Thermal effects on flow and salinity distributions in coastal confined aquifers

Coastal aquifers provide an important hydrologic connection between terrestrial freshwater and oceanic seawater. Understanding the water flow and salinity distributions in those aquifers is essential to manage seawater intrusion and regional groundwater resources. For the last 50\ua0yr, these processes have been extensively examined; however, previous studies typically assumed isothermal conditions and overlooked groundwater-seawater temperature contrasts, as commonly found along the global coastline. Here, we validated a solute and heat transport model against laboratory experiments and then revisited the classical Henry problem for coastal confined aquifers. We found that as the seawater temperature increases or freshwater temperature decreases, the freshwater-seawater interface retreats seaward and thus the freshwater storage increases. This occurs as a result of temperature-induced changes to the fluid density and aquifer hydraulic conductivity. Furthermore, double diffusion of salt and heat can induce two circulation cells in opposing directions in the saltwater wedge once the seawater temperature exceeds that of groundwater by a certain extent. Consequently, the circulating seawater flux and associated travel time change dramatically compared with isotheral conditions. Overall, the effect of thermal forcing can be significant, as demonstrated by a sensitivity analysis considering a large range of groundwater-seawater temperature contrasts. These results highlight the impact of changing thermal regimes on the flow and salinity distributions in coastal confined aquifers and provide guidance for better assessment of water resources in coastal zones