Structural optimization of water distribution systems in South Korea local cities facing depopulation driven hydraulic and water quality challenges

South Korea, rapid aging and regional decline have accelerated depopulation, creating significant challenges for the sustainable management of infrastructure systems. Therefore, small- and medium-sized cities in Korea are vulnerable, as declining demand threatens the long-term reliability of water distribution systems (WDSs). Traditional WDSs are conservatively designed with the expectation of population growth, leading to overcapacity when the demand decreases. This mismatch results in underutilized pipes, prolonged residence times, and associated hydraulic and water quality deterioration. To address these issues, this study introduces a structural reconfiguration framework that identifies and isolates nonessential pipes using spectral graph theory, followed by multi-objective optimization to balance efficiency and reliability. Extended period simulations have revealed that demand reduction in peripheral areas accelerate chlorine decay and induces network-wide hydraulic imbalances. By closing redundant pipes and eliminating unnecessary pipes, the proposed approach reduces maintenance costs while improving the water quality performance. The findings highlight the structural drivers of WDSs degradation under depopulation and provide an analytical framework that supports structural decision-making for urban water infrastructure under depopulation.