2020-10-192024-05-13https://scholars.lib.ntu.edu.tw/handle/123456789/651200Dongsha Island, located at the western side of Dongsha Atoll, is a fortress of Coast Guard in the South China Sea as well as a base of Dongsha Atoll National Park. It also serves as an important stronghold for marine scientific research and education. For the permanent or temporary residents who have tasks on Dongsha Island, safe and adequate freshwater resource is the basic need. But, the natural freshwater resource of Dongsha Island seems to be limited due to the high permeability of coral reef sands that makes the rainwater hardly remaining on the surface but directly infiltrating into the ground. It is effortless for seawater intruding as well that results in the salinization of groundwater. So far, seawater desalting plant on Dongsha Island benefits producing fresh water. However, if groundwater is overdraft, seawater intrusion will be aggravated. This proposal shows the main survey works and achievements of this project entitled "Investigation of Groundwater Resources on Dongsha Island" which is supported by Marine National Park Headquarters, Construction and Planning Agency, Ministry of the Interior. For the sustainable operation of groundwater resources on Dongsha Island, the primary task is to understand the overall condition of groundwater resources, including the overall recharge and inventory of groundwater as well as the exchange between seawater and groundwater through setting up a database of long-term hydrogeological observations. The following list the main survey works and achievements of this project: (1) For understanding the long-term hydrogeological variations of groundwater, the observations of water temperatures, salinities and conductivities as well as groundwater tables will be conducted in 2 groundwater wells with more than 6-month data retrieved. (2) The major anions and cations as well as the isotopic compositions of hydrogen and oxygen in groundwater samples will be measured. In addition, 2 groundwater samples will be analyzed for carbon-14 dating. (3) The short-term observation, covering more than one tidal cycle period, of water temperature, salinity and conductivity will be conducted at a site along the coast of lagoon within Dongsha Island. The major anions and cations, radium isotopes as well as the isotopic compositions of hydrogen and oxygen will be analyzed. Furthermore, the submarine groundwater discharge flux will be estimated. (4) To draw the spatial variations of groundwater table and salinity within Dongsha Island, and further to roughly estimate the inventory of groundwater.Dongsha Atoll; Dongsha Island; Groundwater; Long-Term Hydrogeological Observation; Submarine Groundwater Discharge