Environmental Isotope-based Investigation on Closed System and Hydrological Aspects of Coastal Lakes

Abstract

The coastal lakes, which are designated for inclusion of Rishiri-Rebun-Sarobetsu National Park, are located in the northern part of Hokkaido Pref, Japan and nurse rich ecosystems. This study is associated with environmental isotope-based investigation of hydrological aspects of the coastal lakes. The zero concentration of radioisotope ²²²Rn observed in the lake water means that groundwater discharge might not occur in the lakes. It is thus deduced that the coastal lakes with no direct surface water inflow and outflow are isolated closed lakes and the lake water is of rain water origin. The enrichment of stable isotopic composition of oxygen and deuterium in the lake water clearly implies the evaporation from lake water bodies. The ratio of evaporation loss from the lakes to rainwater input estimated by a static isotopic mass balance model including a kinetic isotopic fractionation effect ranges from 14.4 to 71.9% with an average of 45.1%, and the difference of the evaporation loss in each lake can be explained by lake's morphology. The lakes have a significant influence on groundwater flow system as “recharge lake”, and thus precipitation and groundwater level in the lower region have a dominant impact on hydrological environment of the coastal lakes.