2018 年 24 巻 1 号 p. 15-21
Laura Island is located about 50 km away from downtown in Majuro atoll, which is the capital of the Republic of the Marshall Islands. It has been very important for Majuro atoll to ensure the conservation of its water resources because low and flat Majuro atoll in fact has neither river nor lake. Nevertheless, up-coning, which is the process of partial rising of the freshwater-saltwater interface in the freshwater lens near the water-supply well, was caused by the excess pumping from it during the severe drought in 1998. The up-coning will make the water usage of the freshwater lens difficult. Thus, sustainable water usage, as demonstrated in this present study, is immediately required to prevent the up-coning in the freshwater lens because there is no other water source during drought. SEAWAT model simulation of the groundwater usage was conducted at the central part of Laura Island, including the water-supply well, which was affected by the up-coning. The sustainable pumping discharge from the water-supply well was calculated under the conditions that variable daily pumping discharge without monthly rainfall was assumed. Consequently, due to the increased daily pumping discharge the total amount of pumping discharge was increased. This indicates that it needs more time to recover from the up-coning in the freshwater lens. Therefore, a pumping standard and the appropriate water usage to reduce the pumping intensity is being proposed, which is based on the numerical simulation concerning the occurrence of the up-coning phenomenon in Laura Island during the drought.