Présentations Colloques

Oral Presentation
Session 8.02: Groundwater Development and Protection in Coastal and Volcanic Environments with Complex Geological Structures
Isao Machida Isao
Contrast of groundwater ages around saltwater-freshwater mixing zone
This presentation reports the subsequent investigation on the last presentation “Carbon-14 age of groundwater in freshwater-saltwater mixing zone” in 42nd IAH Congress, AQUA 2015 in Roma. The aim of the study is to check the stability of saltwater-freshwater mixing zone below active groundwater flow in Quaternary unconsolidated fan deposit, which locates the coastal area on the foot of Mt. Fuji. **The unique feature of this study is to obtain the vertical profile of groundwater age from fresh water to saltwater water through freshwater-saltwater mixing zones at the distance of 50m from shoreline. We carried out a drilling up to 350 m in depth with taking groundwater samples at several depths. **The groundwater chemistry and resistivity log indicate that saltwater-freshwater mixing zone ranges from 125 to 150 m depth. Below the 150m depth, saltwater distributes. The ages of freshwater and saltwater have contrasting characters- freshwater age ranges from modern to several hundred years, while the saltwater has 10000 years. On the other hand, the Helium-4 age do not agree with the caron-14 ages for the water in saltwater-freshwater mixing zone. ****