The modification of bioaccessibility methods for radionuclide measurement and human exposure/dose modeling for relevant (non-occupational) Savannah River Site exposure scenarios
P.J. Lioy, C. Schopfer, M.A. Gallo (EOHSI, UMDNJ - R.W. Johnson Medical School and Rutgers University)
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 (0.7%) and bioaccessibility (10.7%), however, were low compared to the arsenic bioavailability (37.8%) and bioaccessibility (65.9%). For this reason, the priority metal in a risk assessment would change if it were based on bioavailability rather than total metal.
The bioaccessibility methods were modified in order to measure radionuclides, which differ in levels of quantification as well as level of desired containment for safety purposes. The soil preparation procedures were adjusted to ensure multiple levels of containment and to utilize only disposable equipment. A soil dissolution procedure was developed to allow safe low-level radionuclide measurement. The modified methods were used to measure bioaccessibility in two SRS site soils, where 90Sr gastric bioaccessibility ranged from 29.7% to 92.6%. 137Cs gastric bioaccessibility was lower than 90Sr, which ranged from 5.6% to 31.1%. These methods allow for an estimation of bioavailability that is low cost, capable of in-situ measurement and quick to perform.
Concentrations of 137Cs and 90Sr in soil, drinking water and food are sampled annually from background areas on the Savannah River Site (SRS) in order to assess the impact of the SRS on outlying areas and the general public. These environmental concentrations were incorporated into an age-specific exposure/dose model of relevant exposure scenarios at SRS. The 90Sr and 137Cs exposure from soil ingestion decreased by two orders of magnitude from the approximate age of 2.5 to adulthood and an order of magnitude from 2.5 to 6 years. Food and drinking water exposures were also higher for both young and older children than adults. On a per weight basis, young children receive higher exposures and doses than adults from food, drinking water and soil.
Bioaccessibility has been shown to be more conservative than bioavailability measurements, and bioaccessibility has been shown to be more representative of bioavailability of toxic materials 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.