Quantitative assessment of water quality in the Vidsichne reservoir (Zhytomyr, Ukraine)

The modern approach to the implementation of water management activities is based on a quantitative assessment of the quality of water resources, which includes a certain set of indicators to reflect the needs of users in the composition and properties of water, to assess their condition, etc. Ecological assessment is a preliminary stage and a condition for ecological standardization of surface water quality. To analyze the compliance of water quality with ecological standards, the obtained results are compared with the values of ecological standards for the corresponding water body. The study carried out a quantitative assessment of water quality in the Vidsichne reservoir, near Zhytomyr (Ukraine). This assessment is based on the introduction of risk weighting factors from certain types of pollutants to the calculation formula of the integral indicator. Monitoring studies on hydrochemical indicators in the reservoir showed an increase in manganese content to 0.40 mg/dm3. To calculate the integral indicator of water quality, manganese and the related level of oxygen dissolved in water, biological oxygen consumption (BOC) were selected. It was established that the water in the reservoir belongs to the fourth quality class, that is, it is polluted. A model map of manganese concentration was built and the ecological coefficient of water quality was calculated, which indicates a 5-fold excess of ecological standards. The use of a graphic method of comprehensive assessment of the state of water in any reservoir is substantiated. The increase in manganese content is due to the seasonal increase in air temperature. The constructed mathematical model of the dependence of manganese content on temperature makes it possible to predict the state of the water body depending on climatic conditions and indicates an increase in the value of this indicator from 12°C and above. Quantitative assessment of water quality in the reservoir based on the sum of ecological indicators reflects the needs of users in the composition and properties of water and makes it possible to assess their condition.


Introduction
Pollution of water bodies is one of the components of the global environmental problem, which is getting bigger every year.Many changes that occur in the chemical composition of water in rivers are caused by climatic changes.The available numerous methods and techniques are directly or indirectly based on the transformation of quantitative indicators into qualitative indices of the ecological state.According to the analysis of numerous studies, the objective assessment of the ecological state of water bodies is based on the combined use of hydrochemical and hydrobiological data [1][2][3].As part of the standard approach, the indicators determined at individual points of the water body are compared with the normative values of the corresponding indicators [4][5][6].However, recently, an alternative method of water quality assessment, which uses biotesting, has become widespread [7].Over the past few years, there has been increasing concern about the decline in the quality and availability of drinking water.This decline in water quality is mostly due to anthropogenic practices such as agriculture, industry, domestic and municipal waste.In order to minimize the recorded impact on water bodies, it is extremely important to understand the overall composition, structure and dynamics of these ecosystems, which allows the implementation of measures (example, more specific procedures, public information and monitoring programs) to ensure the protection of water resources and ensure water quality [8,9].
Water supplied to the city of Zhytomyr centrally from the city water supply is taken from the Teteriv River in the area of the Vidsichne water intake.Water in the reservoir is characterized by unstable quality, especially during the spring flood and summer months.Water quality during these periods deteriorates, according to the level of phytoplankton, manganese and other indicators.
In previous years, illegal sand mining was carried out near the reservoir, which increased the amount of silt in the reservoir and, accordingly, the accumulation of manganese compounds in the reservoir.Natural processes, such as rock erosion, climatic conditions affect the composition of soil and water in the reservoir area.This leads to a change in the chemical composition of water and affects the development of the reservoir ecosystem [10].
According to the Water Framework Directive 2000/60/EC (WFD), the ecological status of a water body is based on specific physical, chemical, biological and hydromorphological parameters.Especially for highly altered water bodies, the parameters are determined according to the type of reservoir [6].
The purpose of the study was to establish and substantiate the quality of water in the Vidsichne reservoir, near Zhytomyr, using a comprehensive assessment based on the introduction of weighted risk factors from certain types of pollutants to the calculation formula of the integral indicator.The selected pollutant was manganese and the associated level of oxygen dissolved in water, biological oxygen consumption (BOC).

Research methods and tools
Three methods were used to quantitatively assess water quality in the Vidsichne reservoir [11,12].This is the determination of the modified water pollution index, the ecological coefficient of water quality graphically and the level of the quality of a water object based on a three-level classification by characteristics.The research used a method of assessing water quality based on the water pollution index, which is common in practice.
The definition of the water pollution index is based on indicators of the chemical composition of water, which makes it possible to use information from surface water monitoring.The calculation was based on the following ingredients: manganese, BOC, oxygen dissolved in water.These indicators were chosen because they are mandatory for surface water calculations.According to the methodology, the average arithmetic value was calculated for each indicator, which was compared with their maximum permissible concentrations for each indicator.The water pollution index (WPI) was calculated according to the formula: where C i is the average concentration of one of the n indicators of water quality, mg/dm 3 ; MPC i is the maximum permissible concentration of one of the n indicators of water quality, mg/dm 3 ; n is the number of indicators [11].
The ecological coefficient of water quality in the reservoir was determined according to the standard method with the construction of a map model in the form of a circular diagram with radii scales [12].The value of division is the MPC of the indicator, and each axis corresponds to the year of observation of this indicator.After constructing the diagram, the ecological coefficient of water (K e ) quality is determined according to the formula: where F f act is the area of the diagram limited by the actual values of the hydrochemical characteristics; F optim is the area of the diagram, limited by the optimal (normative) values, which is found according to the formula: where r is the radius of the circle, which is limited by optimal (normative) values.
The third technique relates to establishing the level of water quality of the water object Vidsichne.For this purpose, a three-level classification was carried out: 1) according to the signs of recurrence of pollution cases; 2) multiples of exceeding standards; 3) creation of general evaluation scores.
The first degree of classification is based on the determination of the level of pollution resistance.As a measure of the persistence of pollution, the value of recurrence of cases of exceeding the MPC (H), which is common in hydrochemical practice, was used, which was found according to the formula: where H is the frequency of cases of exceeding the MPC of the ingredient; N M P C stands for the number of analysis results in which the content of the ingredient exceeds its maximum permissible concentration; n is the total number of analysis results for the ingredient.
After conducting an analysis of contamination based on repeatability, the following characteristics were identified as qualitatively different: • pollution can be observed in individual samples, that is, it can be single; • pollution may be unstable; • pollution may not be dominant, but obviously persistent; • pollution can be dominant, i.e. characteristic.Qualitative expressions of selected characteristics of water pollution were awarded quantitative expressions in points (table 1).The second level of classification is based on establishing the level of pollution, the measure of which is the indicator F of the multiplicity of exceeding the MPC: where, F is the frequency of exceeding the MPC for the ingredient; C is the concentration of the ingredient in the water of the water body, mg/dm 3 ; C M P C is the maximum permissible concentration of the ingredient, mg/dm 3 .
According to the analysis of water pollution of water bodies, four qualitatively different degrees of pollution level were separated according to the frequency of exceeding standards by a separate pollutant: 1) low; 2) average; 3) tall; 4) very high.Qualitative expressions of the selected characteristics were assigned quantitative expressions of gradations in points (table 2).
When combining the first and second stages of water classification for each of the considered ingredients, generalized estimates of water quality are obtained for a certain period of time (table 3).Generalized characteristics are assigned generalized evaluation points S, obtained as a summary of individual characteristics, as an example, table 3 [11].
Informational materials of the State Agency of Water Resources and the State Department of Environmental Protection in the Zhytomyr region served as the initial data for the calculation of environmental indicators and the construction of models.The data of hydrochemical observations of the laboratory of the city water supply were used as the initial information for the study of the qualitative condition of the Vidsichne reservoir, namely, the indicators of manganese, BOC and the content of dissolved oxygen in the water.

Results and discussions
Water supplied to consumers in the city of Zhytomyr is taken from the Teteriv River in the area of the Vidsichne reservoir, which is located at a distance of about 8 km from the city (figure 1).
A certain supply of water accumulates in the reservoir with very little water exchange, which is due to a geographical position and climatic conditions: firstly, insignificant precipitation, and secondly, long-term hot weather in the summer during the last three years.In addition, due to the incorrect operation of dredgers in Vidsichne, the current moved to the opposite bank.A stagnant zone has formed near the water intake, where silt accumulates.This is an environmental problem that needs a separate solution.Water from streams flowing out of swamps enters the reservoir and contains an increased chemical content of metal ions (for example, iron).Sewage pollution of settlements upstream of the reservoir is observed.The specified reasons in the conditions of regulation of the Teteriv River led to a sharp deterioration in the quality of water in the Vidsichne water intake.If during the design period of the city's water supply facilities, the quality of water in the Teteriv River met the requirements for the first class of surface water supply sources by all indicators, but at the present time, due to a number of ingredients, such compliance no longer exists.The analysis of hydrochemical and hydrobiological indicators presented by the chemical laboratory of the city water supply showed that the level of phytoplankton (blue-green algae) and zooplankton in the river water has increased dramatically in recent years.The accumulation of a significant amount of organic nutrients also causes a persistently elevated level of microbiological indicators in the summer-autumn period in comparison with previous years.Such a physical indicator as turbidity increased to an average monthly level of 11.7 mg/dm 3 and a maximum of 14.9 mg/dm 3 during 2020.In addition, prolonged hot weather in the summer period in recent years led to a significant decrease in the content of dissolved oxygen and an increase in carbon dioxide.
As a result of the long-term increase in water temperature in the Vidsichne reservoir, the death of blue-green algae begins, which leads to the deterioration of water quality, and the content of manganese, which reaches 0.40 mg/dm 3 , increases.Therefore, according to the Water Framework Directive 2000/60/EC (WFD), the ecological status of a specific water body is assessed by both comprehensive and specific indicators.Such a characteristic indicator for the Vidsichne reservoir is the concentration of manganese ions [6].
Manganese belongs to microelements that significantly affect the metabolic processes in the body of plants and animals.It is one of the main elements necessary for water oxidation in the processes of photosynthesis and carbon utilization in carbo-oxidation reactions around green algae.In aquatic ecosystems, manganese in concentrations exceeding the maximum permissible level becomes biologically dangerous and can be considered as a toxicant.The main factors that increase the manganese content in water are temperature and hydrobiological indicators, that is, the amount of phytoplankton.
The research used the results of chemical analyses of water quality indicators in the reservoir, which were carried out during 2019-2021.To calculate the water pollution index by formula (1), indicators related to the biological oxygen consumption (total), dissolved oxygen, and manganese were used (table 4).The table 4 shows that for all three years of water research in the Vidsichne reservoir, the pollution index is within 2.80-3.50,which corresponds to the fourth quality class.This means that the water is polluted and under significant anthropogenic influence, the level of which is close to the limit of sustainability of ecosystems.The dynamics of changes in the presented indicators over the years (figure 2) shows a consistently high concentration of manganese in the reservoir.
With a further increase in the concentration of manganese in the water, the ecosystem may lose its balance.
To establish the ecological coefficient of water quality in the reservoir, a model-map with scales-radii for the manganese ingredient was built.The value of dividing such a diagram is the maximum allowable concentration of manganese in drinking water, which is 0.05 mg/dm 3 and the number of axes corresponds to the average annual value of the indicator in 2019, 2020, and 2021 years of the study, respectively ( CMn,2019 , CMn,2020 , CMn,2021 ).For each indicator, according to the methods described above, average annual arithmetic values were calculated (table 5).
The central shaded circle with a radius corresponding to the MPC value is the ecological optimum.All values of average annual concentrations of manganese that fall into this area correspond to standards for drinking water.The other shaded area is the actual   Figure 3 presents a model map of manganese for the water of the Vidsichne reservoir.According to the methodology, to calculate the environmental factor of water quality, the following were calculated: F f act -the area of the diagram, limited by the actual values of average annual manganese concentrations, and F optim -the area of the diagram, limited by the optimal value of manganese concentration in water (formula 3).Plotting the actual change in hydrochemical characteristics on the diagram shows the state of the river water, which can be used to consider possible sources of pollution, and the ratio of the area occupied by the diagram of actual pollution to the area occupied by the optimal values of the standardized indicators gives ecological coefficient of water (K e , formula 2).The values of the first class of ecological classification of the quality of surface water are taken as the optimum.Water quality class: Ivery clean -< 0.3: The area of the figure occupied by the diagram of actual contamination, which is the area of the formed triangle, is F f act = 1.52.Thus, according to (2), the ecological coefficient of water quality is equal to 5.4.This indicates that manganese contamination of water exceeds regulatory  The level of water quality was established according to the method described above for manganese, as the most influential ingredient on the quality of surface water in the reservoir.The calculated value of repeatability of cases of exceeding the MPC is 83% compared to the total number of results (formula 4).This indicates that manganese contamination is dominant, that is, it is characteristic, as the repeatability value is in the range of 50-100%.Such pollution corresponds to the following partial evaluation points (table 3): 1) conditionally expressed -d; 2) absolute values -4 points.
The second level of classification is based on the establishment of the indicator of the frequency of exceeding the MPC for manganese, as the ratio of the average value of the concentration of manganese over three years to its maximum permissible concentration (formula 5).Calculations showed that the value of the multiplicity of exceeding standards is within 2-10, which corresponds to the average level of pollution, the qualitative expression of which is the following partial evaluation points: conditionally expressed -b 1 ; absolute values -2 points.When combining the first and second stages of water classification, we get that the characteristic pollution of the average level is evaluated by generalized points: conventionally expressed -d • b 1 ; absolute values -8 points.And this means that the water in the Vidsichne water intake is highly polluted with manganese (table 3).
All three considered methods of studying the quality of drinking water make it possible to establish the compliance of water with current regulatory standards.They can be used to establish the ecological character of drinking water by any ingredient [12].
Recently, in the conditions of global climate changes, high temperature indicators, there is a constant increase in the manganese content in the reservoir due to a decrease in the dissolved oxygen content.Further studies showed a direct dependence of this indicator on water temperature (figure 4).Using the method of least squares, we obtained the dependence of Mn concentration (y, C(Mn), mg/dm 3 ) on temperature (x, • C) in the form of an approximation: y = ax   The R 2 approximation reliability value is 0.9831.The assessment was carried out using the Excel program.
The built mathematical model indicates a significant increase in manganese content at temperatures from 14 • C to 22 • C and higher.At elevated temperatures, the concentration of manganese in water reaches values that are 15 times higher than the standard values.

Conclusions
The developed procedure for assessing water quality in natural waters allows us to conclude that the water of the Vidsichne reservoir is characterized by unstable quality.According to the content of manganese, the water belongs to the fourth quality class, that is, it is polluted.
It is reasonable to carry out a quantitative assessment of surface quality based on the following indicators: water pollution index, ecological pollution coefficient and water quality level.The graphical method of comprehensive assessment of the state of water is the basis for calculating the ecological coefficient in any reservoir.
The constructed model-map of manganese concentration for the Vidsichne reservoir indicates water pollution, and the environmental quality coefficient -a 5-fold excess of environmental standards.
Mathematical processing of the results of chemical analysis of manganese concentrations during the year showed that the manganese content in water increases with increasing air temperature.

Figure 1 .
Figure 1.Map of the location of the Vidsichne reservoir.

Figure 2 .
Figure 2. Dynamics of changes in the investigated water quality indicators in 2019-2021.

Figure 3 .
Figure 3. Model map for calculating the ecological coefficient of water quality.

Figure 4 .
Figure 4. Dependence of manganese content on water temperature

Table 1 .
Water classification of water bodies according to the signs of recurrence of pollution cases.

Table 2 .
Water classification of watercourses according to the level of pollution.

Table 3 .
Variations in the water quality of watercourses are possible depending on individual ingredients and pollution indicators. 5

Table 4 .
The results of the calculation of water pollution indices for the Vidsichne reservoir.

Table 5 .
Average annual arithmetic values of the investigated indicators.