Predicting the occurrence of 228Ra in ground water

A conceptual model is presented to predict the relative probability of the occurrence of elevated 228Ra in public drinking water supplies (those which serve more than 25 people or 15 connections) using ground water sources. The model is based on an aquifer classification scheme, which is developed from an understanding of the geochemical and radiological behavior of 228Ra and its parent, 232Th. Using this model, all aquifer types are classified as low, medium, or high probability of having elevated 228Ra. Summaries of the available data are discussed to show the actual concentrations found in each type of aquifer. As part of the initial application of this approach to develop a nationwide occurrence profile for 228Ra, all counties in the United States were classified and a map presented to show the distribution of the three classes. Nationwide, 71% of the counties were ranked as low, 18% were ranked as medium, and 11% were ranked as high.