A technique has been developed using 222Rn as a flow path monitor. Its principle advantage over other radiotracers or dyes is that while the Kd of Rn gas in water solution is zero and hence follows the water path, its 210Pb daughter is retained strongly by rock. The immobilized 210Pb is not subject to diffusion, an important consideration in the proposed Nevada Field Test. To evaluate the technique, a series of laboratory scale experiments have been performed. Artificial fissures consisting of glass plates and prepared flat surfaces were coupled to insure well characterized fissures. Water solutions of radiotracer were metered through these fissures and the discharge monitored to produce breakthrough curves as a function of flow rate. The rock surfaces were analysed for radionuclide concentration and the rock sectioned for diffusion profile into the rock.