Literature review and meta-analysis of gastric and intestinal bioaccessibility for nine inorganic elements in soils and soil-like media for use in human health risk assessment.

When soils become contaminated, a human health risk assessment (HHRA) is beneficial for determining whether identified contaminants of potential concern (COPCs) pose a risk to human health. In certain jurisdictions, when calculating doses from soil ingestion during the exposure assessment step of a HHRA, validated bioaccessibility methods may be used to accurately represent a contaminant's relative absorption factor (RAF). Where validated methods do not exist, risk assessors must either a) conduct their own literature search (which can be time consuming), b) create their own validation study (also a time consuming process), or c) use default assumptions (i.e., a RAF of 1). This literature search and meta-analysis characterizes the past 28 years of published analysis for aluminum (Al), antimony (Sb), arsenic (As), cadmium (Cd), chromium (Cr), iron (Fe), lead (Pb), manganese (Mn), and mercury (Hg) from 13 different gastric and intestinal bioaccessibility methodologies. These elements were selected as they are often risk drivers in HHRAs and are sometimes identified as COPCs in HHRAs at contaminated sites. Results were compiled from soil deriving from a variety of sources and varying in physicochemical properties. The mean percent bioaccessibility and 95 percent upper confidence limit on the mean (95 UCL) were calculated to better inform further study and validation of methods for elements additional to Pb, As, and Cd, which may be applicable to future HHRAs. Although there has been plenty of study into effects of various soil and method parameters on elemental bioaccessibility, meta-analysis found limited evidence that these findings are generalizable. Meta-analysis was also limited by little variability in certain parameters, but a more targeted analysis identified some correlative influences of pH and particle size for well-studied elements within the dataset. High bioaccessibility variability indicates that moving away from default parameters to use percent bioaccessibility to represent contaminant RAF must still be completed on a site-specific basis. The study has identified that more bioaccessibility studies for Al, Cr, Sb, Fe, Mn, and Hg, particularly studies that seek to validate bioaccessibility testing of these elements, will be beneficial for more robust and informative future risk assessments.

Source apportionment of bioaccessible lead in soil reference materials using the continuous on-line leaching method and inductively coupled plasma mass spectrometry.

Bioaccessibility (the amount of a contaminant extracted by the gastrointestinal fluids during digestion) is often incorporated in the assessment of contaminated soils and foods. Current methods, including those published by the United States Environmental Protection Agency (US EPA) and United States Pharmacopoeia (USP), use a batch method of analysis which requires hours of extraction prior to instrumental analysis with inductively coupled plasma mass spectrometry (ICPMS). The continuous on-line leaching method (COLM) uses a more direct method of analysis as extracts are sent directly to the ICPMS instrument, which can reduce extraction time and give real-time elution kinetics. For this study, four reference soils (NIST 2710, NIST 2710a, NIST 2711a, and BGS 102) that are typically used with bioaccessibility methods were extracted using the COLM and US EPA and USP gastrointestinal fluids. With the transient time-resolved data from the COLM, differential elution indicating multiple Pb sources was observed in NIST 2710a, NIST 2711a, and BGS 102. Two methods for calculating the Pb isotope ratios to identify these sources included a point-by-point ratio average method and a more precise regression slope method. There was no statistically significant difference between the ratios obtained by these methods of calculation. Furthermore, NIST 2710a and NIST 2711a did not have any statistically significant difference between the Pb isotope ratios of two observed sources. BGS 102 had a significantly different secondary source of Pb, which was identified to be from Pb historically added to gasoline. Investigation into the regions these soil reference materials were sourced from supported this finding as BGS 102 comes from a more densely populated, industrialized area where soil contamination with Pb from gasoline is likely to be more prevalent. This type of bioaccessibility investigation is only possible with the COLM as it gives real time elution information. Incorporation of the COLM into future bioaccessibility studies (and inclusion of other elements for isotopic analysis, like Sr) will lead to more thorough and comprehensive bioaccessibility studies in the future.