Mapping and monitoring seasonal and tidal effects on the salt-freshwater interface using electrical resistivity tomography techniques

Abstract

Understanding the salt-freshwater interface (SFI) migration in the coastal aquifers is of great significance for the study of seawater intrusion (SI). The electrical resistivity tomography (ERT) method in combination with the physicochemical characteristics of groundwater were used to map and capture the SFI change in the dry and wet seasons. And the west coast of Longkou city, Shandong province, China was taken as an example. And the physicochemical characteristics of groundwater mainly analyzed Cl− concentrations, TDS and EC values. Four monitoring locations, 1200 (W4), 2100m (W3), 2600m (W2) and 3200m (W1) away from and perpendicular to the west coastline, were used to collect groundwater samples and conduct continuous ERT surveys in dry and wet seasons of 2020. The SFI was captured with continuous 24-h tidal cycle and 15-day spring-neap tidal cycle. Analysis of the groundwater samples showed that the SFI was approximately located between the W2 and W3, and the groundwater levels for the four monitoring locations were higher in the wet season than that in the dry season. The ERT results were strongly supported via the Cl− concentrations, TDS, EC values and the groundwater level in the dry and wet seasons. And the ERT results indicated that the SFIs were located about 2562m and 2558m away from the coastline in the dry and wet seasons, which means the most inland part of the interface. Due to the delay effects of the 24-h high and low tidal cycle and different tidal amplitudes in 15-day spring-neap tidal cycle, the SFI retreated to seaward in high tide and spring tide, while it intruded to inland in the low tide and neap tide. Continuous monitoring surveys using the ERT had been proven to be suitable to map and capture the SFI change in the west coast of Longkou city in this study. This study can qualitatively and semi-quantitatively evaluate the location of SFI in coastal area, and the determination of the possible farthest SFI location on neap tide and low tide is helpful for the groundwater resources management and SI prevention.