Traditional estimation of submarine groundwater discharge (SGD) relies on isotopic mass balance calculation, which is a form of regional box modeling that cannot resolve spatial heterogeneity. This study proposes an entirely new approach by coupling a general marine hydrodynamic simulation model with a global optimization algorithm. The framework simulates the transport and mixing of isotopic tracers in coastal waters, and optimizes spatially distributed SGD fields to best match observed isotope activities.

The results demonstrate that the estimated SGD exhibits strong spatial heterogeneity, a feature that conventional box models systematically miss. Furthermore, the study reveals that isotope sampling time has a significant effect on estimation accuracy. Specifically, fluctuations in seawater level and a time lag in isotope activity response to water level changes must be considered to ensure representativeness of measured isotope activities.

Overall, the proposed simulation-optimization framework adds a versatile and powerful new tool for more detailed SGD estimation. It represents a major methodological advance beyond traditional box-model approaches, enabling spatially explicit quantification of SGD without requiring additional field instrumentation beyond routine isotope sampling.