Up-coning Caused by Excess Pumping during Drought Had Reduced Storage Volume of Fresh Groundwater Lens on Atoll Island

摘要

Fresh groundwater lenses develop on relatively large atoll islands and play a key role in providing stable freshwater supply. The Republic of the Marshall Islands in the Pacific Ocean sometimes suffer from drought caused by El Nifio. About half of itspopulation lives in the capital,Maj uro， and securing freshwater resources for these people is a crucial issue. There is a fresh groundwater lens (Laura Lens) on Laura Island in Majuro Atoll. In this research, the current status of Laura Lens was investigated as up-coning that mixes seawater into fresh groundwater could render the fresh groundwater lens unusable. Electrical conductivities (ECs) of the groundwater at different depths in the monitoring wells were observed to determine the Laura Lens interface depths. ECs of groundwater surface at resident shallow wells were also observed to determine the planar Laura Lens interface. Locations of these interfaces, which have ECs equivalent to 200 mS/m, were calculated by interpolation. The Laura Lens storage volume from 2010 to 2013 was estimated by inputting the data into Surfer software. The results of groundwater observations showed that up-coning, similar to the one that occurred in 1998 which had resulted in serious drought, could still be observed. It was also found that the Lauraしens storage volume had decreased and that most of the fresh groundwater that seeped into Laura Lens had flowed out into the sea. Therefore, it is necessary to conserve and use the fresh groundwater lens.