Freshwater lens oscillation induced by sea tides and variable rainfall at the uplifted atoll island of Minami-Daito, Jap
- PDF / 2,964,811 Bytes
- 10 Pages / 595.276 x 790.866 pts Page_size
- 6 Downloads / 196 Views
REPORT
Freshwater lens oscillation induced by sea tides and variable rainfall at the uplifted atoll island of Minami-Daito, Japan Heejun Yang 1,2,3
&
Jun Shimada 1 & Tomo Shibata 3 & Azusa Okumura 1 & Daniele L. Pinti 4
Received: 15 January 2020 / Accepted: 12 May 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract In order to evaluate the behavior of a freshwater lens in the uplifted atoll island of Minami-Daito, Japan, groundwater level and electrical conductivity were simultaneously measured in six monitoring wells. The monitoring allowed determination of the position and variability of the top and bottom positions of the freshwater lens. The freshwater lens periodically oscillates with phase lags (delay time) every 3 h following sea tides. Recharge from local rainfall temporarily increases the volume of the freshwater lens but is disturbed by the low-permeability muddy sediments deposited on the central lowland of the island. Changes in the groundwater levels correlate well with rainfall, after first removing semi-diurnal, diurnal, and long-term components of the sea tides from the groundwater level data using a multiple regression analysis. Changes in the water electrical conductivity provide information on the temporal and spatial fluctuation of the freshwater lens. The monitoring scheme for this freshwater lens could be applied to other uplifted atolls, contributing to better evaluation of the potable groundwater resources and to making freshwater use sustainable on other islands. Keywords Freshwater lens . Island hydrology . Water sustainability . Japan
Introduction Oceanic tide provokes fluctuation of the pore pressure in freshwater and saltwater island aquifers (Kim et al. 2005, 2006; Levanon et al. 2017; Martin et al. 2012; Underwood et al. 1992). The fluctuation in pore pressure successively propagates inland, and the propagation yields exponential decrement of amplitudes and linearly phased lags (Dong et al. 2015). The oceanic tide induces oscillation of the groundwater level (Carr and Van Der Kamp 1969; Dong
* Heejun Yang [email protected] 1
Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
2
Soonsimine, 3 Sanbang-lo, Andeok, Seoqwipo, Jeju Island 63530, South Korea
3
Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 3088 Noguchibaru, Beppu, Oita 874-0903, Japan
4
GEOTOP Research Center for the Dynamics of the Earth System, Université du Québec à Montréal, Succ. Centre-Ville CP 8888, Montreal, QC H3C 3P8, Canada
et al. 2015) and the fresh–saline interface (Levanon et al. 2017). The depth to the fresh–saline interface from sea level is dependent on the aquifer permeability, thus the depth increases when permeability decreases (Schneider and Kruse 2003). Rainfall recharge is the main source of fresh groundwater in oceanic islands (Post et al. 2018). As the island hydrogeological system presents a delicate balance between the sea level and the rainfall recharge, recent c
Data Loading...
