In the underground of Nobi plain, sedimentary layers exist more than 1,500 m tiick. In the lowest part of these layers, there is the ultra-deep underground water the temperature of which is about 50° C.
For the purpose of knowipg annual variation on the chemical compositions of these waters, nine wells were selected to investigate water chemically once a year from 1973 to 1977.
The chemical compositions of these waters are as follows, NaHCO3 type of water was found in four wells, NaCI type in other four wells and NaC1-NaHOO3 type in one well.
The annual variations observed on the chemical compositor's of these well waters were only less than 10% during five years for the former two types, while one well of the last type indicates rather large annual fluctuation. As for the last type well, Cl- contents of water changed widely, ranging from 101 to 488 mg/l, and it corresponds to the change of the water type from NaHCO3one to Nag one.
It is assumed that the underground water of NaHCO3 type (122 mg l-I) occurs in the aquifer of upper hodzon in sediments, while that of NaCI type (341 mg C1-/I) exists in the lower one.
The well in which large annual variation was observed, had the strainers at about 250 m length, and the water was seemed to be derived both upper and lower aquifers. It seems that the large annual variation is due to the variable contribution to the well water from two different aquifers.
JAPANESE ASSOCIATION OF GROUNDWATER HYDROLOGY