Background in the context of land contaminated with naturally occurring radioactive material

The financial implications of choosing a particular threshold for clearance of radioactively contaminated land are substantial, particularly when one considers the volume of naturally occurring radioactive material (NORM) created each year by the production and combustion of fossil fuels and the exploitation of industrial minerals. Inevitably, a compromise needs to be reached between the level of environmental protection sought and the finite resources available for remediation. In the case of natural series radionuclides, any anthropogenic input is always superimposed on the inventory already present in the soil; this 'background' inventory is conventionally disregarded when assessing remediation targets. Unfortunately, the term is not well defined and the concept of 'background dose' is open to alternative interpretations. In this paper, we address the issue of natural background from a geochemical rather than from a solely radiological perspective, illustrating this with an example from the china clay industry. We propose a simple procedure for decision making based on activity concentrations of primordial radionuclides and their progeny. Subsequent calculations of dose need to take into account the mineralogical and chemical characteristics of the contamination, which in the case of NORM are invariably reflected in uranium series disequilibrium.