Mean transit time estimation for spring and river waters around Lake Ezu

–A case study using seasonal variation of deuterium excess–

Abstract

We investigated the seasonal variation in the d-values of river water, lake water, and spring water at 11 sites around Lake Ezu, located in the southeastern part of Kumamoto City. We analyzed these seasonal variations using the modified sine-wave method, and we estimated the mean transit time （MTT） of water at each site. As a result, the MTT of river water flowing into Lake Ezu was 2,645 days, the longest among all sites. Compared to the MTT of river water at the point of inflow into Lake Ezu, the MTTs of spring water, groundwater, and river water flowing into the three zones—Upper Lake Ezu, Middle Lake Ezu, and Lower Lake Ezu—were all shorter. Thus, the MTT of lake water flowing out of Lake Ezu was 1,840 days, shorter than that of the inflowing river water.