We have measured the radon concentration profile along a 20 meter deep drilled well with etched track type radon monitors. The well has a diameter of 12. cm. It has water at 11.6 meter below the surface and exhales nearly 100% carbon dioxide of geological origin at rate of about 300 liters/hour. We made radon measurements both underwater and in the gaseous atmosphere. Radon transport models (such as microbubble, macrobubble, eddy diffusion, etc.) were tested to find the best fit to experimental radon profiles. The measuring site is at a geologically active area, and the effect of a small earthquake on the exhalation rate of CO2 from the well was also observed.