Fate of Underground Brine Resources in Beach and Neritic Zones in Shallow Aquifers Driven by Salt Pumps

Underground brine in beach and neritic zones (UBBN), widely distributed on muddy coasts in arid coastal regions, undergoes dynamic salt cycling driven by a “salt pump” system, composed of hydraulic gradients, salinity gradients, and tidal forces. This study investigates the fate of UBBN along the southern coast of Laizhou Bay, China, where salt depletion threatens sustainable resource management. Combining field observations, we developed a bay‐scale numerical model incorporating geomorphological diversity, submarine groundwater discharge hotspots, and sediment heterogeneity to quantify UBBN dynamics during tidal cycles and evaluate the impacts of coastal underground brine (CUB) mining and suspension. Results show that tidal fluctuations control groundwater flow fields, which drive spatiotemporal salt transport and persistent UBBN depletion. Salt outflow rates peak at the sediment‐water interface in beach zones and decline seaward. Discharge hotspots exhibit salt outflow rates 4–8 times greater than adjacent neritic areas. Despite net losses, approximately 9% (offshore)–60% (nearshore) of salt from tidal recharge (evaporated beach salt and seawater) is retained and replenishes the aquifer per tidal cycle, accumulating predominantly in the top‐section of silt cover layers. CUB mining and suspension enhance seawater and evaporated salt influx while reducing leakage from high‐salinity confined brine, effectively slowing UBBN salt loss. These findings advance large‐scale UBBN cycling mechanisms and provide actionable insights for sustainable development of UBBN resources in bay‐scale beach and neritic zones.