Water and Salt Transport Patterns and Controlling Factors in the Seawater Intrusion-Soil Salinization Hazard Chain along Silty Coasts

In multilayer aquifer systems on muddy coasts, groundwater dynamics can jointly drive seawater intrusion and soil salinisation in the upper silty layer. However, their spatiotemporal linkage and responses to hydraulic-head difference and aquifer slope remain poorly quantified. Using Laizhou Bay as a prototype, we constructed a saturated tank model comprising silty layers above and below a sand layer. We varied head difference and slope, and monitored water and salt transport using time-lapse electrical resistivity tomography. Seawater preferentially advanced landward through the sand layer before salt migrated into the upper silty layer, causing soil salinisation. Increasing the hydraulic-head difference accelerated both processes and expanded their affected areas. Salinisation was quantified using dissolved-salt mass in pore water within 30 cm 3 of the upper silty layer. A greater head difference advanced transitions to higher salinisation classes, although salinisation and the landward extent of intrusion were not fully synchronous. At 0° slope, the nearshore upper layer remained non-salinised (< 39.2 mg) for 36 h but exceeded 313.3 mg by 72 h, indicating extreme salinisation. At 1.5°, moderate salinisation (78.3–156.7 mg) occurred by 24 h and severe salinisation (156.7–313.3 mg) by 60 h; the peak then stabilised, with no extremely salinised zone developing within 72 h. These results reveal a hazard-chain sequence of preferential sand-layer intrusion, cross-layer advective and dispersive transfer, and salt accumulation in the upper silty layer. Intrusion distance, the timing of salinisation-class transitions, and fixed-volume dissolved-salt mass provide complementary metrics for assessing this hazard chain under contrasting hydrogeological and coastal settings.