2016 Volume 46 Issue 3 Pages 213-231
To determine the mean residence time of spring water in the Kirishima volcanic area, repeated samplings and measurements of CFCs were conducted during the years 2007 to 2013. The observed relationship between CFC-11 and CFC-12 concentrations (tracer plot) and the time series data of the CFC-12 concentration results suggest that most of the springs were recharged through the mixing of groundwater of various ages. Groundwater flow model, which is an essential to estimate the mean residence time, was assumed based on CFCs results and the geological setting of the study site. Through these assumptions, it was determined that the most suitable groundwater flow model to apply in the study area was a combination of exponential mixing flow and piston flow (EMM+PFM). The mean residence time was estimated using a lumped parameter model, taking the above flow model (EMM+PFM) into account. Consequently, the mean residence time was estimated to range from 1 to 58 years. In the areas around the tops of the mountains, most of the springs showed a relatively shorter residence time of less than 10 years. In contrast, relatively longer residence times of 10 to 40 years were distributed around the foot areas of the mountains. Springs located at the foot of the southern mountain of the Kirishima volcanic area showed the longest residence time (50 to 60 years) among all springs studied in the area, and for those with the longest residence time, their dominant groundwater flux type was piston flow. These springs are distributed at the end of the Takachihonomine lava, which is the youngest volcano in the Kirishima area. The longest residence time may have occurred due to a preferential groundwater flow system, which developed from piston flow in the bottom of the youngest volcanic lava.
