〈研究ノート〉筑波大学陸域環境研究センター構内における地下水と土壌水の酸素・水素安定同位体比の特徴について

摘要

It is necessary to make clear the formation process of the stable isotopic compositions of oxygen and hydrogen in soil water for consideration of the soil water movement. We have taken soil samples in the Terrestrial Environment Research Center (TERC) at eleven times from April 28 to November 15, 2001 and analyzed stable isotopic compositions of oxygen and hydrogen in soil water.There are cyclic variations of the δ18Ο and δD profiles in soil water. Isotopic compositions of soil water near soil surface become isotopically heavy because of the evaporation in particular summer season, thus the isotopic heavy peak is formed near the soil surface. In fall season, there is strongly rainfall by a number of typhoons and autumn rain fronts, so relatively large amount of precipitation are supplied to the soil. At that time, the cyclic variations of the isotopes in soil water move downward wholly, but when rainfall is relatively small amount, the cyclic variations do not move clearly. This result indicate that the recharge occurs only after intense rainfall events of typhoons or autumn rain front. And as a result of the stable isotopic compositions of soil water and groundwater, the piston flow is dominantly in the soil water movement in TERC. Consequently, cyclic variations of δ18Ο and δD in soil water are caused by the evaporation in summer season and transmitted downward by the storm events.