The continuous expansion of underground space development in Seoul has led to a significant increase in artificially discharged groundwater and fluctuating groundwater levels. Nationwide, approximately 140 million tons of discharged groundwater were recorded in 2020, and Seoul has shown a consistent rise due to the cumulative impact of multiple underground construction projects. The city’s groundwater utilization rate has reached 72.2 percent of its available development capacity, higher than the national average of 67.2 percent, indicating substantial pressure on groundwater resources.
Excessive fluctuations in groundwater levels can have serious implications for urban safety and the environment. A declining groundwater level may cause soil shrinkage, land subsidence, and sinkhole formation, while rising groundwater levels can lead to flooding of underground facilities such as subway stations, underground shopping centers, and tunnels, threatening the structural stability of subsurface infrastructure. Continuous and systematic monitoring of groundwater levels is therefore essential. Japan successfully reduced annual land subsidence from over 5 cm to less than 1 mm through strict groundwater abstraction regulations and a dense observation network since the 1950s, while Houston, USA, restored its groundwater levels by enforcing pumping restrictions and strengthening monitoring programs.
According to the Groundwater Act, local governments are required to install observation wells when groundwater hazards are anticipated. Since 1996, Seoul has operated its auxiliary groundwater level observation network, managing 269 wells as of July 2025. However, this number remains insufficient compared to the city’s scale, accounting for only about 8 percent of the 3,366 observation wells operated by local governments nationwide. Although the network spans all 25 districts, spatial gaps still exist in certain areas. Furthermore, limited linkage with the national observation network and the absence of integrated management for discharged?groundwater information have reduced the efficiency and utility of monitoring data.
The central government’s Fourth Master Plan for Groundwater Management (2020) calls for a substantial expansion of both national and local observation networks. The Ministry of Environment plans to increase the number of national groundwater monitoring wells from 688 in 2021 to 4,219 by 2045 and expand the local auxiliary networks from approximately 3,366 to 10,000 sites (including 2,000 auxiliary groundwater level observation wells). While the national network monitors long? term nationwide trends in groundwater levels and quality, Seoul’s auxiliary network functions as a complementary system that provides continuous, fine?scale monitoring at the local level. In alignment with this policy direction, Seoul urgently needs to establish a proactive expansion plan to enhance urban safety and achieve sustainable water management.
This study developed site?selection criteria for new observation wells that optimize limited municipal resources and reflect the city’s unique subsurface characteristics. Based on scientific and objective indicators, priorities for network expansion were identified, and seven categories of high?priority installation areas were selected. These criteria are expected to improve monitoring efficiency and serve as a practical foundation for Seoul’s future expansion plan.
Building upon the proposed criteria, a phased implementation strategy for the auxiliary observation network should be established. In the short term, new wells should be installed in areas with rapid groundwater fluctuations and insufficient coverage; in the medium and long term, the network should be gradually expanded in accordance with urban development trends through the 2040s. Alongside expansion, qualitative improvement is essential?integrating monitoring data management systems, linking discharged?groundwater databases, conducting regular instrument calibration, and strengthening interdepartmental cooperation for broader data utilization. Policy measures such as mandating pre?installation of observation wells in large?scale development projects, promoting interdepartmental collaboration for the reuse of discharged groundwater, and developing open?access data platforms are also recommended.
The expanded network will generate data that can be strategically utilized across multiple domains, including subsidence risk mapping, structural safety assessments of underground facilities, long?term water?resource management under climate change, and protection of groundwater quality through coupled surface?groundwater monitoring. These efforts will minimize adverse impacts from discharged groundwater, promote efficient use of groundwater resources, and reinforce Seoul’s foundation for sustainable water management while enhancing the resilience and safety of its urban infrastructure.