Phthalates in Surface Waters of the Selenga River (Main Tributary of Lake Baikal) and Its Delta: Spatial-Temporal Distribution and Environmental Risk Assessment
1. Introduction
Preserving unique ecosystems, such as Lake Baikal, recognized as a UNESCO World Heritage Site [1], has become increasingly important due to the growing anthropogenic pressure on the natural environment. Out of more than 300 watercourses flowing to the lake, the Selenga River contributes nearly 50% of the water volume and over half of the chemical runoff [2,3]. Unlike other tributaries of Lake Baikal, the Selenga River basin includes mining facilities, numerous agricultural, processing and industrial enterprises, as well as settlements and cities, making the Selenga River a major contributor to water pollution [4,5,6]. The river water enters Lake Baikal with a significant amount of dissolved and suspended terrigenous material. This material is mostly deposited at the river mouth, forming a classic delta with an area of approximately 600 km2 [7,8]. The floodplain and soils in the riverine areas, together with the associated aboveground and aquatic vegetation, serve as a geochemical barrier safeguarding the waters of Lake Baikal. This barrier functions by intercepting sediment load and precipitating various chemical elements and substances [9].
The total length of the Selenga River is 1024 km, of which 409 km flows through Russia and the rest through Mongolia. Covering an extensive catchment area of 447,000 km2, the Selenga River contributes to 82% of the total catchment area of Lake Baikal [6]. The continental climate in this area results in a markedly uneven spatio-temporal distribution of precipitation, with annual means ranging from 230 mm in the middle Selenga to 700 mm in the estuary [10]. During the summer months, the Selenga River is primarily fed by rain. Although spring flooding is weak, floods during July and August can reach high magnitudes. The river is covered with ice from late November to March. The majority of the runoff, up to 94.5% of the annual volume, occurs during the warm period, which spans from May to October. The Selenga River maintains an average annual flow rate of 911 m3/s, equivalent to a runoff volume of 28.7 km3/year [11].
The Selenga River forms a classic delta at its mouth, covering an area of approximately 600 km2. This delta begins 34 km from the mouth, below Kabansk, where the river divides into two main branches. A significant part of the delta is swampy and contains many small lakes. In total, the delta contains more than 30 channels of varying sizes. The area is frequently flooded during periods of high water.
The delta’s primary channels are categorized into three groups: southern, middle, and northern [12]. Alternatively, some authors differentiate the Selenginsky, Sredneustievsky, and Lobanovsky sectors [13]. The largest channels, namely Levoberezhnaya, Kharauz, and Lobanovskaya, carry up to 80% of the water flow in spring and fall, and up to 99% in winter under average water conditions. Smaller channels, such as Galutai, Srednyaya, and Severnaya, typically freeze over in the upper reaches during winter [14]. The maximum flow velocity is observed in the main channels (Kabansk, Kharauz), and lower velocities are observed in relatively small channels.
The Selenga River basin spans across the industrialized and populated regions of Mongolia and Russia (Buryatia). As of 1 January 2023, according to official data, 892,535 people lived in the Russian part of the Selenga River basin. This territory includes 186 rural and urban settlements of Buryatia (which accounts for 85.4% of its population), as well as 44 rural and urban settlements of Zabaikalsky Krai (which accounts for 6% of its population) [15]. The urban agglomeration of Ulan-Ude, which includes the city itself and four adjacent rural settlements, is home to the largest number of residents, with a total population of 484,862 people.
Ulan-Ude has two wastewater treatment facilities, located at the right bank (RBTF) and at the left bank (LBTF) of the Selenga River. These facilities utilize a combined process that involves mechanical pretreatment and biological purification. The average efficiency of mechanical wastewater treatment is 53%, while biological treatment facilities have an average efficiency of 92%. The average daily flow of wastewater to the sewage treatment facilities is approximately 100,000 m3. The treated wastewater is discharged into the Selenga River bed [16]. Research has shown that the pollution level of the Selenga River is higher in industrial areas, particularly near Ulan-Ude, compared to other areas. The decline in water quality is attributed to elevated average annual concentrations of chemical oxygen demand (COD), biochemical oxygen demand (BOD5), iron, fluoride, nitrite nitrogen, ammonia nitrogen, nitrate nitrogen, phenols, and petroleum products [17].
Environmental monitoring programs for water bodies are unable to fully assess changes, current environmental status, and potential risks due to various reasons. In addition to traditional pollutants like polycyclic aromatic hydrocarbon (PAHs), polychlorinated biphenyl (PCBs), and heavy metals, recent years have seen the emergence of new generation xenobiotics such as pharmaceuticals, flavorings, synthetic surfactants, and phthalates.
Phthalates are recognized as a global environmental problem by the international scientific community. A recent study presented the results of phthalates research in Lake Baikal, this showed that in the shallow water area of Selenga River (which is part of Lake Baikal) the concentration of Σ4PAEs was no more than 0.77 µg/L. However, a high content of diethyl phthalate (DEP) was noted [18].
The chemical composition of the water in the Selenga River, the primary tributary of Lake Baikal, has also been investigated. Studies have established the peculiarities of changes in hydrological, hydrochemical, and hydrobiological indicators of the surface water of the Selenga River and its delta [19,20]. Additionally, seasonal dynamics of major ions and trace elements have been observed [21]. The effect of various hydroclimatic factors on the transportation and distribution of metals and metalloids was also demonstrated [22], and the content of petroleum products was determined [23]. A detailed analysis of the distribution of PCBs, PAHs, and organochlorine pesticides was presented in another study [24]. However, there is a lack of information on the qualitative composition and concentration of phthalates, which are recognized as endocrine disruptors and listed as priority pollutants by environmental organizations worldwide.
Reproductive disorders during fetal development have been observed due to phthalate exposure [25]. Phthalates have been found to impair spermatogenesis in African spurred frogs [26], cause premature puberty in female rodents [27], reproductive dysfunction in fish [28], and developmental defects in frogs and zebrafish (Danio rerio) [29,30]. In addition, both chronic and acute phytotoxic effects of phthalates have been documented for algae [31] and various plant species [32,33]. Toxicological and epidemiological evidence underscores the link between phthalate exposure and human health disorders, including obesity, diabetes, and male infertility [34,35].
Considering the aforementioned concerns, it is essential to study the levels of PAE pollution in aquatic ecosystems, to identify their sources and income patterns in the components of the aquatic environment, and to assess the resulting environmental risks.
Therefore, the aim of this study is to identify the spatial-temporal patterns of phthalates distribution in the Selenga River and its delta (which is a natural biofilter for Lake Baikal) and to evaluate the associated environmental risks.
2. Materials and Methods
2.1. Study Area and Sampling
To assess the quality of water and phthalate content in the Selenga River and its delta, samples of surface water (n = 118) were taken from a depth of 0.2–0.5 m at the following sampling sites: in the Selenga River [upstream of Ulan-Ude (1); downstream of Ulan-Ude, at the right (2) and left (3) banks of the Selenga River; at the beginning of the delta—a point downstream of Kabansk (4)], and in the estuarine areas of the Selenga River [Levoberezhnaya (5), Kharauz (6), Galutai (7), Srednyaya (8), Severnaya (9), Lobanovskaya (10)] (Figure 1).
Water sampling was conducted in 2021, 2022, and 2023 in each of the four hydrological seasons of the year: under-ice period (February), open water period—spring (May), summer (July), and autumn (October). Because the sampling depth was within 0.2 to 0.5 m, no special sampling equipment was used. Surface water was sampled directly into bottles pre-rinsed with deionized water. The geographic coordinates of the sampling sites are presented in Table 1. Also in August 2023, samples of water were taken at the RBTF and LBTF wastewater treatment facilities (n = 12) in Ulan-Ude, both before treatment and after treatment (being discharged to the Selenga River).
2.2. Chemicals and Materials
In this study, we used the following standard phthalate compounds: DMP (dimethyl phthalate), DEP (diethyl phthalate), DBP (dibutyl phthalate), BBP (benzyl butyl phthalate), DnOP (di-n-octyl phthalate), and DEHP, or di-(2-ethylhexyl) phthalate (Sigma-Aldrich, Burlington, MA, USA). The above compounds as well as the deuterated surrogate standards (DMP-d4 and DEHP-d4) were purchased from Sigma-Aldrich (Burlington, MA, USA). The EPA Phthalate Esters Mix (Accustandard Inc., New Haven, CT, USA) contained 2000 μg/mL of each phthalate (DEP, DMP, BBP, DBP, DnOP, and DEHP).
All solvents (acetone, methanol, n-hexane, methylene chloride, and ethyl acetate) were pesticide/HPLC-grade. Sample filtration was performed using 0.45 μm pore glass fiber filters without binders from Jiuding High-Tech Filtration, Beijing, China.
The standard solutions were diluted in n-hexane and stored in amber glass bottles at 2 °C in a refrigerator. The target analytes were extracted using C18 SPE ENVI-18 cartridges (6 mL, 500 mg, Supelco, Waltham, MA, USA) using an SPE Vacuum Manifold VM12 (Phenomenex, Torrance, CA, USA). The eluates were dehydrated with anhydrous sodium sulfate (Lenreaktiv, Saint-Petersburg, Russia). Glass, stainless steel, and polytetrafluoroethylene equipment were used for sampling and experimentation. The glassware was washed with hot tap water without detergents, followed by a series of washes with diluted sulfuric acid (H2O:H2SO4 = 2:1 by volume), bidistilled water, acetone, methylene chloride, and hexane. The glassware, with the exception of the volumetric glassware, was dried at 400 °C for 4 h and then washed again with methylene chloride and hexane.
2.3. Laboratory Analyses
2.3.1. Analysis of Parameters of Water Quality
Water parameters such as turbidity, temperature, pH, salinity, dissolved oxygen (DO), ammonium, phosphate, nitrate, nitrite, and total phosphorus (TP) content were measured in a field laboratory on the day of sampling. For these purposes, a photoelectric colorimeter (PE-5400 UV, Ecroskhim, Saint-Petersburg, Russia) and a pH tester (Hanna portable instruments; HI 991300 and HI 98703) were used. A detailed description of the methods and techniques was given in [20].
2.3.2. Determination of PAEs
PAEs in water samples were determined using a validated GC/MS method. Briefly, PAEs were extracted from water samples (0.5 L) in triplicate using the SPE method, with surrogate standards added. After collection, the eluates were dehydrated with calcined anhydrous sodium sulfate. Solvents were then distilled under vacuum on a rotary evaporator to reduce the volume to 1 mL. The eluates were then evaporated to near dryness using a weak nitrogen stream and analyzed by GC/MS. Agilent MassHunter Quantitative analysis software B.07.00 (Quantitative analysis of the environment, MS) was used to calculate the concentrations of PAEs using the absolute graduation method. The correlation coefficients for all six calibration curves exceeded 0.98%. The recovery rates ranged from 77.17% to 116.30%.
Quality assurance and quality control (QA/QC) procedures were in place before the analysis of each batch of samples. The 3σ IUPAC criterion (S/N ratio ≥ 3) was used to estimate the minimum detection limit (MDL). To estimate the minimum quantification limit (MQL), the 10σ IUPAC criterion (S/N ratio ≥ 10) was applied. The MQL ranged from 0.30 to 2.10 ng/L. To calculate the final concentrations, the mean values of the procedural blanks were subtracted from the measured values, and any values below the MQL were regarded as zero. Further details of the validated GC/MS method, such as reproducibility and accuracy data, can be found in our previous study [36].
2.4. Ecological Risk Assessment
2.4.1. Human Health Risk
Hazard quotients (HQ) were calculated to assess the non-carcinogenic risk [37]:
where AE refers to the level of exposure of an adult to PAEs when drinking water, mg/kg of body weight/day; RfD is the individual reference dose of specific phthalate, according to the U.S. EPA [37].
where AE refers to the level of exposure of an adult to PAEs when drinking water, mg/kg of body weight/day; RfD is the individual reference dose of specific phthalate, according to the U.S. EPA [37].
The exposure of the local population to phthalates (AE) was calculated in accordance with [36]. The following are the RfD values for the phthalates studied (µg/kg/day): DEP—800, DBP—100, BBP—200, DEHP—20, and DnOP—10. The sum of their individual HQs was calculated and expressed as a hazard index (HI) to assess the overall non-cancer risk [38].
2.4.2. Freshwater Risk Assessment
The European Technical Guidelines for Risk Assessment were used to calculate risk quotients (RQs) for each individual phthalate. This was carried out for three sensitive hydrobiont species using measured environmental concentration (MEC) and predicted no-effect concentration (PNEC) of PAEs [36]:
4. Conclusions
This study is the first to investigate the levels and spatial-temporal distribution of six priority phthalates detected in the surface waters of the Selenga River and its delta. The dominant phthalates were DBP and DEHP. It was found that the concentrations of phthalates in surface waters were closely related to the hydrological regime of the Selenga River (namely, the amount of precipitation in the catchment area, which forms diffuse runoff during spring–summer–autumn periods). The highest concentrations of phthalates in all years of the study were found in the Ulan-Ude area. During periods of low precipitation, when the diffuse input of pollutants from the watershed decreased, the contribution of local sources (the city’s wastewater treatment plants) became more pronounced.
Although the MACs were exceeded in some cases, the DEHP levels, hazard quotients, and hazard indices in the Selenga River and its delta for all priority phthalates in all seasons during the three study years indicated that there is no risk of phthalate exposure from consumption of Selenga River water. The RQ values indicated that under low-water conditions in 2022, potential toxic effects on hydrobionts were absent or low. However, in 2021 and 2023, the risk of potential adverse effects of DBP and DEHP on aquatic organisms could be higher.
Scientists predict that phthalate levels in the environment will increase in the coming decades and pose a serious threat to living organisms. Therefore, it is necessary to include phthalates in the system of state environmental monitoring, especially when it is related to the Lake Baikal ecosystem.
The obtained results may be useful in preparing scientific recommendations for pollution prevention and developing relevant laws and regulations in the field of environmental management.
