Jong Kwon Im

Climate change has increased the frequency and intensity of extreme weather events worldwide. In South Korea, annual precipitation in 2014–2015 was only 50% of the long-term average, resulting in severe drought conditions. This drought extended water residence time in dam reservoirs, enhancing internal nutrient recycling, degrading water quality, and promoting harmful cyanophyta blooms in downstream reservoirs. Using the Standardized Precipitation Index—for drought assessment, and monthly water sampling—for environmental factors and phytoplankton analyses, this study examined the impacts of drought on water quality and phytoplankton communities in a series of interconnected dam reservoirs (Uiam, Cheongpyeong, Sambong-ri, and Paldang Lakes) within the Bukhan River system from 2013 to 2016. The prolonged residence time during drought facilitated nutrient accumulation and recycling within the reservoirs, intensifying eutrophication and water quality deterioration, alongside a pronounced cyanobacterial dominance and harmful algal blooms. These findings suggest that changes in upstream dam discharges directly influence water quality and ecosystem health in downstream reservoirs and that diverse hydrological changes associated with drought pose a significant threat to water source management. These findings may inform the development of integrated water management strategies for maintaining water quality and protecting water sources during droughts and extreme climatic events.

The environment is a complex system composed of various creatures and pollutants caused by human activity. In this study, a mixture toxicity assessment based on the concentration-addition (CA) model was applied to 51 types of pesticides detected in the Saemangeum Watershed (the Mangyeong and Dongjin Rivers) for one year to evaluate pollution levels. A mixture toxicity assessment was conducted based on the measured environmental concentration (MEC) and the predictive values of basic ecotoxicity data (algae, daphnids, and fish). The risk and the trophic levels that showed the highest sensitivity varied depending on the sampling time and point for each mode of action (MoA) group of pesticides. In particular, the mixture risk for chloroacetamide and thiocarbamate pesticides was high (risk quotient > 1), and the potential risk was highest in autumn. In addition, the driving forces of seasonal changes were examined for samples that exhibited potential risks, and it was found that one to four substances contributed the most to the risk. The results of this study show that an initial risk assessment based on a mixture toxicity assessment can help in managing pesticide pollution in rivers.

This study aimed to determine the seasonal variations in the growth and species composition of phytoplankton in Lake Paldang, South Korea’s largest freshwater body, based on environmental factors. Weekly surface water samples were collected and analyzed for three years (2015–2017). A total of 160 species of phytoplankton were distributed across eight divisions. Diatoms were dominant in all seasons except summer and their seasonal equivalents. All seven indices—Shannon, equitability, Simpson, Menhinick, evenness, dominance, and Berger-Parker—presented a clear trend in September, which may be attributed to rainfall in July. Canonical correspondence analysis (CCA) was employed to analyze the relationship between the occurrence and the abundance of phytoplankton species and environmental factors. The results showed that WTE, TP, PO₄-P, and rainfall were negatively correlated with Ulnaria acus, Asterionella formosa, and Stephanodiscus hantzschii, whereas Scenedesmus sp. and Merismopedia tenuissima presented positive correlations. Microcystis demonstrated positive correlations with TOC, NH₃-N, and HRT, and appeared primarily in the summer and fall of 2015, a dry season. According to the Palmer pollution index, which is calculated based on phytoplankton species, Lake Paldang showed no signs of pollution for the top 99.8% of phytoplankton species, but was evaluated as organically polluted for overall phytoplankton, especially in spring and summer.

Carbamazepine (CBZ) is a widely used pharmaceutical for various purposes, including as an anticonvulsant, antibiotic, and antiepileptic agent, and it undergoes diverse metabolic pathways in both the environment and the human body. Therefore, this study aimed to explore the distribution of CBZ, the presence of its transformation products (TPs), and the transformation pathways in the Mangyeong and Dongjin Rivers in the Saemangeum watershed of Korea using non-targeted screening. The concentration distribution results for CBZ and its TPs showed that the average concentrations in the Mangyeong and Dongjin Rivers were 128.8 ng/L and 89.0 ng/L, respectively. The Mangyeong River exhibited a higher CBZ concentration than the Dongjin River, which was similar to those of the reported CBZ concentrations in other major domestic and international rivers. The types and detection frequencies of the identified TPs exhibited similar trends. The detection frequencies of the TPs decreased in the following order: CBZ-EP > DiOH-CBZ > 10OH-CBZ > 2OH-CBZ > 9-carboxyacridine > 9-acridinecarboxaldehyde. The detection frequency of the main TPs was high, and some were believed to be generated during the water treatment process. The presence of additional TPs (CBZ-O-quinone, acridine, and iminostilbene) was confirmed by the generated molecular networks. This study presents the transformation pathway of the CBZ and provides foundational data for understanding the environmental behavior of TPs, improving wastewater treatment plants, managing water quality, and establishing water environmental policies.

We evaluated the thermal and chemical stratifications of Paldang Lake using Schmidt’s stability index (SSI) and the chemical stratification index (IC-i) with weekly data from 2013 to 2022. The temporal trends of stratification were analyzed alongside correlations with meteorological, hydrological, and water quality variables. Thermal stratification intensified with rising air temperature and sunshine duration, while hydrological factors like discharge and retention time affected SSI during periods with less than five days of water retention. During summer, fewer occurrences of intense rainfall or early rainfall before August led to stronger stratification. In fall, nutrient influx from external sources during summer stimulated algal growth, increasing Chlorophyll-α (Chl-α) concentrations. Summer rainfall had a significant impact on the strength and duration of stratification in Paldang Lake. Annual rainfall patterns and subsequent changes in discharge were key factors affecting the physical environment of the lake, which in turn determined water quality and the extent of algal blooms. We provide insights into the seasonal stratification and water quality variations in temperate river-type reservoirs like Paldang Lake. SSI and IC-i from this research can be applied to understand stratification and mixing dynamics in other lakes.

The Paldang Reservoir (PDR), the largest artificial lake in the Republic of Korea, has recently experienced increased chemical oxygen demand (COD), eutrophication, and algal blooms due to climate change. This study aimed to analyze the hydrological characteristics and water quality of the PDR and assess its spatial and seasonal eutrophication status changes using the Korean-type trophic quality index (TSI_KO) and the seasonal Mann–Kendall Test (SMK). The PDR’s water was graded “good” in terms of biochemical oxygen demand (BOD) and “fair” for COD, total phosphorous (TP), and chlorophyll-a (Chl-a), according to Korean water quality standards. COD and Chl-a had a significant positive monotonic relationship. According to the TSI_KO calculations, the trophic state of the PDR was mesotrophic, but eutrophic during the summer season. TSI_KO (Chl-a) in summer tended to increase with a slope value of 0.22 mg/m³/year. The main causes of eutrophication in the PDR were proposed to be TP and phytoplankton overgrowth. Therefore, lake and reservoir functions need to be assessed, and solutions that can effectively protect them from excessive eutrophication to ensure sustainable clean water use and protect freshwater ecosystems need to be found.

Lake Uiam is situated midway through a basin with dams at both the upstream and downstream ends; considerable environmental fluctuations have been observed here. However, studies on changes in environmental factors and plankton community fluctuations remain limited. This study analyzed the seasonal physicochemical factors and changes in the phytoplankton community structure in Lake Uiam (2015–2016). Organic matter, phosphorus, total suspended solids (TSS), and Chl-a concentrations were high in the summer. Seasonal changes in the dominant taxa followed the typical succession pattern of temperate phytoplankton, with Bacillariophyceae (Ulnaria acus) being dominant in spring and Cyanophyceae (Pseudanabaena limnetica) dominant in summer. However, Cryptophyceae (Rhodomonas sp.) showed unusually high dominance in autumn. Cell abundance showed no seasonal differences. Rhodomonas sp. was negatively correlated with water temperature, suggesting its dominance in spring and autumn. Cryptophyceae showed a significant correlation with Chl-a (0.708 **), indicating its contribution to spring Chl-a concentrations. Cryptophyceae (Rhodomonas sp. and Cryptomonas spp.) commonly appear in spring but are dominant in autumn in Lake Uiam. Despite disturbances from various environmental factors, they showed higher adaptability than other algae, resulting in their consistent appearance and dominance, differing from the general succession patterns of temperate phytoplankton.

Organic matter in lakes is categorized into allochthonous organic matter, such as leaves and sewage effluent, and autochthonous organic matter, generated by microorganisms within the water system. In this study, organic matter composition was analyzed using UV-vis spectroscopy and liquid chromatography-organic carbon detection (LC-OCD). Several allochthonous natural organic matter substances were collected including leaves, green leaves, forest soils, and paddy soils. The organic matter composition analysis in our study sites revealed that humic substances comprised the highest proportion (36.5–42.3%). Also, individual samples at each site exhibited distinct characteristics. This study used a humic substance-diagram (HS-diagram) and principal component analysis (PCA) to trace the sources affecting the river water quality and identify their origins. The humic substances of soil origin predominantly influenced the water quality, with the impact of organic matter significantly pronounced during the July rainfall period. Compared with the PCA results, the contribution of the humic substance (HS, 48.9%) and building block (BB, 42.0%) indices appeared higher between June and July in summer, likely due to non-degradable substances released by heavy rain. In fall, the contribution of low molecular weight neutrals increased from 71.2% to 85.2%, owing to a humic substance influx and decomposition. This study demonstrated the application of estimating the relative contributions of source materials in lakes utilized for drinking and agricultural water to identify sources, aiding in the development of efficient watershed management plans.

The Paldang Reservoir (PDR) in South Korea is vital for supplying drinking water and maintaining ecosystems; thus, a comprehensive understanding of its water quality is necessary. Spatiotemporal changes in reservoir water quality were evaluated by applying Korean water quality indices and multivariate statistical techniques (MSTs). A dataset of 15 water quality parameters at five sites in the PDR were evaluated from 2017 to 2021. The organic matter, suspended matter, total phosphorus (TP), chlorophyll a (Chl-a), and total coliforms in the PDR exhibited a fair grade or higher. Chemical oxygen demand was found to correlate with biochemical oxygen demand, Chl-a, and TP. The average real-time water quality index (RTWQI) and average trophic state index (TSI_KO) of the PDR were excellent and mesotrophic, respectively, and 46% of eutrophic conditions occurred during the monsoon season. For a hierarchical cluster analysis (HCA), the five sites were grouped into three polluted areas and 12 months were grouped into dry and wet seasons. Principal component analysis and factor analysis identified four potential pollution sources (domestic sewage, industrial wastewater, intensive agricultural activities, and livestock wastewater) in the PDR and explained 79.7% of the total changes. Thus, the RTWQI, TSI_KO, and MSTs are useful tools for assessing freshwater quality in Korea, predicting potentially harmful conditions, and potentially assisting policymakers in PDR management.

This study aimed to investigate the irrigation water quality, major pollution sources, and human health hazards by focusing on heavy metal concentrations in the surface water of the Han River watershed, South Korea that supplies water for consumption and irrigation. Here, Mn was found to have the highest mean concentration, with the maximum concentration recorded at IH-2. The heavy metal concentrations were higher during summer and fall than that during spring. The mean concentration of heavy metals was relatively high in the downtown area (1.8 times) and downstream of the wastewater treatment facilities (1.3 times), indicating that the wastewater treatment facilities (WTFs) may be the primary source of pollution. Water at most of the sites were found to be suitable for irrigation. However, the sodium absorption ratio and soluble sodium percentage indicated that IH-2 was unsuitable. The results of the principal component analysis indicated that anthropogenic (vehicle and industrial) activities were the primary sources of metal pollution. Ingestion was identified as the primary exposure pathway in terms of health risks. However, the hazard quotients and hazard index for all pathways were below the safety limit (<1) for children and adults.