental with cold winters with little snow and dry and hot summers.AALS is located at the junction of two climatic subprovinces with different hydrometeorological characteristics -the irrigated massif of the Golodnaya Steppe and the Kyzylkum desert.In the study area, an increase in continental climate can be observed from east to west.Average long-term temperatures range from 0°C in winter to +29°C in summer.Along the basins of lakes Aidarkul and Tuzkan, stretching 180 kilometers in the westnorthwest direction, the air temperature difference is 3 -4 °C, the humidity difference is 4 -5%, which in turn characterizes the increase in continental climate with movement to the west.The thermal regime of lakes is typical for southern shallow water bodies with intense heating in the spring, a high maximum, reaching up to +30°C according to observations, and a long ice-free period.In recent years, ice phenomena have been observed, the duration of which was 10-30 days, but, according to climatic characteristics, the average climatic duration of freeze-up does not exceed 5 -10 days.On average, stable ice phenomena are observed once every 10-11 years [9][10][11].The objectives of the study included water sampling and laboratory analytical work.Under field conditions, the following were determined: water temperature, pH In laboratory conditions, the following indicators were determined: COD, BOD, suspended solids, water hardness, total mineralization, chlorides, sulfates, ammonium nitrogen, nitrite nitrogen, petroleum products, copper ions, chromium (VI), iron (III).Laboratory and analytical studies were carried out in the laboratory of the State Specialized Inspectorate for Analytical Control of the Ministry of Ecology, Environmental Protection and Climate Change of the Republic of Uzbekistan, as well as in the laboratories of the Center for State Sanitary and Epidemiological Surveillance of the Republic of Uzbekistan.Field research, sampling and sample preparation were carried out in accordance with generally accepted methods in Uzbekistan, according to the "List of certified and temporarily approved methods for determining the content of pollutants in natural and waste waters".The objectives of the study included water sampling and laboratory analytical work.Under field conditions, the following were determined: water temperature, pH In laboratory conditions, the following indicators were determined: COD, BOD, suspended solids, water hardness, total mineralization, chlorides, sulfates, ammonium nitrogen, nitrite nitrogen, phosphates, petroleum products, copper ions, chromium (VI), iron (III).Laboratory and analytical studies were carried out in the laboratory of the State Specialized Inspectorate for Analytical Control of the Ministry of Ecology, Environmental Protection and Climate Change of the Republic of Uzbekistan, as well as in the laboratories of the Center for State Sanitary and Epidemiological Surveillance of the Republic of Uzbekistan.Field research, sampling and sample preparation were carried out in accordance with generally accepted methods in Uzbekistan, according to the "List of certified and temporarily approved methods for determining the content of pollutants in natural and waste waters".

Results and discussion
The studies carried out show (Table 1) that the water temperature on the surface at the studied points did not exceed 11 degrees, characteristic of the spring period.The pH value fully complies with the maximum permissible concentrations (MPC) and does not exceed 8.2.Chemical oxygen demand (COD) as well as biological oxygen demand (BOD) are important criteria in water analysis.COD describes the amount of oxygen required for the chemical degradation of pollutants, while BOD indicates the amount of oxygen required for the biological degradation of organic pollutants by microorganisms.COD indicators range from 17.0 to 80.0, which exceeds the MPC values by 2.6 times.BOD 5 indicators vary from 21.4 to 43, which is 3.5-7.2times higher.Suspended substances found in natural water include the following components: clay particles, sand, silt, suspended particles of organic and inorganic origin, plankton and other microorganisms.They are quite concentrated in water and their quantity exceeds the norm by 1-18.2 times, which is undoubtedly influenced by the factors of changing seasons (spring), the regime of wastewater flow from the surface, as well as factors of anthropogenic origin -agricultural work.These particles influence the transparency of water, its light transmittance, and temperature values.Among other things, it affects the absorption process of toxic impurities, the components and distribution of deposits, and the rate of sediment formation.As a result of dumping into lake depressions collector-drainage waters, the lake system was polluted with agrochemicals and the degree of general mineralization increased.Water hardness is the sum of all dissolved substances (except gases) that break down into charged particles -ions.In the studied samples, the total hardness exceeded the maximum permissible concentrations by 5.6-13.1 times.According to the country's regulatory documents, the total mineralization in water bodies of non-drinking value is 1000 mg/l (including chlorides -up to 350 mg/l, sulfates 100-500 mg/l), in the studied samples it exceeded the maximum permissible concentration of 5.4-26.5 times.The concentration of chlorides varied from 491.3 to 4141.2 mg/l, and sulfates from 113.7 to 2296.0 mg/l, several times higher than the maximum permissible concentration and making the water unsuitable for use for all purposes (Fig. 1 and 2) Long-term monitoring of the West Arnasay hydrometeorological station shows that the average annual mineralization of water entering the lake system increased from 8.   In the studied water samples, the assessment of nitrogen concentration showed that ammonium nitrogen varied from 0.98 to 8.35 mg/l, which is 4.1 times higher than the norm.The concentration of nitrite nitrogen ranged from 0.1 to 0.02 mg/l, which corresponds to the maximum permissible concentration.Phosphate concentrations fully comply with the MPC values and do not exceed 0.092 mg/l.Petroleum products exceeded the MPC concentration of 0.  The concentrations of copper and iron (III) did not exceed the MPC values of 1.0 and 0.5 mg/l, respectively, and the concentrations of chromium (VI) exceeded 1.1 times.The gradual increase in concentrations of total mineralization, sulfates, and chlorides of the EPA over a long period of time affected the number of fish and reduced their commercial value.For example, in 1991, the share of catching carp accounted for 45.1% of the volume of fish caught, and in 2010 it accounted for 18.2% of the total volume of fish caught.At the same time, compared to 1991, the total volume of fish production decreased by only 1.2 times, while at the same time, the volume of carp caught decreased more than 3 times compared to 1991.Since 2005, there has been a lack of catfish in catches.Fishing for asp, bream and other herbivorous fish species in the AALS is irregular and is sometimes absent from catches for several years.Some of the main reasons for the decline in fish numbers are: -a gradual increase in water mineralization, which interferes with the natural reproduction of fish; -lack of proper control and analysis of the activities of fishing and fish farming enterprises; -lack of reproduction of fish resources, as well as other work aimed at preserving and increasing fish stocks; -fishing is carried out without scientific justification, tenants do not have data on the number and variety of fish species living in a given reservoir; -stocking of the reservoir with fish is not carried out efficiently and not by all tenants.To preserve the commercial significance of the Aidar-Arnasay lake system, certain measures must be taken, for example, such as a ban on issuing leases of rivers, collectors, canals for fishing, and using them only for recreational fishing, organizing monitoring of the general condition of the lake systems for obtaining an objective assessment within three years, issuing fishing quotas based on information about the state of ichthyofauna in water bodies, etc.

Conclusions
At eight points of the Aidar-Arnasay lake system, studies determined the concentrations of the main indicators of water quality and revealed their significant excesses.The most notable indicator is the total mineralization of water, which exceeds the maximum permissible concentrations of pollutants by 5.4 -26.5 times.To stabilize mineralization, it is necessary to annually discharge 1.5 billion m3ofadditional water into the lake system, the main source of which is the Shardara reservoir, but this requires an intergovernmental agreement with the Republic of Kazakhstan.An analysis of the current state of the Aidar-Arnasay lake system showed that the situation on the lakes is accompanied by clear signs of deterioration in the ecological condition of reservoirs and adjacent areas, the first signs of which are an increase in the mineralization of lake water, a decrease in the number and varieties of fish.The construction of a dam between lakes Aidarkul and Tuzkan will significantly desalinate Lake Tuzkan and create conditions for industrial fish farming.
25 g/l in 2006 to 10.53 g/l in 2020, and to 11.6 g/l in 2022.For comparison, the average mineralization of the world's oceans is 30-35 g/l.Water mineralization in the middle part of the Caspian Sea ranges from 1.4-13 mg/l, in the northern part (Volga delta) from 0.2-11 mg/l, in the Aral Sea water salinity was 110-210 mg/l.

Fig. 2 .
Fig. 2. Total salinity, chlorides and sulphates in water samples from the lakes.
3 mg/l by 2.0-64.5 times, the concentration was especially high near the villages of Navruz and Chimkurga -19.3 mg/l.

Table 1 .
Results of analyzes of water samples from the Aidar-Arnasay lake system.