| Pingtung coastal plain in the southeast region of Taiwan is one of the most active areas regarding the excessive pumping of groundwater, this area suffered from land subsidence for decades. Therefore, a survey data is proposed in this study to calculate the rate of land subsidence caused by the groundwater over-pumping. In this research, a device called multi-layer compaction monitoring well (MCMW) is able to measure the vertical deformation from the earth surface to the depth of 200 m with high accuracy (1-5 mm). The system of this device is used to measure the compaction of aquifer systems by anchoring several magnetic rings to aquifer systems at different depths. There are two MCMWs, which were established at Jiadong and Fangliao areas, respectively. The results taken from MCMWs indicate the fact of land subsidence caused by groundwater pumping in this area with the correlation coefficients between the data of MCMWs and groundwater observation wells range from 0.58-0.95. Therefore, we propose a method that estimates the major compaction zone based on physical theories of strain adjustment, that is the distance changes between two magnetic rings. According to survey data within 200 m depth, we have figured out the major compaction zone at 0-104 m for Jiadong well and at 101-156 m for Fangliao well. The observation results of vertical deformation vary seasonally. In this research, the positive value and negative value were indicated expansion and compaction of soil, respectively. In the dry season, the average deformation rate in the Jiadong and Fangliao well stations are -64.3 and -40.0 mm/yr, respectively. In the wet season, the average deformation rates range from 29.4 to 52.4 mm/yr for Fangliao and Jiadong well. Moreover, pumping and recharge period are around one-year interval. These two areas are strongly influenced by the change of the wet and dry season. |
