Isotopic Characteristics of Hydrogen and Oxygen for Groundwater of Nansei Islands, Japan

Abstract

Isotopic compositions of groundwaters from 13 islands of the Nansei Islands were determined. Mean δD and δ18O values for each island are, respectively, −31 – −20‰ and −5.8 – −4.5‰, decreasing concomitantly with increasing latitude. The oxygen isotopic latitude effect on the groundwaters of the Nansei Islands is estimated as −0.14‰/degree latitude. The temperature effect for δ18O is estimated as 0.20‰/°C. The altitude effect of sampling locations is −0.1‰/100m for δ18O in Yakushima(elevation 1935m). The monthly δD and δ18O values of collected seepage waters from Omoto Tunnel in Ishigaki Island are almost constant for this observation period. Their isotopic composition indicates that precipitation is well mixed during infiltration into the groundwater. The d-parameters for summer precipitation of all islands are almost identical(about 10). However, in winter, the d-parameters(>25) for the northern islands(Tanegashima, Yakushima, and Nakanoshima) were higher than those(<22) for the southern islands.