Reference Gastrointestinal Absorption Fractions for Radionuclides Ingested in Soil

Oak Ridge National Laboratory is conducting a project for the Environmental Protection Agency (EPA) Office of Superfund Remediation and Technology Innovation involving derivation of cancer risk coefficients for ingestion of radionuclides in soil. EPA provides guidance on quantifying gastrointestinal (GI) absorption of a chemical in a given medium for use in risk evaluations including assessments for Superfund sites. Essentially, a medium-specific GI absorption fraction may be applied if and only if there is sound information to support that value. Otherwise, the applied GI absorption fraction should be the best estimate of fractional absorption of the chemical when ingested in highly soluble form. Based on our review of the literature on GI absorption of elements in soil, we have concluded that for many elements there is convincing evidence of substantially reduced absorption in ingested soil compared with ingestion in readily dissolved form. This report summarizes the reviewed information and proposes a comprehensive set of GI absorption fractions for ingestion of elements in soil. For most elements, the proposed GI absorption fractions are lower than the default absorption fractions recommended by the International Commission on Radiological Protection for radionuclides ingested in highly soluble form, even though considerable conservatism has been incorporated into the proposed soil-specific GI absorption fractions.