The authors have been conducting research with the aim of developing methods to preserve and effectively use groundwater resources. This research has been ongoing since 2008 with the focus on Laura Island, Majuro Atoll, Republic of the Marshall Islands. Examination of the groundwater level potentials in pre-existing observation wells at Site No. 6, named by the United States Geological Survey (USGS), showed no difference in the groundwater level potential along the depth. Accordingly, observation well No. 6-N was established by boring to accurately observe the profiles of electric conductivity (EC) of groundwater. However, salt water intrusion occurred, contrary to the observed results for the groundwater level potential. Accordingly, the depth of the salt water intrusion was identified by developing a double packer, observation well No. 6-N was partially sealed and the sealing effect on the observation well was confirmed. This paper will discuss 1) the measures against salt water intrusion which occurred in the observation well, 2) the groundwater flow in an observation well, which was established to measure electric conductivity (EC) profiles of the groundwater, and 3) the depth of the groundwater path estimated by measuring EC profiles with a double packer in the observation well.