Phthalates in Surface Waters of the Selenga River (Main Tributary of Lake Baikal) and Its Delta: Spatial-Temporal Distribution and Environmental Risk Assessment
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 several small lakes. In total, the delta contains more than 30 channels of varying sizes. The area is frequently flooded during periods of high water.
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.
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
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
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%.
2.4. Ecological Risk Assessment
2.4.1. Human Health Risk
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 United States. EPA .
2.4.2. Freshwater Risk Assessment
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 can 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.
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