Thermal effects of freshwater injection on flow and salinity distributions in tidally-affected coastal unconfined aquifers

Abstract

Freshwater injection is a practical and efficient solution to mitigate seawater intrusion in coastal aquifers, but its thermal impacts on groundwater flow and salt transport are still poorly understood. This study investigates the dynamic characteristics of salinity distributions and seawater recirculation in coastal unconfined aquifers subjected to freshwater injection and tides. The results show that the seaward development of thermal plume modifies the transience of upper saline plume and saltwater wedge, significantly prolonging the aquifer response to injected freshwater. Three temporal variations of upper saline plume due to freshwater injection are driven by pressure waves of injected freshwater, thermal plume-induced changes of hydraulic gradients and saltwater wedge, and the hydraulic conductivity within the upper saline plume. Moreover, a delayed reduction in seawater recirculation in response to the thermal impacts of cold-water injection induces a significant and long-lasting overshoot of total water efflux. An increased tidal amplitude facilitates thermal effects on flow and salinity distributions. A deeper injection well extends the flow path of injected freshwater and thermally regulates discharge rates along the beach surface, thereby enhancing overshoot magnitude of total water efflux. These results improve the understanding of underlying processes related to freshwater injection in coastal aquifers and provide guidance for better managing coastal groundwater resources.