Accurate evaluation of the human health risk from ingestion of arsenic in soil or soil-like media requires knowledge of the relative bioavailability (RBA) of arsenic in the soil or soil-like material. In general, studies to date have indicated that the RBA of arsenic in soil is lower than the U.S. Environmental Protection Agency (EPA) default value of 100%. Consequently, estimation of site-specific RBA values can often save substantial costs during site cleanup.
Although RBA can be measured using studies in animals, such studies are generally slow and costly. An alternative strategy is to perform measurements of arsenic solubility in the laboratory. Typically, a sample of soil or sediment is extracted using a fluid that has properties that resemble a gastrointestinal fluid, and the amount of arsenic solubilized from the sample into the fluid under a standard set of extraction conditions is measured. The fraction of arsenic that is solubilized is referred to as the in vitro bioaccessibility (IVBA). The IVBA is then used to predict the in vivo RBA of arsenic in that sample, usually through an empiric correlation model.
The objective of this project was to develop, optimize, and validate an IVBA-based method to accurately predict the RBA of arsenic in soil and soil-like materials.
The technology consists of two parts. In the first part, the IVBA of arsenic is measured. This is achieved by placing 1 g of test material in 100 mL of extraction fluid and extracting for 1 hour at 37°C with constant end-over-end mixing. A sample of the extraction fluid is removed and analyzed for arsenic. The IVBA value is calculated as the mass of arsenic solubilized in the fluid divided by the mass of arsenic contained in the sample extracted. In the second part, the RBA of arsenic is estimated from the IVBA value using an empirical mathematical model:
RBA = a + b IVBA
The parameters of the model (a and b) are derived by fitting the model to a “calibration” data set of test materials that have both a reliable in vivo RBA measurement and a reliable IVBA measurement.
Test materials used to establish the correlation between IVBA and RBA included 20 materials where RBA had been measured in juvenile swine and 17 samples where RBA had been measured in monkeys. Based on extensive and systematic investigation of a wide range of differing extraction conditions, it was found that no single method would yield high quality RBA predictions for the combined data set. However, each data set could be successfully modeled independently. For swine, the optimum extraction fluid is 0.4 M glycine at pH 1.5, and the best fit regression model is:
RBA(swine) = 19.7 + 0.622 IVBApH1.5(R2 = 0.723)
For monkey, the optimum extraction fluid is 0.4 M glycine plus 0.05 M phosphate at pH 7, and the best fit regression model is:
RBA(monkey) = 14.3 + 0.583 IVBApH7(R2 = 0.755)
The finding that the best-fit regression model occurs at pH 7 for monkey and pH 1.5 for swine suggests that there might be significant physiological differences between the animal species that result in this outcome. However, this study did not seek to investigate the reason why different extraction pH conditions yielded a better fit for swine and monkey, so no mechanistic explanation is available at this time.
The within- and between-laboratory precision of the IVBA method was evaluated by triplicate analysis of each of 12 soils for each of three extraction fluids by each of four laboratories. Within-laboratory precision was evaluated by examining the magnitude of the standard deviation for three replicate values for each of the 12 test materials. Within-laboratory precision was typically less than 3%, with an average of 0.8% for all four laboratories. Between-laboratory precision was evaluated by examining the between-laboratory variability in the mean IVBA values for each test soil for each extraction condition. Between-laboratory variation in mean values was generally less than 7%, with an overall average of 3%. These results demonstrate that the method is highly reproducible, both within and between laboratories.
The principal advantage of this IVBA-based method compared to measurement of RBA in vivo is that it is much less expensive and much more rapid. For example, a typical in vivo RBA study may cost up to $100,000 and require several months for assessment of two samples, while a typical IVBA study can perform 40-60 extractions in 1 day at a cost of about $100 per extraction. This has the additional advantage that multiple samples (20 or more) may be collected from a site to ensure a robust characterization of IVBA/RBA across the site. The principal advantages compared to other in vitro methods are that (1) the fluids and extraction conditions are simple; (2) the results have been calibrated against a larger data set than any other method; and (3) the method has been demonstrated to be reproducible both within and between laboratories.
There are no significant issues associated with implementation of this technology. Several commercial and EPA laboratories currently provide IVBA extraction analyses. The method has been developed in close coordination with EPA’s Bioavailability Subcommittee of the Technical Review Workgroup (TRW), and the method is expected to be acceptable to EPA for use in evaluating risks from arsenic at sites where soil, sediment, or other soil-like media contain elevated levels of arsenic.