The Kabu-ido system was a customary institution for groundwater management in a ring-levee area which extends in the southern part of Noubi Plain in the Tokai area of Japan. It was created in 1810s and disappeared around 1910s. In Japanese, “Kabu” means “privilege to do a business” and “ido” means “well”; therefore, “Kabu-ido” can be interpreted as “the special right to dig wells,” or well regulation. At the beginning of Meiji era, there were about 80 ring levees in Noubi Plain. The Kabu-ido system was introduced at least in Fukuzuka, Takasu, Shimogasa, Noginomori and Sunomata ring levees. But whether the system existed in other ring levees remains to be investigated. This paper shows areas where the Kabu-ido system was introduced and discusses natural conditions which worked behind the system.

Atmospheric concentrations of sulfur hexafluoride (SF₆) have continued to rise since the middle of the 20th century and therefore, it is necessary to prevent current atmospheric contamination into groundwater samples for use in age dating of groundwater. On the basis of SF₆ concentrations of groundwater at the same points collected by different sampling methods, this paper focuses on the influence of sampling methods on groundwater dating. Results obtained by using different six methods for water sampling show that inter-method uncertainty for groundwater dating is up to 1.7 years. However, the SF₆ concentration of groundwater sampled in contact with the atmosphere is not necessarily so high, and the influence of elution of SF₆ from sampling tools is not identified. Furthermore, the calculation of SF₆ transfer from gas to liquid phase indicates that during sampling process using a well bailer, theoretical increase in SF₆ concentration of groundwater is less than 1 % corresponding to a residence time of 0.2 years. It is thus deduced that the uncertainty of groundwater dating is mainly due to the heterogeneity of pumped-up groundwater and there is a limited influence of contamination of SF₆ from the atmosphere associated with sampling methods on groundwater dating.