Radon-222 (²²²Rn) concentration in surface water is a useful indicator for groundwater seepage. However, the natural concentration of ²²²Rn in surface water is too low to measure. It is therefore necessary to concentrate ²²²Rn content in water samples before analysis. This is a difficult operation requiring specialized equipment. We developed a method of measurement of ²²²Rn concentration in surface water (a method to concentrate radon by air-circulation) and applied it to detect groundwater seepage in the Ban Phai subwatershed, northeast Thailand. We also measured electric conductivity to determine whether dissolved ions in water were brought by groundwater. In areas of high-elevation (>180 m above sea level), ²²²Rn concentration in surface water was low, indicating that the velocity of subsurface water flow was slow even in the rainy season. This result supported a flow velocity previously calculated from the permeability of surface soil and the hydraulic gradient. Our measurements of ²²²Rn concentrations revealed that groundwater can be obtained by digging to a depth of about 1 m in the river bed in some areas. In areas of low-elevation (<160 m above sea level), electric conductivity increased markedly in the dry season. This was attributed to evaporation, rather than the effect of groundwater seepage, because the ²²²Rn concentration was low, suggesting little influence of groundwater.