Evaluation of a freshwater lens configuration using a time series analysis of a groundwater level and an electric conductivity in Minami-daito Island, Okinawa Prefecture, Japan

Abstract

A freshwater lens configuration was evaluated by using electrical conductivity data in Minami-daito Island, Japan. Groundwater levels, topographical and geological features were discussed in evaluation process. 5 significant constituents (O 1 , K 1 , N 2 , M 2 , and S 2) were detected in spectral analysis results suggesting that ocean tide has significant effect on the groundwater level. The freshwater lens was confirmed at the western side, and at the northeast side. The thickness of those lenses were 10～13m at the central low land, 5～8m at the western side, and 3～6m at the northeast side. The freshwater lens of western side showed the seasonal change due to the difference of aquifer permeability and distribution of the doline ponds or doline.