Water quality in Seoul, South Korea
Seoul is the capital city of South Korea, with a population of approximately 9.6 million. Seoul covers an area of 605 km2, with the Han River flowing through its center (Figure 1). A total of nine WWTPs are located within Seoul and seven of them release effluents to the three streams (Jungnangcheon stream, Tancheon stream, and Anyangcheon stream) which are tributaries of the Han River. The other two WWTPs release effluents to the Han River directly. These WWTP effluents are point pollution sources. Seoul has no indigenous lakes and only two artificial ones: West Seoul Lake and Seokchon Lake.
The Han River drains an area of 35,770 km2 in Korea (Ministry of Environment, https://hrfco.go.kr). The annual mean precipitation of the basin is 1,201 mm and the area-normalized annual water discharge is 706 mm based on the data between 1968 and 2001 (Bae et al., 2008). The Han River has two biggest tributaries: The Bukhangang River and The Namhangang River. The two rivers meet just before the location H1 and flows toward location H2 (Fig 1).
Fig 1. Seoul, the capital of South Korea and the Han River penetrating it. The red line is the boundary of Seoul, and the blue line is the river flow path of the Han River. The blue arrow indicates the direction of the river flow. Green lines are the three main streams (Jungnangcheon stream (JN), Tancheon stream (TC), and Anyangcheon stream (AY)) within Seoul, which enter the Han River. Yellow circles (H1: Paldangdam and H2: Gayang) are sites in the Han River for the water quality monitoring. The characteristics of the Han River watershed is described in et al. (2021a).
The Han River has been studied for its hydrogeochemical characteristics, from its headwaters to the river mouth (Choi et al., 2019; Jin et al., 2018; Lee et al., 2021a; Lee et al., 2021b; Ryu et al., 2007; Song et al., 2017). The mean of total dissolved solids (TDS) in the mainstream were 98 mg L-1, while the levels ranged from 26 to 76 mg L-1 in the Bukhangang River and 125 to 175 mg L-1 in the Namhangang River between summer of 2000 and spring of 2001 (Ryu et al., 2007). The relatively high TDS in the Namhangang River is attributable to the presence of carbonate areas within its basin, despite the predominance of granite and gneiss in the entire Han River basin.
Dissolved greenhouse gas (GHG) concentrations increased from headwaters to the estuary (CO2: 100–9,000 μatm, CH4: 0–4,000 nmol L-1, N2O: 0–1,500 nmol L-1) (Jin et al., 2018). Discrete increases in GHG concentrations specifically within Seoul suggest that urban activities, particularly wastewater treatment plant (WTP) effluents, contribute to these elevated levels.
Δ14C of dissolved inorganic carbon in the Han River ranged from 0 to 30‰, similar to the global average (~0‰), while Δ14C of dissolved organic carbon (DOC) ranged from -55 to 0‰, lower than the global average (~50‰) (Lee et al., 2021a). The lower Δ14C-DOC values in the Han River could be due to WTP effluents from residential areas and/or the mobilization of deep soil carbon from agricultural lands upstream (Lee et al., 2023; Lee et al., 2021b).
Concentrations of rare earth elements such as Gd, La, and Sm were also investigated in the Han River. Gd ranged from 45 to 209 pmol L-1, La from 170 to 375 pmol L-1, and Sm from 24 to 50 pmol L-1 (Song et al., 2017). These levels are 17–181, 7–52, and 2–5 pmol L-1 higher than background levels, respectively, indicating anthropogenic inputs (Song et al., 2017). Gd is commonly used for medical purposes, while La and Sm are utilized as catalysts in industrial facilities (Song et al., 2017).
Concentration of lithium, which is used for batteries and medical drugs, was also 6 times higher in the outlet of the Han River (~250 nmol L-1) than in the upstream level (~40 nmol L-1), suggesting the elevated level of lithium in the coastal ecosystems due to human activities (Choi et al., 2019).
Long-term variation in BOD in the Han River, South Korea
Fig 2. Long-term variation of annual mean BOD in the Han River (Cho et al., 2012; Seoul Metropolitan Government, https://data.seoul.go.kr/dataList/342/S/2/datasetView.do).
BOD5 (just BOD afterwards) is the most basic and informative water quality indicator. BOD of the Han River has been monitored since 1976 in H2. Annual mean BOD of the Han river at H2 ranged 1.3–19.2 mg L-1 from 1976 to 2023 (Fig 2). The mean BOD of H2 was higher (6.9 mg L-1) than that of H1 (1.3 mg L-1) over the period. H2 is the lower reach of Seoul, while H1 is upper reach of Seoul where the water is collected for supplying drinking water to Seoul. The higher BOD in H2 than in H1 suggests that bioavailable organic matter is added to the Han River within Seoul.
BOD of H2 has decreased to 1–2 mg L-1 in 2020s and this is significantly lower than that of 1980s (Fig 2). The decrease in BOD of the Han River is in accordance with increases in WWTP construction within Seoul since middle of 1980s (Cho et al., 2012; Korea Environment Corporation, https://www.data.go.kr/data/3073222/fileData.do). Nowadays, nine WWTPs are in operation in Seoul and nearby, treating up to 6.2×106 m3 of wastewater per day (Korea Environment Corporation, https://www.data.go.kr/data/3073222/fileData.do). WWTPs removes organic matter in wastewater through sedimentation and oxidation thus ultimately lowering BOD of the river, demonstrating the importance of WWTP in enhancing water quality of megacities.
Emerging pollutants in the Han River in Seoul, South Korea
Endocrine disrupting compounds (EDCs) and pharmaceuticals and personal care products (PPCPs) are groups of emerging contaminants that have drawn attention in the Han River. These chemicals include estrogenic, androgenic, thyroidal, antibiotics, analgesics, antiseptics substances which can potentially harm human and ecosystem health.
Fig 3. Mean concentrations of EDCs, PPCPs, and perfluoroalkyl substances (PFASs) in the Han River (Kim et al., 2014; Yoon et al., 2010). Water samples for EDCs and PPCPs were collected once in 2008 at H1 and H2, and four additional locations between the two (Yoon et al., 2010). Water samples for PFASs were collected once in 2010 at H2 (Kim et al., 2014). PFASs were indicated with asterisk in the compound names.
A total of 41 pollutants were selected and their concentrations in river water was analysed for the Han River (Figure 3; Kim et al., 2014; Yoon et al., 2010). Their concentration in the river water ranged from 0 to 1,013 ng L-1 (Figure 3). Concentrations of iopromide (medical agent: X-ray contrast media) and tris(2-chloropropyl) phosphate (TCPP, fire retardant) were relatively higher than those of the other compounds, while concentrations of the other 25 compounds including perfluoroalkyl substances (PFASs) were under 10 ng L-1 (Fig 3).
The sources of the pollutants are likely to be from human activities in Seoul because the concentrations of them are higher in tributaries or WWTP effluents within Seoul, than in the Han River (Kwon et al., 2017; Yoon et al., 2010). The total concentration of ten PFASs, the newly emerging contaminant, was 11 ng L-1 in the Han River (Fig 3). Similar concentrations of PFASs (10–50 ng L-1) are also found in coastal regions of South Korea (Shi et al., 2021).
References
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