Hydrochemical Characteristics and Water Quality Assessment of Irkutsk Reservoir (Baikal Region, Russia)

Abstract

The Irkutsk Reservoir, belonging to the largest unified freshwater Baikal–Angara system, is an important source of drinking water in the region. Therefore, studies of its hydrochemical characteristics are of prime importance in deciding on the role of anthropogenic activity in water quality. The water samples were collected across the reservoir in 2007, 2012, and 2021 and then were analyzed for major ions and trace elements. The data revealed that the distribution of HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, Na⁺ and K⁺ is stable across the reservoir. Trace element concentrations varied from 1.13 to 15.39 µg L⁻¹ for Al, from <DL to 0.39 µg L⁻¹ for Cr, from 0.39 to 23.12 µg L⁻¹ for Mn, from 1.25 to 53.22 µg L⁻¹ for Fe, from 0.005 to 0.100 µg L⁻¹ for Co, from 0.20 to 1.98 µg L⁻¹ for Cu, from <DL to 13.40 µg L⁻¹ for Zn, from 0.25 to 0.48 µg L⁻¹ for As, from 0.004 to 0.127 µg L⁻¹ for Cd, from <DL to 0.195 µg L⁻¹ for Sn, from <DL to 0.0277 µg L⁻¹ for Cs, from <DL to 1.13 µg L⁻¹ for Pb, from <DL to 0.0202 µg L⁻¹ for Th, and from 0.27 to 0.75 µg L⁻¹ for U. The concentrations of all major ions and trace elements in water were below the drinking water standards. CF values showed considerable and high contamination of samples with Al, Mn, Fe, Co, Cu, Cd, Sn, Pb, and Th. PLI values classified the majority of water samples as water with baseline levels of pollutants, and part of the samples was classified as either polluted or highly polluted.

1. Introduction

Surface water bodies, providing drinking water and water for economic use, are a vital resource and are of crucial importance for economic and social development [1,2]. Freshwater resources in these water bodies and their availability can significantly increase the industrial–agricultural and recreational potential of an area, leading to a higher rate of urbanization on the coast. The growing activity of the population, growth in agricultural areas, and industrial development in the reservoir basin led to the influx of pollutants, therefore leading to negative transformations in the quality of water resources [3,4,5]. At present, numerous studies are devoted to the ecological state of surface water bodies, including the identification of water pollution sources, spatial–temporal dynamics of water hydrochemical composition, and assessment of its quality [6,7,8]).

One of the main industries of human economic activity, which is developed at large surface water bodies is hydropower. Despite certain economic advantages, dams give rise to various environmental transformations: changes in the amount of regional precipitation, river discharge from different locations along the river, biodiversity, etc. [9,10,11]. The water hydrochemical composition of reservoirs, including the concentrations of major ions and trace elements, relies on natural (weathering processes, lithology of the basin, the composition of water in tributaries, etc.), and anthropogenic factors [12,13]. The anthropogenic enrichment of water chemical composition is determined both by the creation of the reservoir [14], and the entry of these elements with discharge water from different industries and agriculture, surface runoff, and atmospheric transport from urban areas. Irrespective of the origin, the accumulation of pollutants, in particular heavy metals, is a serious ecological threat [15]. First of all, it is a problem of deterioration in the quality of water used for drinking. Bioaccumulation and biomagnification of potentially toxic elements in hydrobionts of different trophic levels [16] and further consumption of biological resources from such a reservoir may negatively affect the health of the local population [17,18].

The water resources of the Baikal region, with Lake Baikal located in its center, are characterized by fresh and ultra-fresh surface and groundwater [19]. Lake Baikal itself is the largest freshwater reservoir on the planet. In terms of the concentrations of 58 trace elements, the lake is classified as the cleanest lake of the biosphere [20]. The only channel of the surface runoff of Lake Baikal is the Angara River, which carries about 61 km³ annually. The anthropogenic transformation of the hydrological and hydrochemical regimes of the river is related to the construction of a series of water reservoirs, the so-called cascade of the Angara River reservoirs (Irkutsk, Bratsk, Ust-Ilimsk, and Boguchany). Water resources of the Irkutsk Reservoir, which is the first reservoir in the Angara cascade, are used for supplying drinking water to the population of large cities (Irkutsk and Shelekhov) and a number of smaller settlements of the Irkutsk district. The gathered resources of the Irkutsk Reservoir are also used for generating hydroelectric power and navigation, as well as for fisheries and a number of recreational activities.

The major ion composition and the components of the water trophic status were studied at different stages of the Irkutsk Reservoir operation [21,22]. However, the data on water trace element composition of the Irkutsk Reservoir are still scarce. The studies by [23] were conducted to assess average total Ni, Zn, Cu, Pb, V, Co, Mn, Al, and Cr concentrations and to compare them with the trace element contents in other reservoirs of the Angara cascade. The present study focuses on: (a) concentrations of major ions (HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, Na⁺ and K⁺) and trace elements (Al, Cr, Mn, Fe, Co, Cu, Zn, As, Cd, Sn, Cs, Pb, Th, and U) in surface and near-bottom waters of the Irkutsk Reservoir; (b) spatial–temporal dynamics in the concentrations of major ions and trace elements; (c) main natural and anthropogenic factors affecting the hydrochemical composition; (d) assessment of water quality in the Irkutsk Reservoir in terms of trace element concentrations using the single factor pollution index (CF) and the pollution load index (PLI). The studies are of significant practical interest for assessing the water quality of the large unified Baikal–Angara freshwater system and therefore contribute to preserving it as the key source of clean drinking water.

2. Materials and Methods

2.1. Study Site

The creation of the Irkutsk Reservoir (Figure 1), formed by the dam of the Irkutsk Hydroelectric Power Plant in 1956, resulted in flooding the valley of the Angara River between the dam (Irkutsk city) and the river source (Listvyanka settlement). As a result of the filling of the Irkutsk Reservoir, the water level of Lake Baikal increased by approximately 1 m on average. The Irkutsk Reservoir has a water surface area of 154 km² at normal water level (457 m a.s.l.), a volume of 2.1 km³, a length of 55 km, and a width ranging from 0.5 to 3.5 km. The maximum depth of the reservoir occurring close to the dam is 35 m. Water level fluctuations in the reservoir are determined, to a greater extent, by the annual water level changes in Lake Baikal and the regime of the Irkutsk Hydroelectric Power Plant. The amplitude of the Irkutsk Reservoir level variations can reach as high as 3 m; however, during most time of its operation, the reservoir is lowered by 0.14–1.4 m than the normal water level [24]. Since 2001, the water levels in the Irkutsk Reservoir have been limited to the range between 456 m a.s.l. and 457 m a.s.l. in order to minimize the impact of the retention management on Lake Baikal ecosystem [25]. As a result of filling the reservoir, 40 bays were formed. The largest among them are Ershi, Kurma, Kartakoi, Elovyi, and Uladova.

The northwestern part of the Irkutsk Reservoir lies within the Irkutsk–Cheremkhovo plain. At the southeast end of the basin, there are western spurs of the Primorsky Ridge. The northwestern part of the basin is characterized by a hilly, gently rolling landscape with incised valleys and vast flat watersheds. In the eastern part, there are groups of low hills and low ridges with intrefluves lying hypsometrically higher. The left shore is composed of bedrock, represented by the Jurassic sandstones, argillites, and siltstones. The right shore is made up of the Quaternary diluvial–alluvial sediments, containing loess-like loam and sandy loam as well as sand and pebbles, which were subject to abrasion along almost the entire shore [24].

The dam of the Irkutsk Hydroelectric Power Plant is located in Irkutsk city (Figure 1), the largest city in the Baikal region, whose leading industries are aircraft building, metalwork, and electric power generating. There are also metallurgy, forestry, food, pulp and paper, and woodworking industries. All major industrial enterprises are located below the dam of the Irkutsk Hydroelectric Power Plant. The left shore of the reservoir is very steep and therefore, less accessible and untapped. On the shores of such bays as Kurma, Kartakoi, Ershi, and Mel’nichnaya Pad’, there are many settlements and tourist camps. However, from Kurma Bay to the source of the Angara River, settlements and tourist camps are less numerous. At the source of the Angara River, there is an abandoned shipyard that has been in operation since the 1890s. There is also an operating cargo and passenger port (Port Baikal) here. Along the right shore of the Irkutsk Reservoir, you can find a number of settlements, camps, and agricultural fields. The highway runs close to the reservoir connecting Irkutsk and Listvyanka settlements. The tourism industry is particularly developed in Listvyanka settlement, lying on the northeast shore of Lake Baikal, at the source of the Angara River. It should be noted that recreational attractiveness, in turn, entails an anthropogenic load on the environment. Around Listvyanka settlement and adjacent areas, the coastal zone is constantly developing from year to year. At the same time, the lack of a centralized wastewater treatment system in the settlement leads to the increasing anthropogenic load. The creation of the Irkutsk Reservoir provided good conditions for shipping from the dam to Lake Baikal. Currently, water transport for personal use is becoming more popular.

2.2. Sampling and Analytical Methods

There were five sampling campaigns to collect water samples from the Irkutsk Reservoir, conducted in July 2007, 2012, and in May (2021 (M)), July (2021 (J)), and September (2021 (S)) 2021. Sampling sites from S1 to S11 were distributed across the entire water area of the reservoir and were localized in the channel part and bays (Figure 1, Table S1). At the sampling sites with a water depth of over 2 m, the samples were collected from both the surface water layer (0.5 m) and the bottom one (in 1 m layer from the bottom). In 2021, the water samples were additionally collected along the right (S1-r, S11-r) and the left (S1-l, S11-l) shores of the reservoir, in the vicinity of Irkutsk city and Listvyanka settlement.

At each sampling site, three parallel water samples were collected. Replicate samples from the same site were taken within about 100 m of one another and then were well mixed. The mixed water samples were filtered through a 0.45 μm Millipore membrane filter and placed into polyethylene bottles which were prewashed with 3% nitric acid. The primary filtration part was discarded to clean the membrane. For the trace element analysis, the water samples were immediately acidified by addition of HNO₃ (ultrapure «Merk», Darmstandt, Germany). Prior to the analysis, the water samples were stored in the refrigerator.

The chemical analysis of water samples was accomplished at the Center for Collective Use «Isotope-Geochemical Research» located at the IGC SB RAS (Irkutsk, Russia). In the present study, SO₄²⁻, Cl⁻, Ca²⁺ and Mg²⁺, Na⁺, K⁺ were analyzed by the method of capillary electrophoresis using devices of the “Drops” series (Lumex Ltd., St. Petersburg, Russia) and HCO₃⁻ was determined by the titrimetric method. The detection limits for elements were as follows: 6.1 mg L⁻¹ for HCO₃⁻; 0.5 mg L⁻¹ for SO₄²⁻; 0.5 mg L⁻¹ for Cl⁻; 0.5 mg L⁻¹ for Ca²⁺; 0.3 mg L⁻¹ for Mg²⁺; 0.5 mg L⁻¹ for K⁺; 0.5 mg L⁻¹ for Na⁺. Inductively coupled plasma mass spectrometry (ICP-MS) via a high-resolution double-focusing mass spectrometer ELEMENT-2 (Thermo Finnigan, Bremen, Germany) was used to analyze the total concentrations of trace elements. Multi-element standard samples such as ICP Multi-Element Standard Solution-Sol X CertiPUR for Surface Water Testing, Sol XII CertiPUR (MERCK, Darmstandt, Germany), and Combined Quality Control standard IQC-026 (NIST, North Kingstown, RI, USA) were applied to ensure the reliability of the analytical measurements. Water purified by the Millipore-ELIX-3 system (Millipore SA, Molsheim, France) was used to prepare washing, blank, calibration, and analyzed solutions. Three repetitions of the analysis yielded a relative standard deviation of <5%. Otherwise, the measurements were repeated until all the data reached the standard. The following metals were tested in water samples ²⁷Al, ⁵²Cr, ⁵⁵Mn, ⁵⁶Fe, ⁵⁹Co, ⁶³Cu, ⁶⁶Zn, ⁷⁵As, ¹¹¹Cd, ¹²⁰Sn, ¹³³Cs, ²⁰⁸Pb, ²³²Th and ²³⁸U. The detection limits (DL) for the elements were as follows: 0.81 µg L⁻¹ for Al; 0.05 µg L⁻¹ for Cr; 0.37 µg L⁻¹ for Mn; 0.75 µg L⁻¹ for Fe, 0.003 µg L⁻¹ for Co; 0.03 µg L⁻¹ for Cu; 0.64 µg L⁻¹ for Zn; 0.12 µg L⁻¹ for As; 0.002 µg L⁻¹ for Cd; 0.010 µg L⁻¹ for Sn; 0.0003 µg L⁻¹ for Cs, 0.013 µg L⁻¹ for Pb, 0.0003 µg L⁻¹ for Th; 0.001 µg L⁻¹ for U.

The statistical data processing was performed using SPSS statistical software (IBM, Armonk, NY, USA, v. 20.0).

2.3. Pollution Indices

The following generally accepted pollution indices were used to calculate the water contamination in the Irkutsk Reservoir:

• The single-factor pollution index (CF) was used to determine only one element in a sample [26]:

  $$\mathrm{C}\mathrm{F}=\frac{{\mathrm{C}}_{\mathrm{i}}}{{\mathrm{C}}_0},$$

  (1)

  where C_i is the concentration of the analyzed element and, C₀ is the concentration of metal in the control material. The CF value is divided into categories: CF < 1—low contamination, 1 ≤ CF ≤ 3—moderate contamination, 3 ≤ CF ≤ 6—considerable contamination, 6 ≥ CF—very high contamination.
• The pollution load index (PLI) was used to calculate the total contamination in each sample [27]:

  $$\mathrm{P}\mathrm{L}\mathrm{I}=\sqrt[\mathrm{n}]{{\mathrm{C}\mathrm{F}}_1·{\mathrm{C}\mathrm{F}}_2·{\dots ·\mathrm{C}\mathrm{F}}_{\mathrm{n}}},$$

  (2)

  where CF is an individual element pollution index. The PLI is divided into categories: PLI < 0—non-polluted, 0 < PLI ≤ 1—baseline levels of pollutants, 1 < PLI ≤ 10—polluted, 10 < PLI ≤ 100—highly polluted, PLI > 100—progressive deterioration of the environment.

3. Results and Discussion

3.1. Major Ion Composition

In samples collected in 2007, 2012, and 2021, the pH levels varied from 6.4 to 8.6, with an average value of 7.7. Table S2 presents the limits and mean concentrations of major ions and total mineralization (TDS) in 2007, 2012, May, June, and September 2021 in the Irkutsk Reservoir waters.

Table 1. Comparison of major ion concentrations (mg L⁻¹) in water of the Irkutsk Reservoir and other ponds of the Baikal–Angara water system.

	HCO3−	Cl−	SO42−	Ca2+	Mg2+	Na+	K+	TDS	Reference
Lake Baikal	66.3 ± 1.6	0.4 ± 0.03	5.5 ± 0.1	16.4 ± 0.4	3.0 ± 0.1	3.3 ± 0.1	1.0 ± 0.1	~96	[28]
Source of the Angara River	66.2	0.6	5.7	15.4	3.3	4.2		95.6	[29]
Source of the Angara River	59.3−69.3 * 64.2 ± 3.7	<DL−1.21 0.76 ± 0.34	3.9−5.8 4.3 ± 0.5	14.4−15.6 14.0 ± 0.6	3.0−3.6 3.2 ± 0.2	2.9−3.4 3.2 ± 0.2	0.9−1.1 1.0 ± 0.1	86.1−100.1 92.4 ± 4.9	Present study
Irkutsk Reservoir	50.4−73.2 64.0 ± 3.9	<DL−1.27 0.85 ± 0.21	3.7−8.5 4.8 ± 0.8	11.7−17.7 14.9 ± 0.8	2.0−5.7 3.3 ± 0.4	2.7−3.6 3.2 ± 0.2	0.7−1.2 1.0 ± 0.1	50.4−73.2 64.0 ± 3.9
Bratsk Reservoir	71.9	3.0	11.3	19.8	3.9	4.0	1.0	71.9	[30]
Ust-Ilimsk Reservoir	79.6	16.2	5.5	19.1	6.5	6.3	1.1	79.6	[31]

Notes: * Above the line—minimum–maximum value, below the line—mean value ± standard deviation.

shows the data summarized for all sampling campaigns. The data available from monitoring studies of water in the Irkutsk Reservoir (Table 1 and Table S2) showed insignificant spatial and temporal variations in major ion composition. The concentrations of the majority of main ions in the water samples collected during all sampling campaigns were found to lie within ±2 standard deviation (SD) of their mean. Only, the concentrations of SO₄²⁻ in three samples, Ca²⁺ in two samples, and Mg²⁺ in one sample, were found to fall within ±3 SD of the mean. In water samples taken from the Irkutsk Reservoir, the cations were dominated by Ca²⁺ (from 11.7 to 17.7 mg L⁻¹, with a mean value of 14.9 mg L⁻¹) and the anions were dominated by HCO₃⁻ (from 50.4 to 73.2 mg L⁻¹, 64.1 mg L⁻¹ as a mean value). The water type was characterized as HCO₃-Ca.

In the study area, the water samples collected during all sampling campaigns demonstrated low TDS levels: from 78.5 to 104.0 mg L⁻¹. According to our determinations, the average TDS value in the Irkutsk Reservoir (92.5 mg L⁻¹) was less than the world river water median (127 mg L⁻¹) [32]. This value was close to the average TDS level in Lake Baikal (~96 mg L⁻¹) [28] and the source of the Angara River (95.6 mg L⁻¹) [29] (Table 1). The main natural sources of dissolved salts, which change the composition of water over a wide range, include cyclic salts and weathering of minerals [33,34,35]. Similar average concentrations of major ions and TDS values in water columns of Lake Baikal, the Angara River source (S1), and the Irkutsk HPP headwater (S11) (Table 1 and Table S2) suggest that the main source of dissolved substances entering the reservoir is Lake Baikal runoff. Weathering processes and cyclic salts were found to insignificantly influence the major ion water chemistry in the reservoir. Higher concentrations of the major ions in the water column of the Bratsk and Ust-Ilimsk Reservoirs (Table 1), located downstream on the Angara River, suggest a lower contribution from the lake’s runoff and a higher contribution from natural and anthropogenic sources.

As shown by monitoring studies conducted since the beginning of the 20th century, in the pelagic part of Lake Baikal major ion composition exhibits weak seasonal and interannual variability within the accuracy of methods [36,37]). In the Irkutsk Reservoir, in particular at the Angara River source, the levels of main ions, including HCO₃⁻, Cl⁻, and SO₄²⁻ were higher than those in Lake Baikal (Table 1). A similar distribution pattern of main ions was found in the Angara River source during the period of 1997–2003 [38]). Based on the results of long-term monthly investigations, it was shown that HCO₃⁻ concentration at the river’s source could be affected by the fluctuations of water level, whose maxima correlate with minimum ion levels, while SO₄²⁻ concentrations were influenced by large seismic events. The long-term dynamics of the water level in the Irkutsk Reservoir are recorded as the alternation of maximum and minimum cycles. Thus, in the period from 2004 to 2018, the water level in the reservoir was close to the normal water level [39]. Larger water level variations were observed in 2021. Especially pronounced was the drop in the water level to 456.20 m in the first half of May resulting from both the increased discharges of the Irkutsk HPP, and contrast weather conditions of the spring of 2021, with frequent colder weather, extended snowmelt leading to losses of water by evaporation and, as a result, a slow increase in water inflow into the reservoir. In May, HCO₃⁻ exhibited the greatest variations. Moreover, its level was minimal throughout all sampling campaigns (Table S2). From June to September, there was a steady increase in the water level (up to 457.22 m) due to a decrease in discharge flows of the Irkutsk HPP and an increase in the water level of Lake Baikal.

In addition to natural factors, anthropogenic factors are also responsible for the water ionic composition in reservoirs. The anthropogenic impact may lead to increased concentrations of ions, mainly of Cl⁻ and SO₄²⁻ [5,40]. These ions were ascribed to the main pollutants in the southern part of Lake Baikal, whose runoff was found to significantly affect the chemical water composition at the source of the Angara River and in the Irkutsk Reservoir. For several decades, the sulfate concentrations in the water column of the South Baikal and in the Angara River source had been significantly affected by the wastewater from the Baikalsk Pulp and Paper Mill (BPPM) closed in 2013 [41]. In addition to SO₄²⁻, large amounts of Cl⁻ enter (entered) the South Baikal from different sources: wastewater of BPPM, 7.3 ± 0.2 ton/year; wastewater of Ulan-Ude city—5274 ± 648 ton/year; polluted water of Selenga River (Lake Baikal largest tributary) 70.65 ± 9.91 tons/year [42]. A number of suburban settlements along the shores of the Irkutsk Reservoir were likely another source of Cl⁻ in the water of the reservoir. The potassium chloride used as a component of agricultural fertilizers is known to increase Cl⁻ ion concentration in the aquatic system [43]. With the data available, the contribution of fertilizers cannot be quantified. However, in the Irkutsk Reservoir, K⁺ levels vary insignificantly over the entire monitoring period (0.91–0.96 mg L⁻¹ as an average; Table S2); there is no correlation between the ions of chlorine and potassium. Therefore, this study revealed no contribution from agricultural fertilizers to the Cl⁻ concentration at all sampling localities within the Irkutsk Reservoir. A larger influence on Cl⁻ concentrations is likely to be recorded in the water of the bays as the areas adjacent to the bays are more developed.

3.2. Trace Elements Composition

Table S3 and Figure 2 present statistics of trace element concentrations in 2007, 2012, May, June, and September 2021 in the Irkutsk Reservoir waters. Table 2 shows the data summarized for all sampling campaigns.

A large collection of data on trace element geochemistry in surface waters of Africa, Europe, Asia, and North and South America are presented in [44]. The average concentrations of Al, Mn, Fe, Co, Cu, As, Cd, Cs, and Th in the Irkutsk Reservoir were markedly lower than the average world value; Cr levels were lower than their minimum values and the concentrations of Zn, Pb, and U exceeded the corresponding world average but were below the worldwide maximum values (Table 2).

Table 2. Comparison of trace elements concentrations (µg L⁻¹) in Irkutsk Reservoir and other ponds of the Baikal–Angara water system.

		Al	Cr	Mn	Fe	Co	Cu	Zn	References
Irkutsk Reservoir	Min–max	1.13–15.39	<DL–0.39	0.39–23.12	1.25–53.22	0.005–0.100	0.20–1.98	<DL–13.40	Present study
	Mean	4.60	0.10	7.02	12.09	0.017	0.55	4.21
	Median	3.79	0.08	5.26	9.33	0.013	0.45	3.51
	SD	2.93	0.05	5.42	9.51	0.016	0.30	2.71
Source of the Angara River		1.13–6.09 * 3.07	0.06–0.17 0.11	0.76–12.57 5.54	1.25–16.90 7.67	<DL–0.025 0.011	0.28–1.29 0.68	2.02–10.26 4.99
Source of the Angara River		3.89	0.12	2.93	18.8	0.011	0.62	2.11	[45]
Lake Baikal		0.34–1.15	0.07	0.06–0.33	0.26–1.12	0.003	0.16–0.25	0.24–0.56	[46]
Lake Baikal		0.1–1.0 0.38	0.03–0.09 0.07	0.01–0.53 0.13	0.1–1.6 0.38	0.002–0.005 0.003	0.2–1.0 0.21	0.4–4.3 3.2	[20]
Bratsk Reservoir		12.9	0.19	22.9	22.4	–	0.84	3.80	[47]
World average values		0.5–480 32	0.5–11.5 0.7	0.41–113.52 34	1–525 66	0.006–0.260 0.148	0.23–3.53 1.48	0.04–27.0 0.6	[44]
MPC **		500	50	100	300	100	1000	5000	[48]
		As	Cd	Sn	Cs	Pb	Th	U	Reference
Irkutsk Reservoir	Min–max	0.25–0.53	0.004–0.127	<DL–0.195	<DL–0.0277	<DL–1.13	<DL–0.0202	0.27–0.75	Present study
	Mean	0.37	0.025	0.022	0.0034	0.23	0.0017	0.50
	Median	0.37	0.021	0.011	0.0012	0.12	0.0007	0.51
	SD	0.06	0.018	0.031	0.0050	0.25	0.0033	0.07
Source of the Angara River		0.25–0.48 0.37	0.015–0.055 0.029	<DL–0.195 0.032	0.0006–0.0087 0.0043	<DL–1.13 0.33	<DL–0.0053 0.0009	0.40–0.55 0.50
Source of the Angara River		0.46	0.011	0.028	0.0016	0.05	0.0014	0.58	[45]
Lake Baikal		0.40–0.41	0.008	<0.011	0.0017	0.010–0.036	0.0006	0.52	[46]
Lake Baikal		0.30–0.50 0.40	0.001–0.010 0.008	<0.01–0.04 <0.01	0.002–0.008 0.0013	<0.02	0.002–0.020 0.004	0.4–0.7 0.55	[20]
Bratsk Reservoir		0.37	0.023	–	0.0022	0.111	–	0.52	[47]
World average values		0.11–2.71 0.62	0.0006–0.42 0.08	–	0.0006–0.016 0.011	0.006–3.8 0.079	0.001–4.3 0.0055	0.004–4.94 0.37	[44]
MPC		50	1	–	–	30	–	–	[48]

Notes: * Above the line—minimum–maximum value, below the line—mean value; ** Maximum permissible concentrations of trace elements.

Table 2. Comparison of trace elements concentrations (µg L⁻¹) in Irkutsk Reservoir and other ponds of the Baikal–Angara water system.

		Al	Cr	Mn	Fe	Co	Cu	Zn	References
Irkutsk Reservoir	Min–max	1.13–15.39	<DL–0.39	0.39–23.12	1.25–53.22	0.005–0.100	0.20–1.98	<DL–13.40	Present study
	Mean	4.60	0.10	7.02	12.09	0.017	0.55	4.21
	Median	3.79	0.08	5.26	9.33	0.013	0.45	3.51
	SD	2.93	0.05	5.42	9.51	0.016	0.30	2.71
Source of the Angara River		1.13–6.09 * 3.07	0.06–0.17 0.11	0.76–12.57 5.54	1.25–16.90 7.67	<DL–0.025 0.011	0.28–1.29 0.68	2.02–10.26 4.99
Source of the Angara River		3.89	0.12	2.93	18.8	0.011	0.62	2.11	[45]
Lake Baikal		0.34–1.15	0.07	0.06–0.33	0.26–1.12	0.003	0.16–0.25	0.24–0.56	[46]
Lake Baikal		0.1–1.0 0.38	0.03–0.09 0.07	0.01–0.53 0.13	0.1–1.6 0.38	0.002–0.005 0.003	0.2–1.0 0.21	0.4–4.3 3.2	[20]
Bratsk Reservoir		12.9	0.19	22.9	22.4	–	0.84	3.80	[47]
World average values		0.5–480 32	0.5–11.5 0.7	0.41–113.52 34	1–525 66	0.006–0.260 0.148	0.23–3.53 1.48	0.04–27.0 0.6	[44]
MPC **		500	50	100	300	100	1000	5000	[48]
		As	Cd	Sn	Cs	Pb	Th	U	Reference
Irkutsk Reservoir	Min–max	0.25–0.53	0.004–0.127	<DL–0.195	<DL–0.0277	<DL–1.13	<DL–0.0202	0.27–0.75	Present study
	Mean	0.37	0.025	0.022	0.0034	0.23	0.0017	0.50
	Median	0.37	0.021	0.011	0.0012	0.12	0.0007	0.51
	SD	0.06	0.018	0.031	0.0050	0.25	0.0033	0.07
Source of the Angara River		0.25–0.48 0.37	0.015–0.055 0.029	<DL–0.195 0.032	0.0006–0.0087 0.0043	<DL–1.13 0.33	<DL–0.0053 0.0009	0.40–0.55 0.50
Source of the Angara River		0.46	0.011	0.028	0.0016	0.05	0.0014	0.58	[45]
Lake Baikal		0.40–0.41	0.008	<0.011	0.0017	0.010–0.036	0.0006	0.52	[46]
Lake Baikal		0.30–0.50 0.40	0.001–0.010 0.008	<0.01–0.04 <0.01	0.002–0.008 0.0013	<0.02	0.002–0.020 0.004	0.4–0.7 0.55	[20]
Bratsk Reservoir		0.37	0.023	–	0.0022	0.111	–	0.52	[47]
World average values		0.11–2.71 0.62	0.0006–0.42 0.08	–	0.0006–0.016 0.011	0.006–3.8 0.079	0.001–4.3 0.0055	0.004–4.94 0.37	[44]
MPC		50	1	–	–	30	–	–	[48]

Notes: * Above the line—minimum–maximum value, below the line—mean value; ** Maximum permissible concentrations of trace elements.

When studying the trace element water chemistry of the Irkutsk Reservoir, it is reasonable to compare the trace element characteristics of the reservoir with those of Lake Baikal (Table 2). The data on hydrochemistry of Lake Baikal showed that like the ionic, the trace element composition was stable at all depths in the pelagic zone of the lake [46]. At the same time, its hydrochemical characteristics were influenced by natural (multi-component flows of more than three hundred rivers of its catchment area, hot and cold springs [49], and anthropogenic factors as well as by seismic activity [50], etc.). The effect of anthropogenic factors was demonstrated by the chemical composition of snow cover from the lake’s water area near the settlements [51]. The results revealed that the snow chemical composition was characterized by higher concentrations of Mn, Al, Pb, Cu, Fe, and Zn in the southern part, increased Mn, Al, Pb, Cr, Fe, and Zn levels in the middle part, and higher Mn, Al, Pb, Cu, Cr, and Fe contents in the northern part of Lake Baikal. Long-term monitoring of concentrations of major ions [38] and mercury [50] in the Angara River source indicates that hydrochemical characteristics of water from the river’s source characterize the average water composition of the entire Lake Baikal. Therefore, when studying trace element water composition in the Irkutsk Reservoir, the Angara River source is considered separately.

3.2.1. Angara River Source

In sampling campaigns of 2021, the element concentrations in water samples from the Angara River source demonstrated the following ranges: Al (1.13–6.09 µg L⁻¹); Cr (0.06–0.17 µg L⁻¹); Mn (1.22–12.57 µg L⁻¹); Fe (3.94–16.90 µg L⁻¹); Co (<DL–0.018 µg L⁻¹); Cu (0.28–1.29 µg L⁻¹); Zn (2.02–10.26 µg L⁻¹); As (0.25–0.48 µg L⁻¹); Cd (0.015–0.039 µg L⁻¹); Sn (<DL–0.036 µg L⁻¹); Cs (0.0012–0.0087 µg L⁻¹); Pb (<DL–1.13 µg L⁻¹); Th (<DL–0.0202 µg L⁻¹); U (0.27–0.61 µg L⁻¹) (Figure 3).

In different periods of studies [20,46,52], the concentrations of the vast majority of trace elements were in good agreement with each other, except for Zn and Th (Table 2). In the water of Lake Baikal, the Zn concentration measured by Sklyarova [46] (0.56 µg L⁻¹) is much lower than its abundance determined by Falkner et al. [52] (2.9 µg L⁻¹) and Vetrrov et al. [20] (4.3 µg L⁻¹). For thorium, this situation was quite different: Th levels given in [20] (0.004 µg L⁻¹) were an order of magnitude higher than its concentrations measured by Sklyarova [46] (0.0006 µg L⁻¹). In the water of the Angara River source, Zn concentrations were in line with its abundances in Lake Baikal obtained by Vetrov et al. [20], while Th levels were in good agreement with Th contents in Lake Baikal measured by Sklyarova [46]. The concentrations of As and U measured in the Angara source are well comparable with their levels in the water from the pelagic part of the lake (Table 2, Figure 3). The levels of Al, Cr, Mn, Fe, Co, Cu, Cd, Sn, Cs, and Pb in the water of the river source were higher than those in the lake’s water. The results of this study are in line with the data on trace element compositions obtained from 3-year (2006–2008) monthly water monitoring of the Angara River source [45].

At the source of the Angara River in different sampling campaigns of 2021, higher Al, Mn, Fe, Co, Cu, As, Cd, Sn, Cs, Pb, Th concentrations were found along the left shore of the river as compared with its center, while increased levels of Cr, Mn, Fe, Co, Cu, Zn, As, Cd, Sn, and U were observed along the right shore (Figure 3). In June, the water of the middle part was characterized by higher Al, Cr, Fe, Cu, and U contents as compared with the shores. The anthropogenic impact on this part of the reservoir is primarily related to the activity of the cargo and passenger port, whose ships and ferries run between the left and right banks all year round. Antifouling paints, metals, and steel alloys, as well as petroleum products, are an important source of trace elements in the water bodies near the shipyards and yacht clubs [53,54,55]. The area close to the Listvyanka settlement with a well-developed tourist infrastructure may also be a main source of trace elements in this part of the Irkutsk Reservoir. The study of surface and groundwater in the vicinity of the settlement revealed that the anthropogenic impact resulted in the pollution of primarily groundwater, whose subaqual discharge led to higher Mn, Zn, and Pb concentrations in the near-bottom water of the lake’s coastal zone [56]. The Baikal under-ice water taken close to the Listvyanka settlement was characterized by higher Mn, Cu, Co, Al, Fe, and Zn contents as compared with the deep-seated water [57]. The changes in the chemical composition of coastal waters are thought to be related to the dissolution of rocks and soils, mechanical transport of the detrital material from the coastline, and atmospheric transport.

3.2.2. Irkutsk Reservoir

As opposed to major ions, the trace element concentrations showed a more significant spatial–temporal variability (Table 1 and Table 2). In the vast majority of water samples, the concentrations of trace elements were found to lie within mean ± 2SD. We calculated the median, which is unlike the mean and is not sensitive to the outlier measured values.

Table 3. Sampling sites, where trace element concentrations in water exceed median + 2SD.

Sampling Site		Trace Element	Sampling Site		Trace Element
S1	s 1	Al, Cu, Pb—2007 2, Sn—2012, Pb, Zn—2021 (S)	S1-r	s	Pb—2021 (S)
S1-l	s	Cu—2021 (M), Pb—2021 (S)	S2	s	Al, Cr, Th—2007, Sn—2012, Mn—2021 (J), Zn—2021 (S)
				b	Al, Cu—2021 (M), Zn—2021 (S)
S3	s	Cr, Fe, Co, Th, U—2007, Co—2021 (M), Cr, Zn, Pb—2021 (S)	S4	s	Cu—2007, Al, Mn, Co, Cu—2021 (M), Fe, Cs—2021 (J), Fe, Zn, Cr—2021 (S)
	b	Zn, Pb—2021 (S)		b	Sn—2021 (S), Al, Fe, Co, Zn—2021 (S)
S5	s	Al, Mn, Fe, Pb –2012, Mn, Fe—2021 (J), Mn—2021 (S)	S6	s	Sn—2012, Mn—2021 (M), Cr, Cs—2021 (J), Zn, Sn—2021 (S),
	b	Co—2021 (M), Cs—2021 (J), Fe, Zn—2021 (S)		b	Sn—2012, Sn—2021 (S)
S7	s	Zn—2021 (S)	S8	s	–
	b	Zn—2021 (S)		b	–
S9	s	Th—2007, Cu, Zn—2021 (M), Cr, Fe, Co, Sn, As—2021 (J), Sn—2021 (S)	S10	s	Fe, Th, U—2007, Al—2021 (J), Pb—2021 (S)
	b	–		b	Cu—2012, Mn, Cs—2021 (J)
S11	s	Sn—2012, Cd, Sn, Cs—2021 (J)	S11-r	s	Cd, Cs—2021 (J)
	b	Sn, Pb—2012, Cd, Cs, Pb—2021 (J), Pb—2021 (S)	S11-l	s	Sn—2012, Mn—2021 (S)

Notes: ¹ s—surface water, b—bottom water; ² sampling campaigns in 2007–2007, 2012–2012, May 2021–2021 (M), July 2021–2021 (J), and September 2021–2021 (S).

illustrates sampling sites, where the trace element concentrations were above 2SD of the median. In some samples, trace element concentrations were found to exceed the median + 2SD: Al—7 samples; Cr—6 samples; Mn—8 samples, Fe—8 samples, Co—6 samples, Cu—7 samples; Zn—12 samples; Cd—3 samples; Sn—12 samples, Cs—7 samples; Pb—11 samples; Th—4 samples.

There are several features that characterize the influence of Lake Baikal runoff on the water trace element composition in the Irkutsk Reservoir. Firstly, during all sampling campaigns, median concentrations of Al, Cr, Mn, Co, As, Th, and U across the reservoir were close and the median Cu, Zn, Cd, Sn, Cs, and Pb contents were lower than those measured at the source of the Angara River. Secondly, the concentrations of Zn and U in the water of both the Irkutsk Reservoir and Lake Baikal exceeded the average value for the world’s surface water (Table 2). Higher U concentrations (similar to maximum), as compared with the average world values, and increased Zn levels (similar to mean ones) in the water of Lake Baikal characterize its geochemical background in the area surrounding the lake [46]. Thirdly, the concentrations of trace elements in the water of the Irkutsk Reservoir decreased in the following order Fe > Mn > Al, Zn > Cu, U > As > Pb > Cr > Cd, Sn > Co > Cs > Th, which is similar to the pattern found for the water at the Angara River source.

When comparing the channel part and the bays of the reservoir (Table S3), the number of elements showing higher concentrations was larger in the surface water of the bays. It is, in particular, true for the Kurma Bay (S4), whose water area is widely used for fishery, Burduguz (S3), and Uladova (S5) Bays (Table 3). In the channel part, higher concentrations of trace elements are primarily found opposite the Mel’nichnaya Pad’ settlement (S9). Water transport, which is rapidly developing at present, was another anthropogenic factor influencing the water hydrochemistry of both the Irkutsk Reservoir and the Angara River source. In addition, a number of suburban settlements, which are not connected to the urban sewer system or septic tanks for treating household waste, are located along the coastline of the bays in the immediate vicinity of the water. Untreated waste can significantly affect the water quality in waterways connected to the reservoir [58]. In the absence of wastewater treatment facilities, the entry of pollutants into the Irkutsk Reservoir with both surface (meltwater and rainwater flowing from coastal settlements) and underground runoffs can be significantly higher. The elements, whose concentrations were higher at the majority of sampling sites included Mn, Fe, Al, Zn, Sn, Pb, Cu, and Cs (Table 3). Giri and Singh [59] related the enhanced Mn, Pb, and Zn levels in the waters of the Subarnarekha River (India) to industrial wastes and transport pollution. In coastal waters of China, Pb and Zn abundances were found to be closely associated with motor transport [60]. The Pb and Zn levels in the Aibi Lake are mainly influenced by agricultural fertilizers, emissions from traffic, and/or urban construction in its basin [61].

The results from this study indicate that enhanced Al, Mn, Fe, Cu, Zn, As, and Cd levels were observed along the right and left banks of the Irkutsk Reservoir, close to Irkutsk city (Figure 4). In July and September, Pb concentrations significantly increased in relation to May. A variety of anthropogenic activities in urban areas led to the accumulation of a wide scope of trace elements, including Cd, Cu, Pb, and Zn [62] in the upper soil layer. From the area near Irkutsk, trace elements of anthropogenic origin enter the reservoir with the surface runoff. Therefore, the trace element concentrations can increase, primarily in the aquatic environment along the shores. In the vicinity of Irkutsk, the anthropogenic impact on the left shore is mainly due to the city beach, while on the right shore, this impact is related to a passenger port and parking for private water transport (about 200 small vessels and yachts). Herewith, near Listvyanka settlement, the navigation takes place all year round, but near the HPP dam, water transport activity and the growing number of tourists are observed in the warmer season (July–August).

Another source of trace elements in the reservoir, in particular major elements of the Earth’s crust, is the abrasion of shores. The creation of the reservoir facilitated the processes of shore erosion, both in the reservoir itself and on the shores of Lake Baikal. Within the Irkutsk Reservoir, the shores, which are not exposed to erosion, lie close to the source of the Angara River, while the abrasive shores formed in the Jurassic sandstones and Quaternary sediments make up about 54% of the reservoir’s shoreline [63]. The composition of the bottom sediments in the reservoir reflects the geochemical specifics of rocks, which compose the shores exposed to impacts from processes of erosion. Along the banks composed of the Jurassic sandstones, the bottom sediments showed the dominance of Mn, Cr, V, Zn, Cu, while on the banks composed of the Quaternary diluvial loess loams the bottom sediments contained higher abundances of Mn, Co [64]. It was found that, with the material from the coastal erosion, the reservoir receives 4700 tons of Fe, 154 tons of Mn, and about 220 tons of other trace elements each year [64]. The terrigenous material, entering the bottom of the reservoir due to erosion processes, is able to suspend in the water column as particles of different fractions for a long time leading to higher Al, Mn, Fe, Co, as well as Cr, Zn, Cu levels in the water of the Irkutsk Reservoir.

The retrospective impact of human activity on the reservoir can be assessed by studying the composition of bottom sediments, which are active accumulators of elements of anthropogenic origin [65,66]. In the Bratsk Reservoir, the decrease in anthropogenic impact led to better water quality characteristics [47]. However, the results of layer-by-layer sampling of its bottom sediments showed the accumulation of the vast majority of trace elements in the middle layers of the bottom sediments formed during the period of the greatest anthropogenic impact [67]. In this regard, the bottom sediments of the Irkutsk Reservoir have not been sufficiently studied. The most complete information on the chemical composition (30 elements) of its bottom sediments is given in Jagus et al. [68]. In four sectors of the reservoir (Tal’tsy, Patrony, Novogrudinina, and Mel’nichnaya Pad’ settlements) we analyzed 10 samples of bottom sediments. Amongst the elements discussed in this study, Co, Pb, and Cr are worthwhile and noteworthy: Co and Pb demonstrate different spatial distribution patterns and Cr had higher concentrations as compared to its levels in other dammed reservoirs worldwide. At the same time, in Cartagena Bay, the bottom sediments taken at stations related to the repair and maintenance of ships were characterized by high concentrations of Cr, Cu, As, and Cd while sediments from stations receiving inputs from petroleum plants displayed high Pb levels [69]. Therefore, it can be suggested that higher concentrations of trace elements in near-bottom water layers of the Irkutsk Reservoir, particularly close to the dam (Table 3), may be a result of their release from bottom sediments.

3.3. Correlation Analysis

The Pearson correlation coefficient is widely used to determine general sources of elements’ entry [70,71]. A number of significant positive and significant negative correlations were obtained using the information from the study of trace element composition of water in the Irkutsk Reservoir (

Table 4. Statistically significant correlations between concentrations of major ions and trace elements in water of the Irkutsk Reservoir.

	2007	2012	2021 (M)	2021 (J)	2021 (S)
HCO3−	(+ **)Ca2+	(+ **)K+	(+ **)K+, Ca2+, U; (– **)SO42−, Mg2+, Al, Cr, Fe, Th	(+ *)Ca2+; (– *)Al	(+ *)Mg2+, As; (+ **)Fe
Cl−	(+ **)SO42−, (+ *)Mg2+, (– *) Ca2+		(– **)Zn	(+ **)SO42−, Cd, Cs; (+ *)Na+, Pb	(+ *)Cu
SO42−	(+ **)Cl−, Mg2+	(+ *)Co	(+ **)Mg2+, Al, Mn, Fe, Pb, Th; (– **)HCO3−, K+, Ca2+, U	(+ **)Cl−; (+ *) Mg2+, Cr, Cu; (– *)K+, U	(+ *)Mg2+, Al, Mn
K+	(+ **)Al, Co; (+ *)Cd, Pb	(+ **)HCO3−; (+ *)U; (– **)Fe; (– *)Mn	(+ **)HCO3−, Ca2+, U; (– **)SO42−,Al, Cr, Mn, Fe, Pb, Th	(– *)SO42−, Zn
Na+			(+ **)Cs; (+ *) Cd	(+ *)Cl−; (– *) U	(+ **)Fe: (+ *)Mg2+; (– *)Ca2+
Ca2+	(+ *)HCO3−, Cr (– *)Cl−	(– **)Mg2+, Al, Fe, Zn	(+ **)HCO3−, K+, U; (– **)SO42−, Mg2+, Al, Cr, Fe, Pb, Th; (– *)Mn	(+ *)HCO3−	(+ *)Cd; (– **)Fe; (– *)Na+
Mg2+	(+ **)SO42−; (+ *)Cl−	(+ **)Fe, Zn; (+ *) Al; (– **)Ca2+	(+ **)SO42−, Ca2+, Al, Mn, Fe, Th; (+ *) Pb, (– **)HCO3−, U	(+ **)Th; (+ *)SO42−	(+ **)Fe; (+ *)HCO3−, SO42−, Na+
Al	(+ **)K+, Mn, Fe, Co	(+ **)Mn, Fe, Zn; (+ *)Mg2+, Co, Th; (– **)Ca2+	(+ **)SO42−, Mg2+, Cr, Mn, Fe, Pb, Th; (– **)HCO3−, K+, Ca2+; (– **)U	(+ **)Cu, Th	(+ **)Fe, Co, Cu; (+ *) SO42−
Cr	(+ *)Ca2+		(+ **)Al, Fe, Pb, Th; (– **)HCO3−, K+, Ca2+, U	(+ **)Fe, Co, Cu, As, Sn; (+ *)SO42−, Pb	(+ **)Zn, Pb
Mn	(+ **)Al, Fe, Co, Th	(+ **)Al, (+ *)Fe, Co, Zn (– *) K+,U,	(+ **)SO42−, Mg2+, Al, Fe, Pb; (+ *)Th; (– **)K+; (– *)Ca2+,U		(+ *)SO42−
Fe	(+ **)Al, Mn, Co	(+ **)Mg2+, Al, Co; (+ *)Mn, Zn; (– **)K+, Ca2+	(+ **)SO42−, Mg2+, Al, Cr, Mn, Pb, Th; (– **)HCO3−, K+, Ca2+, U	(+ **) Cr, Co, Cu, As, Sn	(+ **) HCO3−, Mg2+, Al, Co; (+ *)Na+, Zn; (– **) Ca2+
Co	(+ **)K+, Al, Mn, Fe	(+ **)Fe, (+ *)SO42−, Al, Mn		(+ **)Cr, Fe, Cu, Sn; (+ *)As	(+ **)Al, Fe; (+ *)Cu
Cu	(+ **)Cs		(+ **)Sn, Pb	(+ **)Al, Cr, Fe, Co, Sn; (+ *)SO42−, Zn	(+ **)Al, (+ *)Co, Cl−
Zn	(+ **)Cd, Pb	(+ **)Mg2+, Al, As; (+ *)Mn, Zn, Cd; (– *)Ca2+	(– **)Cl−	(+ *)Cu; (– *)K+	(+ **)Cr, Cs; (+ *)Fe
As		(+ **)Zn, Cd		(+ **)Cr, Fe, Sn; (+ *)Co	(+ *)HCO3−, Pb
Cd	(+ **)Zn, Sn, Pb; (+ *)K+	(+ **)As; (+ *)Zn	(+ *)Na+	(+ **)Cl−, Cs, Pb	(+ *)Ca2+
Sn	(+ **)Cd, Pb	(+ **)Th	(+ **)Cu	(+ **)Cr, Fe, Co, Cu, As
Cs	(+ **)Cu		(+ **)Na+	(+ **)Cl−, Cd, Pb	(+ **)Zn, (+ *)Pb
Pb	(+ **)Zn, Cd, Sn; (+ *)K+		(+ **)SO42−, Al, Cr, Mn, Fe, Cu, Th (+ *)Mg2+; (– **)K+, Ca2+	(+ **)Cl−, Cd, Cs; (+ *)Cr	(+ **)Cr; (+ *)As, Cs
Th	(+ **)Mn	(+ **)Sn; (+ *)Al	(+ **)SO42−, Mg2+, Al, Cr, Fe, Pb; (+ *)Mn; (– **)HCO3−, K+, Ca2+, U	(+ **)Al; (+ **)Mg2+
U		(+ *)K+; (– **)Mn	(+ **)HCO3−, K+, Ca2+, (– **)SO42−, Mg2+, Al, Cr, Fe, Th; (– *)Mn	(– *)SO42−

Notes: *—p < 0.05, **—p < 0.01. Elements that are correlated during 3–4 sampling campaigns are given in bold.

). It was found that correlation relationships between elements were not stable across sampling campaigns in different years. At the same time, there were pairs of elements preserving significant correlation during 3 or 4 sampling campaigns. Amongst major ions, these pairs included HCO₃⁻ vs. Ca²⁺, SO₄²⁻ vs. Mg²⁺. The positive correlation between HCO₃⁻ vs. Ca²⁺ in the Irkutsk Reservoir, found also in the Boguchany Reservoir, being the Angara Cascade Reservoir as well [72]), is characteristic of freshwater calcium bicarbonate composition [73]. Of the trace elements, the pairs with a positive correlation included: Al–Mn, Al–Fe, Al–Co, Al–Th, Mn–Fe, Co–Fe, and Pb–Cr. Aluminum, which is the most abundant metal in the Earth’s crust, is noteworthy [74]). The positive correlation of Al with Fe, Mn, Co, as well as of Fe with Mn, Co, Mg²⁺ over several sampling campaigns showed that these elements can enter the water environment of the Irkutsk Reservoir as a result of weathering of the parent material, pedogenesis, and erosion activity. The data concerning Th concentrations in rocks, composing the drainage area of the Irkutsk Reservoir, are still lacking. At the same time, maximum Th levels in the bottom sediments of the Irkutsk Reservoir (12.9 ppm) exceeded the values of the geochemical background of sedimentary rocks in the region [68]. The significant positive correlation between Th and Al over three sampling campaigns might indicate that bedrock, composing the shores of the reservoir, is probably the main source of thorium entering the reservoir. The positive correlation between Pb and Cr, determining the relationships between these two elements, reflects that the combustion of petroleum or gasoline used for water transport is probably the main mechanism by which these trace elements enter the water reservoir.

3.4. Trace Element Pollution Status and Analysis of Water Quality

Water supply to the population in Irkutsk and smaller settlements of the Irkutsk district is provided from the water intake, located in the Ershi Bay. Residents of suburban settlements living on the shores of the reservoir, in the spring, summer, and autumn use submersible pumps for pumping water from nearby bays. Currently, in Russia, the requirements for the quality of drinking water are based on maximum permissible concentrations (MPC) of trace elements and are regulated by the Sanitary Rules and Norms for Drinking Water [48]. Concentrations of all the elements under consideration in the water of both the Irkutsk Reservoir and Lake Baikal are substantially below the standards for drinking water (Table 2).

An important tool that helps to assess surface water pollution is the choice of a criterion that should be used as a control material (geochemical background) [75]. At the same time, such a criterion is to be chosen on both local and global scales [71]. As the Irkutsk Reservoir is a part of the unified Baikal–Angara water system, the summarized hydrochemical characteristics of Lake Baikal which include the average concentrations of trace elements at the source of the Angara River, can be used as a control material.

The calculation of the single-factor pollution index (CF) is shown in

Table 5. Pollution indices (CF) in Irkutsk Reservoir water.

Trace Element	2007	2012	2021 (M)	2021 (J)	2021 (S)
Al	0.8–5.3	1.0–5.2	0.6–3.7	0.4–4.3	0.5–4.8
Cr	0.6–1.8	0.4–1.3	0.5–1.6	0.2–3.0	0.1–1.7
Mn	0.1–2.8	0.5–3.9	0.2–4.2	0.4–4.4	0.3–3.5
Fe	0.6–6.3	0.1–3.3	0.3–3.3	0.4–3.5	0.5–5.9
Co	0.8–2.5	0.5–4.2	0.1–10.0	0.6–3.5	0.6–1.9
Cu	0.4–1.7	0.3–1.7	0.4–3.0	0.3–1.7	0.4–1.5
Zn	0.1–0.9	0.4–1.7	0.4–1.6	0.5–1.4	0.5–2.8
As	0.9–1.4	0.7–1.0	0.7–1.2	0.9–1.3	0.7–1.2
Cd	0.2–0.8	0.6–1.3	0.5–1.4	0.6–4.7	0.5–1.6
Sn	0.2–0.7	0.2–6.3	0.2–2.1	0.2–3.2	0.2–2.7
Cs	0.1–0.9	0.0–0.6	0.1–0.4	0.3–6.9	0.2–1.7
Pb	0.1–2.2	0.2–2.3	0.0–1.1	0.0–2.1	0.3–3.6
Th	2.4–12.6	0.2–1.2	0.1–1.1	0.1–1.2	0.2–0.8
U	0.5–1.5	0.7–1.0	0.4–1.1	0.9–1.1	0.9–1.2

. Water samples of the Irkutsk Reservoir were classified as low contaminated: in 2007 by the concentrations of Cd, Sn, Zn, and Cs; in 2012, May 2021—by the concentration of Cs, and in September 2021—by the concentration of Th. Water samples were classified as low and moderately contaminated: in 2007 by concentration of Cr, Mn, Co, Cu, As, Pb, and U, in 2012—by concentration of Cr, Cu, Zn, As, Cd, Pb, Th, and U, in May 2021—by concentration of Cr, Zn, As, Cd, Sn, Pb, Th, and U, in July 2021—by concentration of Cu, Zn, As, Pb, Th, and U, in September 2021—by concentration of Cr, Co, Cu, Zn, As, Cd, Sn, Cs, and U. Part of the water samples revealed considerable and very high contamination: in 2007 by concentrations of Al, Fe, Th, in 2012—by concentrations of Al, Mn, Fe, Co, and Sn, in May 2021—by concentrations of Al, Mn, Fe, Co, and Cu, in July 2021—by concentration of Al, Cr, Mn, Fe, Co, Cd, Sn, and Cs, in September 2021–by concentrations of Al, Mn, Fe, and Pb.

The PLI index in 2007 (0.000–0.407), 2012 (0.000–0.223), May 2021 (0.000–0.155), July 2021 (0.000–0.910), September 2021 (0.000–0.333) classified the vast majority of samples as waters with a baseline level of pollutants. The exceptions included samples of surface water from S5 site in 2012 (PLI = 7.2, polluted water), S11 in July 2021 (PLI = 39.6, highly polluted), S11-r July 2021 (PLI = 11.6, highly polluted), S5 July 2021 (PLI = 1.4, polluted), as well as surface (PLI = 1.9, polluted) and near-bottom (PLI = 5.3, polluted) waters S4 in September 2021.

4. Conclusions

This study focused on the hydrochemical parameters of the Irkutsk Reservoir, which is a unique natural site due to it belonging to the unified Baikal–Angara freshwater system. The spatial–temporal dynamics in HCO³⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, Na⁺, and K⁺ concentrations in the Irkutsk Reservoir showed that the major source of dissolved substances in the reservoir is Lake Baikal runoff. The comparison of Al, Cr, Mn, Fe, Co, Cu, Zn, As, Cd, Sn, Cs, Pb, Th, and U levels in the waters of Lake Baikal, Angara River source and Irkutsk Reservoir suggests that the water trace element composition of the reservoir also carries the main characteristics of Lake Baikal water. At the same time, spatial heterogeneity determined by local increases in the concentrations of trace elements in the reservoir water is highlighted. Transformation of Lake Baikal water is already evident in the vicinity of the Listvyanka settlement (Angara River source): the water samples taken here showed higher Al, Cr, Mn, Fe, Co, Cu, Cd, Sn, Cs, Pb, and Th concentrations. Downstream, from this settlement to the Irkutsk HPP dam, the frequently observable concentrations of Al, Cr, Mn, Co, As, Th, and U remained at a level close to their levels in the water of the Angara River source. The levels of Cu, Zn, Cd, Sn, Cs, and Pb were lower relative to the Angara River source, but they were still higher than the concentrations in the water of Lake Baikal. In the absence of significant industrial facilities along the shores of the Irkutsk Reservoir, the water trace element composition is mainly influenced by anthropogenically loaded areas and water transport. The concentrations of trace elements, mainly Al, Fe, Mn, Co, and Th, are greatly influenced by the shore abrasion processes, which became faster with the creation of the reservoir.

The concentrations of all major ions and trace elements in the surface and near-bottom waters of the Irkutsk Reservoir were substantially below the drinking water quality standards. Therefore, in terms of concentrations of components under study, the water is of high quality and may be used for drinking. At the same time, the pollution indices calculated relative to the Baikal water showed that the water samples from the Irkutsk Reservoir were contaminated with trace elements. By the CF values, considerable and very high contamination was found for Al, Fe, Mn, Fe, Co, Cu, Cr, Cd, Sn, Th, and Pb during different sampling campaigns. According to the PLI values, water samples from Uladova Bay (2012 and 2021, July) and Kurma Bay (2021, September) can be classified as polluted, while the samples from the middle and the right bank, in the vicinity of Irkutsk city, as highly polluted.

Our results show that dense constructions of houses along the reservoir coast, as well as the development of recreational infrastructure and navigation, have a significant impact on the water quality in the reservoir. Local higher concentrations of trace elements in the water of the Irkutsk Reservoir, which were not repeated in the interannual dynamics, suggest that the water of the Irkutsk Reservoir still copes with the anthropogenic load. The impact of anthropogenic activity could likely be more significant in the coastal areas of the reservoir. The lack of water treatment facilities makes the source of drinking water supply not protected from the anthropogenic impact. Human activity in the reservoir and on its coasts may affect the unique physical and chemical properties of the water, as well as the aquatic flora and fauna inhabiting the reservoir. All of the above underlines the need to take environmental protection measures aimed at preventing irreversible changes affecting the water quality and aquatic organisms, primarily endemic Lake Baikal species, poorly tolerant to habitat transformation.