Numerical simulation and assessment of seawater intrusion under coastal brine extraction: implications for groundwater management in the Weibei Plain, China.

Seawater intrusion poses a growing threat to groundwater sustainability in coastal aquifers, particularly in regions affected by intensive groundwater extraction and coastal brine extraction. Based on regional hydrogeological conditions and long-term monitoring data, a three-dimensional variable-density groundwater flow and solute transport model was developed for the phreatic and confined aquifer systems within 150 m depth of the Weibei Plain (Shandong, China). Four pumping scenarios were designed to evaluate the impacts of groundwater extraction patterns on seawater intrusion dynamics. The results indicate that long-term and spatially unbalanced groundwater extraction, including freshwater pumping in the landward freshwater zone and brine extraction in the seaward coastal saline zone, has contributed to the formation of a dual-cone depression system separated by a groundwater divide, thereby altering the regional flow regime and increasing the potential for seawater intrusion. The simulated Cl- distributions indicate that the intrusion boundary changed heterogeneously along the coast, with landward displacement in some sectors and seaward displacement in others, while the total intrusion area increased overall. Freshwater overextraction in the landward freshwater zone was identified as the primary driver of intrusion expansion, whereas brine extraction in the seaward coastal saline zone modified the hydraulic gradient and influenced the position of the groundwater divide. Scenario analysis demonstrated that combined management strategies were more effective than single measure approaches. In particular, a coordinated scheme involving a 30% reduction in freshwater extraction in the landward freshwater zone and a controlled 30% increase in brine extraction in the seaward coastal saline zone produced the smallest simulated increase in both the spatial extent of seawater intrusion and Cl- concentrations among the tested scenarios. These findings highlight that integrated regulation of freshwater and brine extraction provides a feasible pathway for mitigating seawater intrusion while considering brine resource utilization, offering scientific support for sustainable groundwater management in coastal areas with brine extraction.