<p>High-flux groundwater seepage occurred during deep excavation at a landfill construction site on coastal reclaimed land in South Korea. In this study, the cause of the groundwater seepage was analyzed using integrated conceptual and numerical modeling techniques. Ten candidate hydrogeological conceptual models (HCMs) were developed by combining various configurations of subsurface hydrogeological units (HGUs) and different magnitudes of the seepage phenomenon. Numerical groundwater models, based on these HCMs, were built using a three-dimensional stratigraphic model derived from borehole data to simulate the distribution of HGUs and groundwater seepage. Among the numerical models, the one that best reproduced the observed water levels, seepage, and drawdown was selected and further refined through model calibration. The final HCM provided insights into the site’s hydrogeological system and enabled a forensic interpretation of the groundwater seepage mechanism. The high-flux groundwater discharge was attributed to a groundwater upwelling phenomenon that hydraulically connects deep permeable media to the excavation surface. Weathered bedrock located approximately 22&#xa0;m below the ground surface were found to be sufficiently permeable to serve as a source layer for the seepage. Through case studies of additional excavation and drainage strategies, the integrated conceptual–numerical modeling approach demonstrated its potential as a reliable predictive tool for designing response strategies to groundwater seepage. This approach also facilitates interdisciplinary communication and supports adaptive site management throughout the project lifecycle, highlighting its practical engineering relevance.</p>