Proposal of groundwater dating method by using recharge temperature

The residence time of groundwater is one of the most important information to know migration of groundwater. 14C and 4He are powerful tools for dating groundwater with 10000 to 30000 years residence time. However, in some cases, 14C and 4He can not provide accurate residence time of groundwater because residence time estimated by 14C and 4He is affected by geochemical reaction or He flux. Thus another indicator have to be developed to estimate residence time such as 10000 to 30000 years, even in such cases. In this study, recharge temperature was used as a indicator of residence time of groundwater. Concentration of Krypton (Kr) and oxygen isotope ratio are used as indicators of temperature and depth profile of these indicators were investigated. Local maximum value of Kr concentration and minimal value of oxygen isotope were found at same depth indicating groundwater at this depth recharged in local lowest temperature. The residence time of groundwater at this depth can be estimated as 18000 years because this lowest temperature is considered to be related to LGM (the Last Glacial Maximum) that took place 18000 years ago. Thus extremal values of temperature indicator can indicate the groundwater that recharged 18000 years ago. Furthermore, the relationships between temperature indicator and 4He concentration were investigated. He accumulation rate in groundwater can be estimated by comparison between He concentration and local maximum value of Kr concentration. The residence time of groundwater can be by using 4He concentration and 4He accumulation rate calibrated by temperature indicator. Thus by using extremal values of temperature indicator or by using 4He concentration and 4He accumulation rate calibrated by temperature indicator, residence time of groundwater can be estimated even in the cases 14C and 4He can not provide accurate residence time. (author)