This paper shows the results of investigation of radon concentration and ionic composition (Na⁺, K⁺, Ca²⁺, Mg²⁺Cl-, SO42-, HCO3-, NO3-) of leakage water and reservoir water at the initial filling of Nishonai Dam in Aomori Prefecture. Nishonai Dam is a rockfill dam constructed in a tuff and hot spring area.
The results showed that the nitrate concentration of leakage water was very high, and the typical influence of filling of the reservoir was the decrease of nitrate concentration of leakage water caused by the intrusion of reservoir water. Therefore, the monitoring of nitrate concentration can be taken as an effective way to detect the arrival of the reservoir water at Nishonai Dam.
The variation of radon concentration showed that the influence of filling appeared on the bed rock under the core and filter zone more rapidly than the bed rock under the rock zone.
There are large differences on water quality between leakage water and reservoir water, which indicate that the flow velocity of leakage water from the reservoir is very slow.