Analysis of water movement through an unsaturated soil zone in Jeju Island, Korea using stable oxygen and hydrogen isotopes

SummaryIn order to analyze water movement through an unsaturated soil zone in Jeju Island, Korea, temporal variations in the oxygen-18 and deuterium isotopes of precipitation and soil waters at three different depths were monitored for about one year. The stable oxygen and hydrogen isotopic compositions of the soil waters are plotted between or near the two local meteoric water lines for summer and winter precipitation indicating that the soil waters were recharged from the year-around precipitation, and evaporation was negligible even during hot summer season in the study area. Mean residence times of the soil waters were then estimated from the δ18O, δD, and deuterium excess or d-values of the precipitation and soil waters using two well-mixed or exponential models (EM) and one exponential piston-flow model (EPM). The regression results show that the combination of the exponential piston-flow model and the d-values of the precipitation and soil waters gives the most reasonable estimation of the mean residence times of the soil waters in the study area. It suggests that the deuterium excess or d-values can be used as a better parameter than the δ18O or δD values alone for estimating mean residence times of subsurface waters not only in Korea but also in other Northeast Asian countries. The estimated low moisture contents and hydraulic diffusivities of the soils at the three different depths also indicate that the boundary between the upper fine-grained soil and the lower coarse-grained soil may act as a capillary barrier, and the soil waters probably flow slowly through micropores rather than rapidly through macropores in the unsaturated soil zone.