Evaluation the freshwater-saltwater interface using geophysical and geochemical investigations within the single borehole

Abstract

<p>The Korean peninsula has three sides facing the sea, and many monitoring wells for seawater intrusion observation installed along these coasts. The electrical conductivity (EC) profiles of these monitoring wells are found to vary greatly depending on the hydrogeological characteristics of the coast. The research area is located in Seocheon, Chungnam, where two wells are about 1.5 km away, one borehole is freshwater and the other boreholes have high electrical conductivity. In the well where the high electrical conductivity is measured, the electrical conductivity increases rapidly at a depth of about 90m. The well is located about 50 meters from the coast, and the change in water level reflects tidal changes well. We conducted a variety of field surveys and laboratory tests to clarify whether seawater intrusion occurred in the freshwater-saltwater boundary at the depth of about 90 m and whether the freshwater-saltwater boundary was affected by tides. Applied methods are electrical resistivity survey, geophysical well logging and geochemistry. The geophysical survey line was located along the coast centered on the well and measured every six hours. As a result of the survey, it was difficult to identify the change of electrical resistivity due to tidal changes in the lower alluvial zone, that is, the rock formation. These result indicates that the permeable fracture zones are rarely distributed in the rock below the survey line, so the change of the formation resistivity of according to the tidal change is very small. Geophysical well logs such as optical televiewer, spectral gamma log, and water quality logs (i.e., temperature, EC, pH, DO, and ORP) were measured, and groundwater samples were collected according to depth and analyzed for T, EC, pH, DO, ORP, and 222Rn. We found it difficult to identify permeability fracture zones in the temperature gradient and optical televiewer logs. The electrical conductivity is generally around 4,000 uS/cm and increase rapidly from about 90m to 48,000 uS/cm. The water quality logs and the analysis of groundwater samples showed pH range of about 7 to 7.5, DO of 7 to 14 mg/L, and OPR of -80 to 180 mV. pH does not change with depth, DO decreases slightly, and ORP decreases rapidly around the fresh-saltwater interface. The analysis of 222Rn shows that, to the median depth of the borehole, it is similar to the intensity of natural gamma rays and decreases rapidly across the lower freshwater-saltwater interface. Finally, we measured the electrical conductivity logs repeatedly at the interval of 30 minutes in the lower borehole where the freshwater-saltwater boundary was formed, and identified that the profiles of electrical conductivity change with tides.</p>