It is essential to analyze the interaction between surface water and groundwater in order to use water effectively and predict water quality. The conventional method of analysis, however, measures only the flow of a stream and can not determine groundwater seepage accurately. Since the concentration of Radon-222 (222Rn) in groundwater is much higher than in surface water, the use of 222Rn was examined as an indicator for the analysis of the interaction between surface water and groundwater. Measurement of the 222Rn concentration in surface water was conducted to detect groundwater seepage into a stream. Furthermore, the simultaneous movement of water both into and out of a stream from the underlying strata was quantified by solving the 222Rn and water balance equations.