Hydrogeological characteristics and mechanisms of dynamic variations in the saltwater-freshwater transition zone in a coastal aquifer, East Sea

Abstract

The hydrogeological characteristics and mechanisms of dynamic variations in the saltwater–freshwater transition zone were investigated using physical and geochemical approaches in a coastal aquifer, East Sea. The study site is located along a fully open coast of East Sea, where waves, tides, and precipitation influence the fluctuations in groundwater level (GWL), electrical conductivity (EC), and groundwater temperature (GWT). In addition, since the aquifer consists of estuarine deposits and fractured rocks with anthracite, not only mixing process and geochemical reaction in saltwater–freshwater transition zone, but also the oxidation of sulfide and reduction of hydroxide have influences on the groundwater quality. Long-term monitoring and time series analyses of GWL, EC, and GWT were used to understand variations of the saltwater–freshwater transition zone and physical mechanisms of dynamic changes within. Correlation, spectral analysis, and multi-variable regression analysis of monitoring data were performed to deduce the relationships between hydrological factors. The general shape of the transition zone is determined by the seasonal variation of GWLs, but temporary fluctuations in the transition zone are dominated by local rising GWL events. The distinct increases in the GWL were closely related to the high wave heights. Different patterns of EC change were detected in the shallow and deep zones, which indicate that the transition zone is highly dynamic. At shallow depths, the EC values are temporarily increased by the wave setup and tidal fluctuations during the rising GWL events, but the EC at greater depths is reduced by the seaward or downward movement of the relative fresh groundwater. In exceptional cases, during extreme increases in the GWL resulting from seawater flooding, the rapid downward flow of the flooding saltwater through the well bore causes synchronous EC fluctuations at all depths. The governing geochemical processes were estimated to be the mixing of saltwater and freshwater, the oxidation of iron sulfide and the reduction of iron hydroxide. The mixing process has intensively influences on the concentrations of the major ions, whereas the oxidation of iron sulfide and the reduction of iron hydroxide control the concentrations of the minor ions and trace elements. The mixing ratios of seawater and fresh groundwater, calculated using stable isotopes of oxygen and hydrogen, EC value, and chloride ion, range from 0.2% to 55.4%. The high concentration of soluble iron is assumed to be caused by sequential geochemical reactions. The dissolved oxygen with relatively high concentration in the recirculated saline water has promoted the oxidation of iron sulfide, which decreases the pH of groundwater in the saltwater–freshwater transition zone. The consumption of dissolved oxygen and low pH values make the favorable environment for the reduction of iron hydroxide. Mn has similar chemical properties to Fe, but has showed a peculiar high concentration due to the difference of the redox conditions between Fe and Mn. The cation exchange by saltwater intrusion, dissolution by acidity, and reduction or precipitation by redox condition led to the enrichment of some ions and deficiency of others. The enrichment of Ca is dominated by cation exchange in landside and dissolution by acidity in seaside.