Supply rates of 224Ra and 212Pb from zircon and monazite grains to the aqueous phase are experimentally determined by dispersing these minerals in 1N nitric acid and PRL (Physical Research Laboratory) groundwater. In nitric acid medium, recoiled 224Ra and 212Pb behave conservatively and are not removed by solid surfaces. However, in groundwater, 212Pb is removed from the aqueous phase. A model relating the actual supply ratio of 212Pb/224Ra, the measured activities of 224Ra and 212Pb in the leachates at different times of contact and the first order removal rate constant of 212Pb is developed. At steady state, the supply ratio of 212Pb/224Ra in nitric acid from zircon and in groundwater from monazite is evaluated to be a unique value 2.3¿0.3 which agrees with the model predicted value of 2.67 based on alpha recoil supply of the nuclides and their precursors. The residence time of 212Pb in groundwater, with respect to its first order removal, is estimated to be of the order of 39 minutes. ¿ American Geophysical Union 1995