Wet Deposition, Reduction, and Bioaccumulation of Mercury in Freshwater Reservoirs in South Korea

Abstract

The biogeochemical processes within freshwater ecosystems involve a complex interplay of physical, chemical, and biological factors, influencing the movement and accumulation of mercury. Factors such as water flow rates, turbulence, evaporation, and precipitation patterns impact the distribution of mercury. Faster water flow can redistribute mercury particles, altering their concentrations. Chemical reactions can convert inorganic mercury into organic forms, making it accessible to organisms. Biological interactions contribute to bioaccumulation, with microorganisms and plants forming the foundation of the food web. Mercury accumulates along the food chain, posing potential risks to both freshwater ecosystems and human health. Understanding these processes is crucial for managing mercury pollution, conserving freshwater ecosystems, and preserving water quality. In chapter2, understanding the partitioning and associated factors of mercury in wet deposition is crucial for predicting the fate and transport of atmospheric mercury. In this study, we collected weekly rainfall samples in Gwangju, a suburban area in the southwest of South Korea, over a five-year period from 2017 to 2022. The volume-weighted mean concentration of total mercury (THg) in rainfall was 4.6 ng L-1, which was similar to or lower than levels in remote control areas or rural regions in China, and notably lower than urban concentrations. A significant negative correlation was observed between precipitation amount and THg (p < 0.0001), providing valuable insights into the effective removal processes of atmospheric THg during initial precipitation events. As known, Gaseous Elemental Mercury is primarily removed during wet precipitation. Therefore, theoretically, atmospheric THg concentrations should be higher during humid summer months. However, our research results showed that THg concentrations were highest during the winter season. Additionally, there were positive correlations between THg concentrations and atmospheric particulate matter, especially PM2.5, ammonium ion, and nitrogen dioxide. These findings underscore the importance of particulate-bound mercury in atmospheric THg removal, highlighting the close relationship between THg concentrations in precipitation and the formation of secondary fine particles, particularly during high-winter PM2.5 events. In the high PM2.5 winter and spring periods, THg concentrations were significantly correlated with lead, cadmium, and nickel, suggesting industrial emissions, metal smelting, and petroleum refining as potential sources. Gwangju, as a suburban area, is influenced by long-distance transport from external sources, impacting mercury's external input and long-distance transport. This study provides essential data to enhance understanding and management strategies regarding mercury's external influx and long-distance transport. In chapter 3, this study investigated the behavior and sources of atmospheric methylmercury (MeHg) through wet deposition in a suburban area of South Korea. Weekly samples of wet deposition were collected from January 2017 to December 2022 in Gwangju, and the volume-weighted mean (VWM) concentration of MeHg was determined to be 0.045 ng L-1. The annual wet deposition flux of MeHg was found to be 0.045 ng m-2, indicating a significant pathway for the transport of atmospheric pollutants into local ecosystems. Seasonal variations were observed, with the highest VWM concentrations of MeHg found in winter and the highest wet deposition fluxes in summer. The study identified a statistically significant positive correlation between PM2.5 and MeHg concentrations, with MeHg concentrations increasing as fine dust concentrations increased in spring and winter. The sources of MeHg in seasonal rainwater were investigated using cluster analysis of backward trajectory and concentration weighted trajectory analysis. Air masses inflow from the northwest were identified as a significant source of MeHg in spring rainwater, while photodemethylation of dimethylmercury and domestic fuel combustion emissions were found to contribute to MeHg in summer and winter, respectively. Additionally, the study found that high MeHg concentrations in spring rainwater were likely due to the migration of atmospheric PMeHg over long distances. To confirm in-situ methylation of mercury in rainwater, this study conducted methylation incubation on bulk rainwater samples collected for each season. The results showed a high methylation rate in summer, which was in contrast to the observed seasonal MeHg concentration. Moreover, the potential role of dissolved organic matter (DOM) as a methyl donor for mercury methylation in rainwater was tracked for understanding methylation in rainwater. This study discovered that in summer, a protein-like DOM of marine origin contributed to the methylation of mercury. Meanwhile, in winter, a humic-like DOM of terrestrial origin was found to contribute to the methylation process. These findings suggest that seasonal variations in the type of DOM present in rainwater may be responsible for the observed seasonal variations in mercury methylation rates. However, it's important to note that other factors such as temperature, pH, and microbial communities may also contribute to the methylation process. Further research is needed to better understand the mechanisms behind in-situ methylation of mercury in rainwater and its potential implications for environmental health. This study provides valuable insights into the behavior and sources of atmospheric MeHg in a suburban area of South Korea. The study emphasizes the potential risks of atmospheric pollutants to local ecosystems and human health, highlighting the importance of monitoring atmospheric mercury pollution. In chapter 4, dissolved gaseous mercury (DGM) formation in lake water primarily results from photochemical processes mediated by dissolved organic matter (DOM). In this research, we investigated the impact of various DOM components on the rate constant (kr) of Hg(II) photoreduction in lake surface water. We collected data on kr and the fluorescence properties of DOM from three Korean lakes exhibiting different trophic states. Through excitation-emission matrix fluorescence spectroscopy and parallel factor analysis, we identified three major fluorophores: plant-derived terrigenous humic-like DOM (C1), autochthonous DOM (C2), and soil fulvic-like DOM (C3). Principal component analysis (PCA) loading matrix revealed that kr increases when bulk DOM contains high flavin-like and soil-derived fulvic-like fractions. Pearson's correlation results corroborated the PCA analysis, showing a strong positive correlation between kr and the soil fulvic-like DOM component (r = 0.92) and the redox index (r = 0.92). These findings were further confirmed using a partial least squares-regression model that successfully predicted kr (r = 0.99) by incorporating multiple DOM components. Overall, our results indicate that kr can be modeled by considering the fluorescence intensities of diverse DOM components. This modeling approach has the potential to enhance Hg biogeochemical models, allowing for more accurate predictions of Hg redox rates in various lake systems. In chapter 5, in this study, we conducted a comprehensive analysis of total mercury (THg) and methylmercury (MeHg) concentrations, alongside various environmental factors, in two mesotrophic lakes and one oligotrophic lake. Our investigation focused on understanding the factors influencing mercury accumulation in freshwater ecosystems, particularly in fish. We examined these factors in water, particles, sediments, and biota (phytoplankton, zooplankton, and fish) to gain deeper insights into mercury dynamics within these environments. Our results showed significant variations in THg concentrations among the lakes, with higher levels in one of the mesotrophic lakes, likely due to pollution sources downstream. We identified positive correlations between MeHg and total phosphorus (TP) and sulfate ions in surface waters, indicating their influence on MeHg concentrations. Additionally, sediment-associated microorganisms and organic matter played a role in enhancing MeHg transformation and bioavailability. We also observed differences in mercury accumulation in phytoplankton, zooplankton, and fish across the lakes, highlighting the impact of trophic levels and dietary sources. This study sheds light on the complex interactions influencing mercury dynamics in freshwater ecosystems and provides valuable insights for environmental conservation and mercury management efforts. These findings can inform the development of strategies to control mercury pollution, monitor water quality, ensure fish safety, and protect aquatic ecosystems. Furthermore, they offer a basis for policymaking aimed at regulating and mitigating the impact of mercury contamination in freshwater environments.