Analysis of seasonal differences in tidally influenced groundwater discharge processes in sandy tidal flats: A case study of Shilaoren Beach, Qingdao, China

Abstract

Tidally influenced groundwater discharge in tidal flats is an important route by which land-based pollutants enter the sea. Seasonal changes in groundwater flow fields create differences in the process and amount of groundwater discharge in sandy beaches. To explore this subject, we selected Shilaoren Beach in Qingdao as our research area and used electrical resistivity tomography (ERT) and hydrogeological measurement methods to monitor the groundwater discharge process in the same area in both May (rainy season) and November (dry season) in 2019. The study found that there are obvious upper saline plumes (USPs) and freshwater discharge tubes (FDTs) in the sediment layer in both seasons. When sea level falls, the USPs sink and lateral extent decreases due to groundwater discharge, and the FDTs develop from deep below to the beach surface, and divide the USP into multiple sections. During the flood tide, the pore water salinity of USPs increases and lateral and vertical ranges expand rapidly. The groundwater discharge process differs seasonally. In the dry season, the vertical extent of USP is larger around high tide and the distribution range of FDT is more unstable. During the ebb tide, the pore water salinity in USPs and FDTs changes from the moment when seawater starts receding, however, in the dry season, it changes more slowly during the early stage of the ebb tide and more rapidly near the end the ebb tide. The difference of hydraulic driving force is the main factor leading to the difference of distribution range and pore water salinity change of USP and FDT in different seasons. The volume of groundwater discharge also varies with seasons, and the discharge in rainy season is about 1.21 times of that in dry season. In the rainy and dry seasons, the groundwater discharge gradually increases from the land to the sea, and the maximum discharge location is near the low tide line.