Temporal Dynamics and Sources of Heavy Metals in an Aquatic Ecosystem: An Applied Study ^†

Abstract

This study investigates the sources and distribution of heavy metals in the Styr River, particularly in the area influenced by the cooling water blowdown from the Rivne Nuclear Power Plant (Ukraine). The concentrations of eight heavy metals (Zn, Cd, Pb, Cu, Ni, Mn, As, and Cr) were measured over a period from 2018 to 2022. Monthly water samples were collected and analyzed using an inductively coupled plasma optical emission spectroscopy (ICAP 7400 Duo, Thermo Fisher Scientific, Waltham, MA, USA). The results show that the average concentrations (M ± SD) of the heavy metals decreased in the following order: Cu (6.43 ± 1.82 ppb), As (5.1 ± 0.2 ppb), Zn (4.67 ± 1.14 ppb), Mn (4.03 ± 2.81 ppb), Ni (3.3 ± 0.8 ppb), Cr (1.06 ± 0.22 ppb), Pb (1.05 ± 0.11 ppb), and Cd (1.01 ± 0.03 ppb). Seasonal and annual variations in metal concentrations were observed, with notable decreases in Zn, Cu, and Mn in 2021, likely due to anthropogenic activities. Pearson correlation analysis and cluster analysis were employed to explore relationships between the metals. The findings suggest that certain metals, such as Pb, Cr, and Ni, share common sources, likely industrial emissions or urban pollution, while others, such as Cd and As, have more isolated sources. This research highlights the complex interplay of natural and anthropogenic factors influencing heavy metal levels in the Styr River.

1. Introduction

The predominantly adverse effects of heavy metals on organisms, including their toxicity, persistence, and bioaccumulation, have been the subject of global public concern in recent years [1,2]. Heavy metal pollution in the environment is increasing [3]. There are two main sources of heavy metals in the environment that have a significant impact on heavy metal levels. These are natural sources, such as weathering of bedrock and volcanism, which are controlled by geology and lithology, and anthropogenic activities, including mining, metal smelting and refining, energy production and consumption, and waste incineration [4]. The presence of heavy metals in excess of allowable limits is often associated with a range of disadvantageous effects in humans, other organisms and the environment at large. The allowable safe limits of heavy metals in water samples are associated with low health risks in humans [5].

Traces of heavy metals, such as zinc (Zn), cadmium (Cd), lead (Pb), copper (Cu), nickel (Ni), manganese (Mn), arsenic (As), and chromium (Cr), in surface waters may increase due to anthropogenic activities [6]. Cooling water systems are widely used in industrial applications; however, the cooling water blowdown produced by various industries and district cooling facilities contains high concentrations of different chemicals [7]. In cooling water systems, processes such as heat transfer and concentration through evaporation occur, leading to an increased presence of chemicals from natural sources in the cooling water blowdown. Additionally, physical and chemical impacts in cooling water systems result in corrosion processes, which further contribute to the accumulation of chemicals, including heavy metals, that are part of the construction materials of cooling systems. The discharge of water from an open cooling water system of a power plant into natural water bodies can reach a flow rate of up to 1.0 m³/s per 1000 MWh of electricity generated [8]. The increase in chemical levels due to water discharge is the result of anthropogenic activities, subjecting the water body to significant anthropogenic pressure.

In this study, we identified the sources of heavy metals by measuring their levels in a natural water body receiving the cooling water blowdown. The purpose of this study is to analyze the temporal distribution of eight heavy metals (Zn, Cd, Pb, Cu, Ni, Mn, As, and Cr) in the waters of the Styr River, within the impact zone of the cooling water blowdown from the Rivne Nuclear Power Plant (Ukraine).

2. Materials and Methods

The Styr River is located in northwestern Ukraine and has a length of 483 km, a drainage basin of 13,130 km², and an annual mean flow rate of 69 m³/s. Similarly, surface water samples were collected from the Styr River after water discharge from the Rivne Nuclear Power Plant. The sample campaign was organized every month from 2018 to 2022 at a distance of 300 m from the mixing zone with return water. The sampling was performed according to procedural standards [9,10]. Clean high-density polyethylene bottles were used for the sample conservation after rinsing with each sample. Sampling was performed manually by plunging the bottle into the water source at a 10 cm depth and allowing it to fill without air. Samples destined for the major and trace elements analysis were acidified with 65% HNO₃ until pH 1–2 in order to prevent precipitation and retention on the walls of the vessels. All samples were preserved by refrigeration in thermal boxes and protected from the sunlight. Samples were instantly transported to the laboratory for analysis within 24 h.

An inductively coupled plasma optical emission spectroscopy (ICAP 7400 Duo) was used for analyzing the metal concentrations, following [11]. Analytical lines (nm) Zn (213.857), Cd (226.502), Pb (220.353), Cu (324.754), Ni (231.604), Mn (257.610), As (193.696), and Cr (267.716) were used. In order to assure the quality of results, equipment was calibrated with standard solutions. All methods were checked against the laboratory’s internal quality standards, each result as two measurements, the variation between results, and the relative measurement error not exceeding the normalized values.

The statistical processing of the research results involved determining the arithmetic mean (M), the range of data series (min–max), and the standard deviation ± (SD). In addition, the correlation of the concentration of heavy metals was determined using the Pearson correlation coefficient (r) according to [12,13]. This study also uses cluster analysis (CA) to explore the relationships between different heavy metals according to [14]. The JASP software package (Version 0.14.3) was used for the statistical processing.

3. Results and Discussion

The arithmetic mean concentration (M ± SD) of eight heavy metals in the water of the Styr River within the influence zone of the cooling water discharge from the Rivne Nuclear Power Plant (NPP) decreases in the following order: Cu (6.43 ± 1.82 ppb), As (5.1 ± 0.2 ppb), Zn (4.67 ± 1.14 ppb), Mn (4.03 ± 2.81 ppb), Ni (3.3 ± 0.8 ppb), Cr (1.06 ± 0.22 ppb), Pb (1.05 ± 0.11 ppb), Cd (1.01 ± 0.03 ppb). The concentrations of As, Cd, Pb, and Cr during the monitoring period corresponded to the lower limit of detection (5.0, 1.0, 1.0, and 1.0 ppb, respectively), with only a few samples exceeding the lower measurement threshold (Figure 1).

Annual variations in the concentrations of the eight heavy metals in the Styr River, within the zone influenced by the Rivne NPP’s cooling water discharge, were not significant. However, in 2021, significant decreases in the concentrations of Zn (M = 1.03 ppb), Cu (M = 1.42 ppb) and Mn (M = 1.72 ppb) were noted. In contrast, the same year saw an increase in Ni concentration (M = 0.5 ppb) and Cr (M = 0.32 ppb). These variations can be attributed to localized anthropogenic pressure, particularly due to intensified industrial or agricultural activities in that year [15].

Seasonal variations in the concentrations of heavy metals are significantly influenced by water flow rates. In this region, water flow peaks during winter and spring (due to snowmelt, floods, and water dilution) and reaches a minimum during summer and autumn (due to evaporation caused by higher temperatures). However, concurrent processes such as sedimentation and bioaccumulation of heavy metals by aquatic organisms complicate this relationship. As a result, no clear seasonal patterns were observed for Mn, Cd, and Cu concentrations (Figure 2). During the summer, increased concentrations of Pb (M = 0.21 ppb) and Cr (M = 0.31 ppb) were recorded. It is known that Pb and Cr pollution sources are predominantly anthropogenic, and their concentration increases during the warm season due to water evaporation in rivers. Conversely, Cu and Zn concentrations decreased during the summer. It is also known [16] that these elements are bioaccumulated by aquatic organisms, and their reduced concentration in the warmer months may be explained by biological uptake processes (Figure 2). Moreover, some metals can be extremely toxic to biota (e.g., Hg, Pb, As, and Cd), while others are essential for the proper functioning of organisms; for instance, Zn and Cu play a vital role in phytoplankton growth [17].

A correlation matrix was used to identify relationships between the eight heavy metals from 2018 to 2022 (Figure 3). A weak positive correlation was found between the concentrations of Zn and Mn (0.466), as well as Cu and Pb (0.63), indicating that these metals may share common sources or influencing factors. A weak negative correlation was identified between Mn and Ni (−0.55) and Cr and Zn (−0.436). Weakly correlated heavy metals may have different natural or localized anthropogenic sources that do not overlap to a large extent [18]. Heavy metals such as Mn, Cu, and Zn participate in metabolic regulation mechanisms and are components of vitamins, enzymes, and hormones, but in excessive amounts, they can disrupt essential physiological functions. Others, such as As, Pb, Cd, Ni, and Cr, have distinctly toxic properties and exhibit carcinogenic and mutagenic effects [19]. The consistently strong positive correlations between Cu and Pb and Zn and Mn suggest that these metals have similar origins and migration pathways. The distinct negative correlations highlight the separate sources or processes for certain metals, underscoring the complexity of their environmental behavior.

According to Council Directive 2020/2184 [20], the following parametric values (in ppb) are established for Zn (5000), Cd (5), Pb (10), Cu (50), Ni (20), Mn (50), As (10), and Cr (50), above which concentrations should not be exceeded for water intended for human consumption. It can be stated that, for the water of the Styr River during the period 2018–2022, none of the concentrations of the eight heavy metals exceeded these parametric values, and thus, the water can be considered suitable for human consumption (Figure 1 and Figure 2). However, the simultaneous presence of Cu and Zn may indicate the introduction of corrosion products from the MNZh-5 alloy (used in the heat exchange surfaces of the main condensers in the cooling system) into the water discharge of the Rivne NPP [21], though this does not exceed the established limits.

The cluster analysis (CA) of heavy metal concentrations in the Styr River reveals a complex interplay of sources. Industrial activities, urban runoff, and potentially agricultural practices appear to be significant contributors. The clusters for Mn, Cu, Ni, and Zn are spread across multiple peaks, suggesting diverse sources or fluctuating levels across different samples. This indicates a combination of natural background levels and potential anthropogenic inputs. As shows a single prominent cluster (cluster 3) with minor overlaps from other clusters. Pb presents a primary peak within cluster 4. Cr shows a main peak in cluster 4 with additional minor peaks, suggesting a significant source with some variability across samples. Cd exhibits a sharp peak in cluster 3 with smaller contributions from other clusters. Metals such as Pb, As, Cd, and Cr demonstrate clear signs of anthropogenic influence, while Mn, Cu, Ni, and Zn display more variability, implying both natural and anthropogenic sources (Figure 4). The clustering patterns suggest that certain metals may share common origins.

For instance, the clustering of Pb, Cr, and Ni in cluster 4 suggests a shared source, likely industrial emissions or urban pollution. Similarly, the presence of Cu, Zn, and Ni in multiple clusters suggests overlapping sources, such as mixed industrial activities and urban runoff. Metals like Cd and As, which show more isolated clustering patterns, suggest specific sources influencing their concentrations. Sharp peaks of Cd, Cr, and As in cluster 1 indicate distinct sources, likely industrial or agricultural.

The analysis of heavy metal concentrations in the water of the Styr River within the influence zone of the Rivne NPP cooling water discharge highlights several critical environmental and anthropogenic dynamics. The study revealed that while the concentrations of heavy metals remained below the parametric thresholds for water quality as per Council Directive 2020/2184, their patterns and relationships indicate complex interactions between natural and anthropogenic factors. The observed ranking of heavy metal concentrations suggests that Cu, As, and Zn are the dominant pollutants in the study area. The elevated levels of Cu and Zn may be linked to corrosion products from the MNZh-5 alloy used in the Rivne NPP cooling systems. Despite their essential roles in biological systems, such elements, when present in excess, can lead to toxicity and disrupt aquatic ecosystems. The study identified noteworthy seasonal and annual variations in heavy metal concentrations, which provide insight into the interplay between environmental conditions and pollution sources. Seasonal fluctuations in water flow rates, linked to snowmelt, flooding, and evaporation, appeared to drive variations in metal concentrations. These findings underscore the necessity of continuous monitoring to understand and manage these fluctuations, as the ecological and health implications of even sub-threshold concentrations can be profound over time [22]. The correlation matrix revealed significant relationships among certain heavy metals, suggesting shared origins or common influencing factors. Notably, Pb, Cr, and Ni clustered together, implicating industrial emissions, whereas Cd and As exhibited more isolated clustering, likely linked to specific industrial or agricultural inputs. Despite the concentrations of all studied heavy metals falling below the thresholds established for potable water, their persistent presence raises environmental concerns. Long-term exposure, even at low concentrations, can have detrimental effects on aquatic ecosystems. In addition, the joint action of elements can cause the phenomenon of synergy or summation, capable of having a toxic effect on biota. Future research should focus on temporal and spatial monitoring, examining metal concentrations with increasing distance from the NPP, complemented by bioindication, which together will allow for subtle variations and the identification of new sources of pollution. Furthermore, these studies could be conducted using index decomposition analysis (IDA) [23], as it is a useful method for quantifying the relative contribution of different factors to the change in environmental impact.

4. Conclusions

The study of heavy metal concentrations in the Styr River, within the zone influenced by the Rivne Nuclear Power Plant’s cooling water discharge, revealed that none of the eight monitored metals (Zn, Cd, Pb, Cu, Ni, Mn, As, and Cr) exceeded the parametric values established by Council Directive 2020/2184, ensuring that the water remains suitable for human consumption. Temporal and seasonal variations in metal concentrations were observed, with some fluctuations in 2021 attributed to localized anthropogenic activities. While the presence of Cu and Zn suggests possible corrosion from industrial equipment (MNZh-5 alloy), this introduction remains within permissible limits. Cluster analysis indicated a complex interaction of natural and anthropogenic sources, particularly for metals like Pb, As, Cd, and Cr, which show clear signs of pollution from human activities. In contrast, Mn, Cu, Ni, and Zn display more variability, suggesting both natural and industrial sources. Overall, the study highlights the need for continued monitoring to manage environmental risks effectively. While the concentrations of heavy metals currently remain within permissible limits, their persistence in the aquatic environment raises concerns regarding cumulative effects on ecosystem health. Long-term exposure, even at sub-threshold levels, can contribute to bioaccumulation and potential ecological disruptions.