A model is derived to predict the abundance of ²²²Rn in ground water in contact with a rock of known uranium content. The model assumes that secular equilibrium is attained in the rock-water system as a whole, but is independent of any microscopic geometric properties of the system. The key variables in the model are bulk properties such as porosity, uranium content of the rock, emanating efficiency, and rock density, all of which are measurable. Thus, the model is simplified by the averaging effects of a macroscopic view of the system. Although less rigorous than other models presented in...