The bioaccessibility of selected radionuclides and heavy metals: An investigation of bioaccessibility, bioavailability and natural soil characteristics

Childhood soil ingestion can contribute greatly to inorganic contaminant exposure because many metals are enriched in the upper surface of soil, which is the main contact region. Risk assessments of contaminated soils are based on total metal measurements, however the strong binding of metals in soil can reduce the oral bioavailability, or the fraction of metal to reach systemic circulation. Since oral bioavailability is dissolution-limited, we can estimate bioavailability using human gastrointestinal dissolution models that measure bioaccessibility. Bioaccessibility measurements are more cost efficient than bioavailability studies and more physiologically relevant than strong acid leaching procedures.; In the present studies, bioaccessibility was measured and compared to bioavailability using a Standard Reference Soil from the National Institute of Standards and Technology. In comparison, the total lead concentration was much higher than total arsenic concentration, using an aggressive acid leaching method. The lead bioavailability and bioaccessibility were low compared to the arsenic. For this reason, the priority metal in a risk assessment would change if it were based on bioavailability rather than total metal. The addition of organic acids to the gastrointestinal fluids increased the intestinal bioaccessibility of chromium and lead, but did not considerably affect that of 90Strontium and 137Cesium. The bioaccessibility of Aluminum, Chromium and 137Cesium was significantly correlated to soil physico-chemical characteristics. These relationships can be used to prioritize remediation according to soil type. In the exposure/dose calculations, the young children (2.5 years) carried the highest burden of both dose and exposure than older children (6 years) and adults.; In the present studies of As and Pb, bioaccessibility has been shown to be more conservative than bioavailability measurements, and have shown bioaccessibility to be more predictive of bioavailability than typical regulatory based strong acid leaching techniques. The bioaccessibility methodology was successfully modified to allow measurements of low levels radioactive contaminants, which can be used safely to model human exposure and dose from soil ingestion pathways. In conclusion, bioaccessibility measurements can improve prioritization of contaminated sites as well as improve risk assessments by increasing their accuracy.