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Freshwater-salt water interface (FSI) location is very important information for decision maker in managing coastal aquifer system, however, its temporal change have been hard to get using conventional method such as EC monitoring at one or several fixed depths, geophysical logging or remote sensing techniques. A FSI tracking device, which has density between freshwater and salt water and hence can moves up and down as the freshwater-salt water transition zone moves, is used to get a temporal change data for the interface during several different types of pumping tests, which were performed at coastal monitoring wells in Seocheon, middle west of Korean Peninsula. Four short period pumping tests, three long-period pumping tests, one step-drawdown test, one reverse step-drawdown test were performed at different pumping rate ranging 19.86 to 48.71 m3/d for different pumping period of 60 minutes to 2851 minutes. Time series data shows that the Interface-Egg rises up from -86.0 to -77.6 m amsl after 24-hours pumping and to -40.8 m amsl after 2-days pumping and freshwater lens thickness is getting thinner from 88.1 m to 78.4 m after 24-hours pumping and then 42.3 m after 2-days pumping. These salt water up-coning phenomena are supported by EC profiles which were logged before and after the whole pumping periods. Time series data tell us that salt water upconing in the pumping well happens quickly and recovers at a very slow rate which is about 1.5 cm/d at 3 months after stopping pumping. The time series data of groundwater head and the interface-Egg’s location also shows that there is a tidal influence between pumping periods. The FSI tracker is expected to be practically applied to coastal aquifer management preventing from salt water intrusion, especially at dynamically pumping area for agricultural and/or domestic water supply.