A pilot study to assess the feasibility of using naturally-occurring radionuclides as mass balance tracers to estimate soil ingestion

The relatively few soil ingestion studies that have been conducted to date to support soil ingestion rate values used for contaminated site human health risk assessments (HHRAs) typically have measured mass balance elemental tracers (e.g., Al, Si, Ba, Ce, Mn, Ti, V, Zr), found in soil to estimate soil ingestion. This pilot study, involving a canine subject fed a known amount of tracer on a daily basis, assessed the use of alternative mass balance tracers, specifically naturally occurring radionuclides of the 238U and 232Th decay series, to estimate soil ingestion. A novel method of estimating soil ingestion via difference in isotopic ratios between the two decay series in food and soil was also assessed. The results of the study showed that the mean 214Pb and 212Pb activities measured in fecal samples were greater than what was contained in the soil inoculant, suggesting that the tracers were not being significantly absorbed in the GI tract. The mean daily soil ingestion rates, calculated after subtracting the contribution of tracers in the soil inoculant, were 3.9 g d−1 (standard deviation 3.6 g d−1) for the isotope tracers, and 1.9 g−1 (standard deviation 2.1 g d−1) for the 3 most reliable elemental tracers. The differences were not statistically significant and further evaluation of isotopic tracers for soil ingestion studies is warranted. Similarly, soil ingestion estimates calculated using the Isotope Ratio Method were not significantly different than when calculated using 212Pb; however, the Isotope Ratio Method was observed to positively bias the soil ingestion estimates by approximately 50%.

► A mass balance soil ingestion pilot study was conducted in a canine subject.
► Soil ingestion was calculated using element and 238U and 232Th decay series isotopes as tracers.
► Mean soil ingestion rates ranged from 1.5 and 3.9 g d−1 using the mass balance approach.
► Soil ingestion estimates using element and isotope tracers were not significantly different.