Thermal effects on coupled density-driven solute transport in a layered heterogeneous coastal aquifer

Variable-density flow and solute transport in porous media are fundamentally governed by groundwater–seawater thermal gradients and permeability heterogeneity. However, the coupled influence of temperature-induced variations in viscosity and density, together with stratification, on solute transport remains poorly understood. This study integrates laboratory experiments with numerical simulations to investigate solute transport dynamics under non-isothermal conditions in a stratified coastal aquifer containing a subsurface dam. Spatiotemporal variation, migration pathways, spreading characteristics, and solute mass fluxes are systematically evaluated under varying aquifer–ocean temperature contrasts and layered permeability configurations. Results reveal that cooling freshwater causes the saltwater wedge to contract while the upper saline plume expands. This phenomenon is driven by thermally induced changes in fluid density, viscosity, and hydraulic conductivity. Warmer freshwater reduces viscosity, enhancing localized hydraulic conductivity and accelerating flow within the freshwater zone. However, it simultaneously suppresses particle movement within the saltwater wedge through enhanced vertical density stratification. Conversely, colder freshwater markedly decelerates solute transport and reduces maximum mass discharge nearly threefold. Solutes released from the lower layer are more susceptible to mixing with residual saltwater upstream of the subsurface dam, with the average residence time prolonged by 1.8 times. The total efflux of upper-released solutes exhibits a distinct double-peak pattern due to post-discharge retention caused by the low-permeability layer. Sensitivity analyses demonstrate that thermal forcing and permeability exert substantial impacts on plume spreading and mass discharge. Collectively, these findings provide fundamental physical insights into the coupled thermo-hydrodynamic mechanisms governing density-dependent solute transport in heterogeneous porous media.