Temporal and spatial distribution patterns of upper saline plumes and seawater-groundwater exchange under tidal effect

Abstract

Under tidal influence, an active exchange process occurs between seawater and groundwater in the coastal aquifer system. However, in-situ studies and research on the temporal and spatial distribution patterns of upper saline plumes (USPs) and seawater-groundwater exchange process are lacking. In this study, we selected the Shilaoren beach in Qingdao as the study area, and used pseudo-3D electrical resistivity tomography (ERT) and hydrogeological measurement methods to monitor the spatial distribution of groundwater salinity and the seawater and groundwater exchange process. We found that during the ebbing tide, USPs were divided into several smaller USPs driven by groundwater discharge. Subsequently, during the flooding tide, high-salinity seawater recharge was observed, and several smaller USPs gradually aggregated into a complete USP. In coastal aquifers, the exchange of seawater and groundwater not only occurs through the beach surface and seepage face, but also includes high-permeability underground channels. Seawater and groundwater exchange occurs more rapidly through these underground channels randomly distributed at depth, which also leads to spatial differences in the seawater and groundwater exchange process. Additionally, affected by underground channels, the groundwater discharge flux shows spatial heterogeneity. In areas with underground channels, the groundwater discharge flux significantly increases. Therefore, the non-seepage face area has a similar groundwater discharge flux to the main groundwater discharge area, namely, the seepage face. In the intertidal zone of the study area (70 × 60 m), the total groundwater discharge flux through the seepage face is 3407.6 m3, and no significant difference is observed in the groundwater discharge flux at various parts of the seepage face. In the non-seepage face area, the groundwater discharge is highly spatially heterogeneous; the total groundwater discharge flux is 3252.0 m3, and approximately 60% of this occurs through densely developed underground channels.