Percolation of water from a pond may cause a landslide in a hilled rural area. Before measures to prevent such landslides can be planned, it is essential to identify reservoir-water influent from the pond. We describe an analytical method that uses 222 Rn-and water-balance equations to quantify groundwater effluent and reservoir-water influent simultaneously, which is impossible using the conventional method that measures only surface-water inflow and outflow. We selected F pond, Nagano Prefecture, as the study site. We estimated the rate of 222 Rn dispersion to the atmosphere by assuming a stagnant film between water and air with thickness inversely proportional to the rate of dispersion. By a laboratory experiment, we estimated the film to be about 830 µm thick and found that the film thickness was not influenced by wind velocities less than 1.5 m s -1 . A preliminary investigation suggested that reservoir-water was mixed very well. The groundwater effluent and the reservoir-water influent during the investigation were calculated to be 0.67 × 10 -3 m 3 s -1 and 0.41 × 10 -3 m 3 s -1 respectively, by making 222 Rn-and water-balance equations and solving them. This analytical method is expected to be useful for not only prevention of a landslide but also for effective use of water and prediction of water quality.
There is a freshwater lens which is stored in porous Pleistcene Ryukyu Limestone aquifer at Tsuken island in Okinawa prefecture. The volume of the freshwater lens were estimated approximately 97,000m3 in this study.In order to avoid the intrusion of saltwater during pumping, a radial well type intake facility with vertical pit (14m in depth and 3.5m in diameter) and four horizontal collector pipes (each 25m in length) was designed and constructed. After that the continuous pumping test during 31 days was proceeded.As the result, amount of the freshwater yields by the intake facility are 7,000m3 which correspond to 3.4 times larger than that of a vertical pipe well (150mm in diameter and 27m in length) where located close to the intake facility.
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