Evaluation of the quality indicators of collector-drainage water for reusing purposes: a study from Sirdarya province, Uzbekistan

. Currently, in the countries of the Aral Sea basin, due to the negative effects of climate change and the unreasonable use of water resources, the reduction and pollution of existing water resources, the increase in demand for water and its protection are required. Due to the increase in the amount of mineralization due to the discharge of the mineralized collector-drainage waters formed in the irrigated areas to the rivers, their use is limited due to the pollution of the upper and lower streams of the river basins. Researches revealed that the mineralization of Central Mirzachol drainage water has increased by 3.5 times, the total hardness has increased by 4 times, the amount of cation and anion and heavy metals is also high. It was found that the quality of the water of the Central Mirzachol drainage is the worst, and the water of the Boyovut well has a relatively good quality indicator. As a result of the discharge of collector-drainage waters to the Syr Darya River, the main water source of the province, the total hardness of the river reaches 2.0 times, the mineralization is 1.2 times, the amount of cations potassium, sodium, calcium and magnesium, the amount of sulfates and chlorides from anions, heavy metals: manganese, caused the amount of zinc, tin, copper to exceed the permitted standards.


Introduction
The area of irrigated land in the Aral Sea basin has increased from 3.5 million hectares to 8.0-8.5 million hectares since 1950, and at the same time, development and expansion of previously unused land from the two main rivers of the region, Syr Darya and Amu Darya and a sharp increase in the use of water resources of this river for irrigation purposes, along with the drying up of the Aral Sea, led to a decrease in the quantity and quality of freshwater resources in the sea basin and a violation of the eco-hydrological balance of the region [1][2][3][4].
Today, the degradation of irrigated lands in the Aral Sea basin as a result of unreasonable use of water resources, the decrease in the quantity and quality of existing water resources due to salinity have caused a number of environmental problems and threats in this area [3][4][5].This situation is exacerbated by climate change, changes in hydrological and hydrochemical regimes, population and habitat expansion, and pollution of land and water resources [1][2][3][4][5].
One of the characteristics of irrigated agriculture in the arid regions of the Aral Sea basin is the generation of large amounts of collector-drainage water (CDW) on agricultural land.In most regions of the Aral Sea basin, industrial and municipal waste water, as well as wastewater from animal husbandry, are discharged into collector-drainage networks [4][5][6][7][8][9].95% of the water discharged into collector-drainage networks (CDN) is irrigation water, and 5% is sewage and industrial wastewater [4][5][6][7].
Globally, 500 km³ of waste CDW is generated due to inefficient use of water in irrigated lands, and this amount is 25-30% of the total used water volume [4,8].In 1993-1994, the total amount of CDW generated in the former Union states was 49-50 km 3 , of which 36-40 km3 per year in the Aral Sea basin [6].CDW accounted for an average of 30% of the total water used for irrigation purposes due to the rapid development of newly irrigated lands in the region [4][5][6][7][8][9].
The average amount of CDW produced in the countries of the Aral Sea basin varies depending on the level of aridity of the year.In the period 1980-2009, multi-year observations showed that the highest amount of annual generated CDW was 32.4-36.0km 3 , the average amount was 27.3-32.4km 3 , and the lowest amount was 24.2-27.35km 3 [4,9].
In the Amudarya basin, including the Karakum canal zone, together with the Murgob and Tedjen irrigation fields (Turkmenistan), 17-18 km 3 /year of CDW was formed, and the mineralization of these waters was 1.8-14.2g/L [4,5] .The largest amount of CDW is produced in the Tuyamoin irrigation fields (downstream): 4.71 km3/year, with an average mineralization of 4.2 g/L [4].These waters are sulfate-chloride-calcium-magnesiumsodium in terms of ion composition.Most of the CDW in the Tuyamuyun irrigation area is dumped into the Sarikamish depression on the border of Uzbekistan and Turkmenistan through inter-republic collectors [4,8].
In 2004, the total annual amount of salts in CDW in the Aral Sea basin was estimated as 143.5 million tons, including 42.5 million tons in the Syr Darya River and 81 million tons in the Amudarya River basins [1,9].About 10% of the available land resources in Uzbekistan are irrigated lands, and 90% of the total used water is used for agricultural purposes [6,9].There is a possibility that the increase in demand for water resources in the country and climate change will lead to a significant decrease in the country's water supply [4,8,9].
Due to the dumping of CDW, mineralization rates along the rivers are increasing, especially in the lower reaches of the rivers.Until the end of the 1960s, the mineralization of the water of the Amu Darya and Syr Darya rivers did not exceed 1.0 g/L even in the lower reaches.Currently, the mineralization of river waters ranges from 0.3-0.6 g/L in the upper part of rivers and 1.7-2.0g/L in the lower part [4,[6][7][8][9].An increase in the mineralization of irrigation water leads to a decrease in the yield of agricultural crops, and every 0.1 g/L increase in the level of mineralization causes a loss of up to 134-147 US dollars per hectare in the Amudarya basin, and 70-150 US dollars in the Syr Darya river basin, compared to the initial value of the crop yield.[4].
The formation of very large amounts of highly mineralized CDW has become a serious environmental problem for the region.Sources of CDW pollution include mineral fertilizers applied to irrigated fields, residues of agrochemicals, and salts leached from soils with irrigation water [4,10].
However, CDW is often used for irrigation, fisheries, recreational areas, and other similar purposes [4,11,12].For these purposes, the degree of mineralization and ion content of water are the most important quality indicators, especially for assessing suitability for irrigation.The use of water with a high concentration of sodium and chloride ions for irrigation poses a great risk by increasing soil salinity [4].
Although river water quality indicators are well studied in island basin regions and other regions of the world, CDW waters are still poorly studied [4][5][6][7][8][9].A large amount of collector-drainage water contaminated with harmful salts, pesticides, and residues of mineral fertilizers can have a negative impact on the environment and become a serious environmental problem for the natural environment of the region [4][5][6][7][8][9].Kulmatov et al. determined and evaluated the dynamics of changes in the quantity and quality of CDW formed in the irrigated lands of Jizzakh and Navoi provinces.For the first time, the amount of heavy metals in the CDW of these provinces was determined [8,13].
Chembarisov carried out research on monitoring changes in the quantity and quality of CDW in Uzbekistan [5,9,12].Sirdarya province is one of the regions of Uzbekistan where agriculture is the main economic direction and is a semi-arid region prone to very high soil salinity [11,12].Studying the characteristics of mineralization and chemical composition of CDW, developing practical recommendations for reducing their amount and reusing them will significantly alleviate the environmental and socio-economic problems of the province.
Taking into account the above, this research work is dedicated to determining the quality of CDW generated in the territory of Sirdarya province, to study the factors that cause changes in mineralization and chemical composition, and to determine the possibilities of reuse.

Study area
Sirdarya province is located on the left bank of the Syr Darya River and is prone to high soil salinity.The province borders Kazakhstan, Tajikistan, Tashkent and Jizzakh provinces of Uzbekistan.
The area of the province is 4,300 km2, and it occupies a large part of the Mirzachol desert.The province is divided into nine administrative districts: Sirdarya, Sayhunabad, Gulistan, Boyovut, Khavos, Mirzaabad, Aqoltin and Sardoba districts [11].The climate of the province is sharply continental, characterized by dry and hot summer.Precipitation is mainly observed in winter and spring seasons.Hot wind (harmsel) often blows and dries the soil, adversely affecting the development of plants [14].
According to Uzgidromet [14], the average annual air temperature at the Sirdarya meteorological station between 2009 and 2019 was 14.8 0C, the highest average annual temperature was 15.82 0C in 2016, and the lowest was 13.72 0C in 2014, Oqoltin the average annual temperature at the meteorological station is 15.43 0C, the highest annual average temperature is 16.51 0C in 2016 and the lowest is 14.2 0C in 2014, the average annual temperature at the Yangier meteorological station is 15.44 0C, the annual average temperature is the highest index was 16.570S in 2016 and the lowest index was 14.37 0S in 2014.
According to Uzgidromet, the average annual rainfall during 2007-2019 was 318 mm.The highest amount of precipitation was observed in 2010, 389 mm, and the lowest amount of precipitation was observed in 2021 -208 mm.Due to the relatively arid climate of the province, the amount of precipitation is relatively low (Figure 1).

Materials and methods
In the Sirdarya province, the amount of CDW is determined by the Lower Syr Darya Irrigation Systems Basin Department under the Ministry of Water Management.Primary data on regional collector networks were obtained from the materials of this department [15,16].The obtained data were subjected to deep statistical processing.
The addition of several small collectors to the large collector ditches and the change in the quality and quantity of their water indicate the need to organize several hundred monitoring points (Figure 2).This, in turn, requires a lot of time and a lot of money.For this reason, the main large collectors of the province were selected to take samples.Samples from the Syr Darya River were taken from the river entering and exiting the province.
A special field expedition was organized by the authors of the article in June-July 2021.Samples were taken from the Syr Darya River, which is the main source of water for large collectors of the province, in clean polyethylene bottles of 1.5 L (Figure 2).Polyethylene containers were rinsed 3 times with sampled water.

Characteristics of regional collector-drainage networks and sampling points
Since the topography of Sirdarya province is an undulating plain, CDN penetrated into every region of the province.According to the information of the Lower Syr Darya BAIS, the province has more than 100 km of highway, more than 215 km of inter-district, more than 140 km of inter-farm and about 7 km of internal CDN, and their technical condition is different.
Water samples: from the upper and lower part of the Syr Darya River; Taking into account that the waters of the Main drainage and Shuruzak ditches flow into the Syr Darya River, from the upper and lower parts of these ditches; From the head (point of settlement of Ettisoy and Boyovut ditches) and lower part of Central Mirzachul ditch (CMD); It was taken only from the head of Ettisoy, Sardoba and Boyovut trenches.Yettisoy, Sardoba and Boyovut ditches are discharged into the CMD, and CMD to the Aydar-Arnasoy lake system.
Taking samples from the upper and lower parts of rivers and large ditches provides the basis for the evaluation of changes in water quality over a distance, and for determining the factors affecting it.In addition, since the Syr Darya River has a transboundary nature, it is important to constantly monitor the quality changes of the river water entering and leaving the province.
The main part of the Syr Darya River belongs to the territory of Shirin Water Consumers' Association (WCA), and the lower part belongs to Shalikor WCA; Main drainage head Beshbulok WCA, lower S. Rakhimov WCA; Head of CMD Beruni WCA, Lower Independent WCA; Head of Shuruzak Uzbekistan WCA, lower Sholikor WCA; Ettisoi head Development WCA; Sardoba Dehkanabad WCA; and Boyovut Uch Khahraman are within WCA boundaries.

Analysis of quality indicators of CDW
The levels of CDW mineralization are presented in Figure 3.The obtained analyzes show that the mineralization index of the water sample taken from the head of the Shuruzak ditch did not exceed the permissible norms (PN) (992 mg/L), while mineralization in all other samples exceeded the PN.Mineralization indicators increased from PN in the Boyovut ditch by 1.2 times, in the Sardoba ditch by 3.3 times, in the lower part of Shuruzak ditch, in Yettisoy and Main drainage ditches by 2.1 times, in the main part of CMD by 3.5 times and in the lower part by 3.3 times (Figure 3).This is considered a bad indicator.
The highest mineralization amount of 3532 mg/L CMD was observed in the head (Figure 3).The high amount of mineralization in the upper part of the CMD, when the waters of the Boyovut and Ettisoy ditches coincide with the point of inflow, and the lower amount of mineralization in the lower part compared to the upper part of the CMD, may depend on factors such as time, distance, and changes in the amount of water.
The high level of mineralization in CMD can be caused by the water containing mineral fertilizers, residues of agrochemicals and salts washed off by irrigation water into this ditch, as well as the fact that this ditch flows from Mirzaabad and Oqoltin districts, where the soil salinity index is high compared to other districts in the province.Hardness of water is one of the indicators that determine its quality.It was observed that total hardness values exceeded PN in all samples (Figure 4).In terms of total hardness values, the highest amount (34 mgEq/L) of the ditches was in CMD, and the lowest amount (11.8 mgEq/L) was in Boyovut ditch (Figure 4).The high level of total hardness in the CMD ditch can be attributed to the discharge of a large number of inter-district, inter-farm and internal collector waters into this ditch.The amount of pH in water samples is an important indicator in the fields of medicine, biology, chemistry, agronomy, and engineering, as well as in the field of ecology.The results of the analysis showed that the pH values of all samples increased up to 1.2 times PN (Figure 5).The highest value of pH value was 8.24 in the Sardoba well, and the lowest value was 7.45 in the Boyovut well (Figure 5).The highest amount of calcium in regional collector-drainage waters was 318.64 mg/L, observed in the lower part of CMD, and it exceeded PN up to 2 times.The smallest amount was 122.24 mg/L in the head of Shuruzak ditch.The amount of this cation in the main part of Shuruzak ditch, Boyovut and the Main drainage ditches did not exceed PN (Table 1).
The highest magnesium content of 217.66 mg/L was found in the lower part of CMD and was 5.4 times higher than PN.The smallest amount of 59.58 mg/L corresponded to Boyovur ditch and exceeded PN by 1.5 times.The amount of magnesium, different from other cations, exceeded PN in all analyzed ditch waters (Table 1).Iron content did not exceed PN in all analyses.Nitrogen ammonium cation was detected only in Sardoba pit (0.77 mg/L), i.e. it exceeded PN up to 2 times (Table 1).The highest amount of sulfate exceeding the PN in all wells was 1778.52 mg/L in the head of the CMD, up to 4 times the PN, and the smallest amount was 518.62 mg/L in the Boyovut well, slightly more than the PN (Table 1).The amount of chlorine increased by 1.6-1.7 times the PN in the head part of the Shurozak ditch, the head and lower part of the CMD (469.71-505.16mg/L), and it was less than the PN in the Sardoba, Ettisoy and Shuruzak ditches.The amounts of nitrogen nitrite and nitrogen nitrate did not exceed PN.The largest amount of these anions was observed in the CMD of the analyzed ditch waters, and the least in the Shurozak ditch (Table 1).The amounts of some chemical elements in CDW and their PN are given in Table 2.
The research results revealed that the amount of potassium (K) and sodium (Na) elements in CDW is much higher than other elements.The highest amount of potassium element was observed at 9.7 mg/L in the Sardoba well, and the lowest amount was 6.0 mg/L at the Ettisoy well.Potassium content in all samples did not exceed PN (Table 2).
The highest amount of sodium element is 550.0 mg/L in the Sardoba well, 4.6 times the PN, the lowest amount is 118 mg/L in the Boyovut well, which does not exceed the PN.It was 123 mg/L in the head of the cholera pit, which is very close to the PN level (Table 2).Manganese, copper, zinc, chromium, nickel, lead, and particular amount of other elements exceeded PN.

Analysis of water quality indicators of the Syr Darya River
Since the Syr Darya River is the main water source of the province, it has a transboundary character (it enters the Sirdarya province from the Republic of Tajikistan and exits through the region to the Republic of Kazakhstan), it is very important to determine the quality indicators of the Syr Darya River water.Tables 3 and 4 show the quality indicators of samples taken from Syr Darya River.Mineralization, hydrogen index (pH), total hardness, cation and anion amounts and permissible limit (PN) amounts in the river water are given for comparison (Table 3).The results of the study showed that the amount of рN in the water of Syr Darya River did not exceed PN (Table 3).The total hardness increased from PN to 1.7 times in the upper part of the river and 2.0 times in the lower part.Mineralization indicators of river water did not exceed PN in the upper part of the river, but in the lower part it increased 1.2 times the PN.
The results showed that the amount of cations calcium, iron, nitrogen ammonium and anions nitrogen nitrite and nitrogen nitrate did not exceed PN in the composition of river water, the amount of sodium from cations, magnesium up to 2.2 times, and sulfate from anions exceeded PN up to 1.4 times (Table 3).The results of the research showed that the amount of all the quality indicators determined in the river water increased in the lower part compared to the upper part of the river.Among the analyzed ditches, Shurozak and the Main drainage, as well as a number of small ditches flowing into the river, the mineralization in the lower part of the river is up to 1.2 times the PN, the total hardness is up to 2.0 times, the amount of sodium from cations is up to 1.25, magnesium is up to 2.2 times, and the amount of sulfate is from anions.may have caused an increase of up to 1.4 times.

Conclusions and recommendations
A large amount of collector-drainage water flow in the province is formed due to the low efficiency of irrigation methods and techniques, and consists mainly of irrigation, leaching, and drainage water.Boyuvut collector-drainage water quality is good compared to other collector-drainage water quality.Mineralization, total hardness, potassium, sodium, calcium and magnesium from cations, sulfate and chlorine from anions are 3-5 times less than the amounts found in other drainage waters.
In terms of quality, the Central Mirzachul collector-drainage water had the worst quality, mineralization was 3.5 times higher than the permitted norm, and total hardness was 4 times higher.The amounts of cations, anions and heavy metals were 2-3 times higher than the rest of the well waters.
As a result of the discharge of mineral fertilizers, residues of agrochemicals used on the irrigated fields, and salts washed out from the soils of the irrigation water, Shuro'zak and the main receiving ditch water into the Syr Darya River, which is the main source of irrigation water of the province, the total hardness of the river water is up to 2.0 times, the mineralization is up to 1.2 times, caused the amount of cation and anion, sodium, manganese, zinc, lead, chromium and nickel to exceed the permitted limit.
It is not recommended to reuse Central Mirzachol collector-drainage water.Boyovut, Ettisoy and Bosh receiving collector-drainage waters are recommended for irrigation and other purposes in years of water shortage due to their relatively low mineralization.It is necessary to improve the technical condition of the regional collector-drainage networks in order to increase the productivity of crops, reduce waterlogging and soil salinity, and prevent the rise in the level of seepage water.

Figure 3 .
Figure 3. Amount of mineralization in collector-drainage waters.

Figure 4 .
Figure 4. Amount of total hardness in the collector-drainage water.

Table 1 .
Amount of cations and anions in collector-drainage waters, mg\L.

Table 2 .
Amount of chemical substances in collector-drainage waters, mg/L.

Table 3 .
Mineralization, pH, total hardness, cations, and anions in the water of the Syr Darya River.

Table 4 ) . Table 4 .
Amount of chemical substances in the Syr Darya River.