Submarine groundwater discharge (SGD) represents the subsurface flow of water entering the ocean1,2. SGD impacts the coastal environment in various ways, depending on factors such as discharge scale, groundwater quality, and human proximity to the discharge zone3. On the Korean Peninsula, groundwater extraction for agricultural, industrial, and domestic purposes is increasing, potentially impacting the SGD process in coastal aquifers4,5,6. Quantifying SGD estimates and examining their spatiotemporal variability are crucial. At a local scale, common field techniques for assessing of fresh SGD include seepage meters7,8,9, piezometers10, and the use of natural tracers such as radium and radon11,12,13. However, direct measurements are limited by spatial coverage constraints, posing significant challenges in quantifying fresh SGD due to its high heterogeneity.
Computational methods provide a valuable tool for predicting fresh SGD at various spatial scales, from global (e.g., ref. 14,15) to regional perspectives (e.g., ref. 16,17,18). South Korea has been actively involved in calculating both local and regional SGD estimates, utilizing both direct seepage measurements and indirect isotopic methods6,13,19,20,21. However, there has been no attempt to estimate fresh SGD along the entire Korean Peninsula coast using the water balance approach in combination with advanced land surface models. Previous studies have applied land surface models to estimate SGD using the Global Land Data Assimilation System (GLDAS) to calculate water budgets for coastal aquifers and approximate net recharge rates on a coarse resolution14,15,18. Despite the improvements, GLDAS still struggles to capture the complex topography of coastal regions due to spatial limitations when applied to the Korean Peninsula. Previous assessments of fresh SGD have primarily focused on long-term averages, failing to provide the temporal evolution of fresh SGD rates or assess the impact of climate change on SGD variations across coastal catchments14,15.
Recently, the Korea Land Data Assimilation System (KLDAS) has been developed within the NASA Land Information System framework22, benefiting from the incorporation of local precipitation forcing datasets and soil texture maps23,24. The KLDAS supports an effective hydrological monitoring system, providing continuous regional high-resolution water balance variables across the Korean Peninsula. In this study, we undertake a comprehensive analysis focusing on four key aspects. Firstly, we quantify fresh SGD rates along the entire coast of the Korean Peninsula using a water balance approach through KLDAS for the period 1980-2016. We investigate the spatial pattern of average fresh SGD rates, considering factors such as net precipitation and coastal drainage length across regions. Secondly, we assess the influence of climate change on fresh SGD rate variations over a 37-year model period. We analyze annual and seasonal trends using the Mann-Kendall trend test. Finally, we examine the susceptibility of coastal areas in South Korea to offshore contamination and saltwater intrusion, accounting for land use development over decades.
Regional analysis of fresh SGD rates
Korea comprises numerous small-sized islands, primarily located in the West Sea and South Sea, accounting for a significant portion at 43.6% of the total. Despite their substantial count, the SGD from these minor islands remains relatively small across all regions due to their limited drainage lengths (Extended Data Fig. 1). We exclude these minor islands to prevent potential statistical biases (Fig. 1). The topography of the Korean Peninsula significantly influences its annual rainfall patterns. The southwestern region, positioned on the windward side of high mountains, receives abundant rainfall, while the northeastern region, surrounded by mountains, experiences lower precipitation levels (Extended Data Fig. 1b). The drainage length, representing the average distance from any location in the coastal catchment to the shoreline, tends to be smaller in the southwestern region with small-sized islands (Extended Data Fig. 1c). Various land surface models were employed to calculate water budgets for coastal aquifers and estimate fresh SGD along the Korean Peninsula (Extended Data Table 1). The ensemble average for the entire domain was determined to be 279 m2/year, with the uncertainty in fresh SGD estimated at 16.3 m2/year.
The patterns of fresh SGD are influenced by a combination of drainage geometry and climate conditions (Fig 2). In the North Korean region, including areas like Pyongan, Hwangnam, and Hamgyong, both net precipitation and fresh SGD exhibit lower values. Conversely, the western and southern coasts of South Korea experience higher levels of net precipitation and fresh SGD. The impact of drainage length becomes apparent when examining the East and West coasts. For instance, net precipitation levels are similar in the Chungman and Gangwon regions, but fresh SGD rates are approximately 47% greater in the Chungman region. This disparity can be attributed to the presence of extensive coastal drainage systems within complex terrains. Substantial tidal flats are present along the West coast, where tidal differences play a crucial role in influencing high SGD rates5. Additionally, the West coast is predominantly composed of sandy tidal flats, which can increase fresh SGD rates. The nutrient runoff via groundwater on large continental shelves affects potential ecological systems and needs continuous monitoring. Jeju Island exhibits notably high levels of SGD. Groundwater on the island originates from volcanic rock aquifers, primarily consisting of permeable basalts. The island's significant elevation changes and permeable volcanic bedrock, coupled with its limited river drainage infrastructure, contribute to the elevated rates of SGD. This aspect is particularly relevant as groundwater contamination plays a pivotal role in influencing coastal ecosystems and contributing to eutrophication.
Impact of climate change on the fresh SGD rates
SGD exhibits a rising trend across the entirety of the Korean Peninsula (Fig. 3a). This pattern aligns with the observed trend in net precipitation (Fig. 3b). Most regions in South Korea do not display statistically significant trends, either increasing or decreasing. However, discernible statistically significant trends are evident in the Incheon-Gyeonggi region and various areas within North Korea.
Seasonal patterns in fresh SGD rates (Figs. 4a-4d) closely mirror the precipitation dynamics, exhibiting concentration predominantly during the summer months. Positioned within the temperate monsoon climate zone, the Korean Peninsula distinctly experiences the seasons of spring, summer, autumn, and winter. Spring and autumn are marked by clear and dry weather influenced by migratory high-pressure systems, while summer sees hot and humid conditions due to the impact of the North Pacific high-pressure system. Conversely, winter is characterized by cold and dry conditions influenced by the continental high-pressure system. Notably, during the summer season, rainfall constitutes 54% of the annual precipitation. This discernible trend is most conspicuous in the watersheds along the western and southern coasts of the Korean Peninsula during the summer (Fig. 4b). Conversely, watersheds along the eastern coast consistently exhibit low SGD rates throughout all seasons. The primary contributing factor to this phenomenon is the higher relative rainfall observed on the western and southern coasts compared to the eastern coast. Furthermore, the intricate coastline features in the western and southern seas, characterized by longer drainage lengths, make fresh SGD rates highly responsive to variations in precipitation. Examining the seasonal trends of fresh SGD rates (Figs. 4e-4h), notable distinctions between South and North Korea become apparent. Coastal watersheds in North Korea show a consistent trend of increasing fresh SGD rates across all seasons. In contrast, regions of South Korea, including the western and southern coasts and Jeju Island, distinctly exhibit rising trends in fresh SGD during both the spring (Fig. 4e) and winter (Fig. 4h), while trends observed during the summer (Fig. 4f) and fall (Fig. 4g) are subdued. The spring season is influenced by climate change, resulting in heightened rainfall in these areas. Additionally, reduced soil moisture freezing due to rising temperatures during winter leads to increased surface and subsurface water inflow.
Impact of decadal land use changes on coastal vulnerability
Fresh SGD estimates also reveal potential contamination threats to the ocean and fisheries, upon which coastal populations heavily rely (Fig. 5a). Our definition of vulnerability generally holds true when increases in onshore aquifer contaminant concentrations coincide with a higher fraction of developed land and when the coastal supply rate correlates with elevated fresh SGD rates. Coastal areas with above-average fresh SGD and land development are particularly vulnerable to groundwater-borne contamination, accounting for 15% of the entire coastline (Fig 5a). Vulnerable regions include the western and southern parts of the Korean Peninsula. Notably, almost all of the coastal catchments in the Jeju Island area are susceptible to contamination. According to the Land Cover Change Map between 1980 and 2010 from the Ministry of Environment of South Korea (https://egis.me.go.kr), developed and agricultural lands increased by 2% and 24%, respectively, in the Jeju Island area, which is situated along the coastline. This developmental expansion contributes to elevated nutrient loads in the groundwater. Given that Jeju Island is a volcanic island and heavily relies on groundwater for its water resources, and considering that a majority of its population resides along the coast, addressing this issue is crucial. The reliance of drinking water, agriculture, and industrial activities on freshwater derived from underground sources and surface runoff underscores the need for continuous monitoring of nutrient inputs from fresh SGD21.
Fig. 5a also illustrates the vulnerability of coastal aquifers to saltwater intrusion. Saltwater intrusion can occur in areas where groundwater is discharged into the ocean through underground conduits as part of SGD. Vulnerability to saltwater intrusion is particularly high in the southwest regions (i.e., Jeolla and Gyeongnam) (Fig. 5a). Coastal aquifers in these areas cannot be directly used for agriculture, such as cultivating salt-sensitive crops. These regions are especially susceptible to excessive groundwater extraction and salinity damage during droughts caused by climate change. According to the 2022 Marine Infiltration Survey Report, compiled by the Korea Rural Community Corporation (https://www.groundwater.or.kr), a total of 225 in-situ gauges in South Korea show that 31% of gauges continuously decrease the level of coastal aquifers, and 38% of gauges continuously increase electrical conductivity. Furthermore, ongoing global warming trends contribute to a consistent rise in sea levels, which poses additional challenges to coastal lowland agricultural areas, such as an increasing risk of flooding and saltwater intrusion. The Korea Hydrographic and Oceanographic Agency (www.khoa.go.kr) conducted an analysis of sea level height data gathered from 21 coastal tide gauging stations. The findings also indicate that over the past 34 years (1989 to 2022), the sea level along the Korean coast has experienced a rise at a rate of 3.03 mm/year, culminating in a total average increase of approximately 10.3 cm. In Jeju Island, both the western and eastern regions are vulnerable to offshore contamination and saltwater intrusion. As mentioned earlier, these areas rely primarily on groundwater to meet their resource demands. If groundwater extraction reduces fresh SGD below a critical threshold, salinization can occur25.
Fig. 5b illustrates the changes in developed and agricultural land in South Korean coastal catchments since 1990 and their corresponding impact on vulnerability to offshore contamination. Note that the North Korea region was excluded from the study due to limited access to datasets. The overall proportion of developed and agricultural land has consistently increased from 1990 to 2020. Specifically, the developed area has continuously expanded, while agricultural land has steadily decreased. These changes have exacerbated the vulnerability of offshore contamination-prone regions by 38% since 1990 with the issue notably concentrated in the western and southern coastal regions.
The analysis of GRACE TWS anomalies26 also reveals a decreasing pattern of groundwater across coastal catchments. Extended Data Fig. 2 depicts Fig. S2 depicts the anomalies in TWS (mm) over coastal catchments from GRACE during 2002–2016, along with their piecewise linear trends. TWS represents an integrated measure of the terrestrial water budget, encompassing groundwater, soil moisture, surface water, ice, snow, and water in vegetation. While TWS does not directly measure groundwater storage, changes in groundwater storage trends can be identified through TWS fluctuations. Coastal catchments do not contain streams, and TWS is more similar to groundwater storage compared to non-coastal catchments. The figure indicates no significant change until 2011, but from 2011 to 2016, a notable depletion of TWS (-0.79 mm/month) is evident. This issue primarily arises as domestic agriculture shifts towards high-value facility farming, leading to increased groundwater usage in these coastal areas.
Implications for expanding the perspective
The outcomes of this research provide a fundamental foundation and support for a nationwide coastal vulnerability monitoring system. Our regional LSMs enable the continuous assessment of regional recharge rates and provide land-derived SGD rates along the coast of the Korean Peninsula. To enhance the accuracy of SGD rates using LSM approaches, it is essential to integrate local meteorological forcing datasets and land surface parameters into our regional LSM. This integration will help derive hydrologic variables relevant to SGD and allow for more precise topography for improved delineation of coastal catchments. Additionally, incorporating groundwater flow models on a local scale is necessary to explicitly address lateral groundwater flow.
Our LSM-based SGD estimations should be integrated into the existing groundwater measurement network. The Groundwater Information Management and Service Center (https://www.gims.go.kr) must carefully consider various SGD management factors. In addition to field measurements, satellite observational data and remote sensing techniques can complement proactive coastal vulnerability monitoring alongside the LSM27. GRACE data are valuable for providing groundwater storage anomalies across coastal catchments. The thermal infrared sensors of optical satellite missions can resolve the spatial variation of SGD rates and detect potential groundwater discharge areas along the coast of the Korean Peninsula.