Refining the risk assessment of metal-contaminated soils

The effects of soil intake on the growth performance, rumen microbial community and tissue mineral deposition of German Mutton Merino sheep

Soil ingestion by livestock is common in grazing ecosystems, but few studies have been conducted to assess its effect on the animal organism. The topic is worthy of attention because these potential effects are likely to be enriched in the food chain and interfere with animal and human health. In this study, we present an indoor feeding trial conducted based on a completely randomized design to comprehensively evaluate the effects of simulated soil ingestion during grazing on nutrient digestibility, rumen fermentation, and microflora, and mineral deposition in the organs and tissues of sheep. Eighteen Mutton Merino crossbred sheep (42.7 ± 2.34 kg) were randomly allotted to three treatments and fed diets containing 0% (Control), 5% (SOIL5), and 10% (SOIL10) for 62 d, including a 7-d metabolism trial. It was found that soil intake altered the rumen fermentation in sheep, as evidenced by a decrease in total volatile fatty acids (VFA) and acetate concentrations in rumen fluid of 50.6% and 51.3%, respectively (p < 0.01), with soil proportion in the diet increased from 0% to 10%. Soil ingestion also reduced the species richness of rumen bacteria, with the relative abundance of Bacteroidetes decreasing significantly (p < 0.01), while that of Firmicutes and Proteobacteria increased considerably (p < 0.05). In terms of mineral elements deposition, higher levels of iron (Fe) were detected in the spleen and liver, and a higher concentration of copper (Cu) and zinc (Zn) in the liver were found in sheep fed a diet containing 5% soil compared to the other two groups (p < 0.05). Moreover, the concentrations of lead (Pb) in the liver and kidney, and arsenic (As) in the heart were also clearly increased after ingestion of soil (p < 0.05). Our findings indicate that although soil intake had no significant effect on the growth performance of sheep, it altered ruminal fermentation and increased the risk of excessive Fe, Pb, and As in their organism. This study supplies a theoretical basis for risk assessment of soil ingestion in grazing livestock.

An investigation of the effect of gastrointestinal microbial activity on oral arsenic bioavailability

In vitro gastrointestinal (GI) microbial activity in the colon compartment facilitates the arsenic release from soils into simulated GI fluids. Consequentially, it is possible that in vitro models that neglect to include microbial activity underestimate arsenic bioaccessibility when calculating oral exposure. However, the toxicological relevance of increased arsenic release due to microbial activity is contingent upon the subsequent absorption of arsenic solubilized in the GI lumen. The objectives of this research are to: (1) assess whether microbes in the in vitro small intestine affect arsenic solubilization from soils, (2) determine whether differences in the GI microbial community result in differences in the oral bioavailability of soil-borne arsenic. In vitro GI microbial activity in the distal small intestine increased arsenic release from soils; however, these effects were unlikely to be relevant since they were transient and demonstrated small effect sizes. In vivo arsenic absorption for juvenile swine was unaffected by antibiotic treatment. Therefore, it appears that microbial effects on arsenic release do not result in increased arsenic bioavailability. However, it remains to be seen whether the results for the limited set of soils described herein can be extrapolated to arsenic contaminated sites in general.

French children's exposure to metals via ingestion of indoor dust, outdoor playground dust and soil: contamination data

In addition to dietary exposure, children are exposed to metals via ingestion of soils and indoor dust, contaminated by natural or anthropogenic outdoor and indoor sources. The objective of this nationwide study was to assess metal contamination of soils and dust which young French children are exposed to. A sample of 484 children (6 months to 6 years) was constituted in order to obtain representative results for young French children. In each home indoor settled dust was sampled by a wipe in up to five rooms. Outdoor playgrounds were sampled with a soil sample ring (n=315) or with a wipe in case of hard surfaces (n=53). As, Cd, Cr, Cu, Mn, Pb, Sb, Sr, and V were measured because of their potential health concern due to soil and dust ingestion. The samples were digested with hydrochloric acid, and afterwards aqua regia in order to determine both leachable and total metal concentrations and loadings by mass spectrometry with a quadrupole ICP-MS. In indoor settled dust most (total) loadings were below the Limit of Quantification (LOQ), except for Pb and Sr, whose median loadings were respectively 9 and 10 μg/m². The 95th percentile of loadings were 2 μg/m² for As, <0.8 for Cd, 18 for Cr, 49 for Cu, <64 for Mn, 63 for Pb, 2 for Sb, 56 for Sr, and <8 for V. Median/95th percentile of loadings in settled dust on outdoor playgrounds were 2/16, <0.8/1.3, 17/53, 49/330, 99/424, 32/393, 2/13, 86/661 and 10/37 μg/m² for As, Cd, Cr, Cu, Mn, Pb, Sb, Sr, and V respectively. In outdoor playground soil median/95th percentile of concentrations (μg/g) were 8/26, <0.65/1, 25/52, <26/53,391/956, 27/254, 0.7/4, 54/295, 23/57 for As, Cd, Cr, Cu, Mn, Pb, Sb, Sr, and V respectively. These results are comparable with those observed in other countries. Because of their representative nature, we can assess children's exposures to these metals via soil and dust and the associated risks in urban and rural environments. Ratios of leachable/total concentrations and loadings, calculated on >LOQ measurements, differed among metals. To a lesser extent, they were also affected by type of matrix, with (except for Cd) a greater leachability of dust (especially indoor) compared to soils.

Bioaccessibility of metal cations in soil is linearly related to its water exchange rate constant

Site-specific risk assessments often incorporate the concepts of bioaccessibility (i.e., contaminant fraction released into gastrointestinal fluids) or bioavailability (i.e., contaminant fraction absorbed into systemic circulation) into the calculation of ingestion exposure. We evaluated total and bioaccessible metal concentrations for 19 soil samples under simulated stomach and duodenal conditions using an in vitro gastrointestinal model. We demonstrated that the median bioaccessibility of 23 metals ranged between <1 and 41% under simulated stomach conditions and < 1 and 63% under simulated duodenal conditions. Notably, these large differences in metal bioaccessibility were independent of equilibrium solubility and stability constants. Instead, the relationship (stomach phase R = 0.927; duodenum phase R = 0.891) between bioaccessibility and water exchange rates of metal cations (k(H₂O)) indicated that desorption kinetics may influence if not control metal bioaccessibility.

Sequential digestion for measuring leachable and total lead in the same sample of dust or paint chips by ICP-MS

House lead exposure is generally assessed using total lead, except in France, where acid-leachable lead is used for routine regulatory purposes. In order to allow an international comparison of French lead dust contamination, a sequential digestion protocol is developed to determine both leachable and total lead on the same sample with a two-step digestion stage: firstly, hydrochloric acid is added to the sample at 37°C to solubilize leachable lead; then nitric acid is added to an aliquot at 95°C to solubilize residual (i.e., non-leachable) lead. Both sample fractions are analyzed with inductively coupled plasma mass spectrometry (ICP-MS). The sum of the two fractions allows to determine total lead. This new protocol has been tested with wiped dust (n = 111) and paint chip (n = 46) samples collected in houses (n = 16). The leachability of lead ranged from 63 to 100% in dust and from 4 to 100% in paint. These findings confirm the strong variability of lead leachability in houses samples and thus the importance of considering it for lead poisoning prevention. This double determination of leachable and total lead for each wiped dust or paint sample appears to be a reproducible, simple, low-cost protocol and thus a useful tool for international comparison of house dust lead contamination.

Nutritional status and gastrointestinal microbes affect arsenic bioaccessibility from soils and mine tailings in the simulator of the human intestinal microbial ecosystem

In vitro gastrointestinal models, used to measure the metal(loid) bioaccessibility for site specific risk assessment, are typically operated under fasted conditions. We evaluated the hypothesis that fed conditions increase arsenic bioaccessibility on three reference soils (NIST 2711, NIST 2709, and BGS 102) and the bulk and <38 mum size fractions of a mine tailing. The three nutritional states included a fed state with a carbohydrate mixture, a second fed state with homogenized crowberries (Empetrum nigrum), and a fasted state. The carbohydrate mixture increased arsenic bioaccessibility from four of five samples in the simulator of the human intestinal microbial ecosystem (SHIME) stomach but only three of five samples in the SHIME small intestine and colon. In contrast, crowberries increased arsenic bioaccessibility from four of five samples in the SHIME small intestine but had variable affects in the SHIME stomach and colon. The effect of nutritional status on arsenic bioaccessibility was potentially mediated via ligand-promoted dissolution in the SHIME stomach and small intestine. The displacement of arsenic with phosphate was potentially present in the SHIME small intestine but not the SHIME stomach. Microbial activity increased arsenic bioaccessibility relative to sterile conditions from four of five samples under fasted conditions and three of the five samples under fed conditions, which may suggest that in vitro gastrointestinal (GI) models operated under fed conditions and with microbes provide a more conservative estimate of in vitro bioaccessibility. However, for some samples, the arsenic bioaccessibility in the SHIME colon (with microbial activity) was equivalent to values observed in a separate physiologically based extraction test under small intestinal conditions (without microbial activity). These results suggest that the incorporation of microbial activity into in vitro GI models does not necessarily make estimates of arsenic bioaccessibility more protective than those generated using in vitro models that do not include microbial activity.

Bioaccessibility of lead in high carbonate soils

Metal bioaccessibility is not currently considered in the French human health risk assessment procedure. For contaminants such as Pb this parameter could have a significant effect on the risk calculation. From the literature, it seems that Pb bioaccessibility is strongly controlled by the occurrence of cerrussite in soils: with a high cerussite content showing a positive correlation with high Pb bioaccessibility. Assuming this hypothesis, Pb contained in carbonated soils could show strong human bioaccessibility. This study aims to examine the link between high carbonate soil minerals, focussing particularly on cerrussite and the Pb bioaccessibility. Four soils were sampled in a high carbonate area in France contaminated with Pb due to mining activities. Pb bioaccessibility was measured using the in vitro RIVM protocol. In parallel, the solid phase Pb distribution was determined using chemical sequential extraction for each sample and physical analysis on the more contaminated sample. Results showed that Pb bioaccessibility was significantly different among soil samples. As shown in literature, cerussite (PbCO3) was a highly bioaccessible phase and 40% of Pb in the waste material was present in this form. However, cerussite alone did not account for all of the Pb which was also present in highly stable minerals containing sulphur. Pb associated to these minerals is also likely to be significantly bioaccessible. Indeed, sequential extraction showed that most of the Pb was associated with the sulphur-compartment. This study shows that Pb bioaccessibility in high carbonate soils can be low (down to 20% of the total soil Pb content) and is not correlated with cerussite soil contents even if the concentration of this mineral is relatively high. Consequently, risk management of Pb contamination should not focus only on high carbonate soils but also on Pb contained in other minerals which is also likely to be significantly bioaccessible.

Determining speciation of Pb in phosphate-amended soils: method limitations

Determining the effectiveness of in situ immobilization for P-amended, Pb-contaminated soils has typically relied on non-spectroscopic methods. However in recent years, these methods have come under scrutiny due to technical and unforeseen error issues. In this study, we analyzed 18 soil samples via X-ray diffraction (XRD), selective sequential extraction (SSE), and a physiologically based extraction test (PBET). The data were compared against each other and to previous data collected for the soil samples employing X-ray absorption fine structure spectroscopy coupled with linear combination fitting (XAFS-LCF), which spectroscopically speciates and quantifies the major Pb species in the samples. It was observed that XRD was incapable of detecting pyromorphite, the hopeful endpoint of the immobilization strategy for reduced Pb bioavailability in our studies. Further, the SSE and PBET extraction methods demonstrated an increase of recalcitrant Pb forms in comparison to the XAFS-LCF results suggesting that SSE and PBET methods induced the precipitation of pyromorphite during the extraction procedures. The theme of this paper illustrates the experimental concerns of several commonly employed methods to investigate immobilization strategies of amended, metal-contaminated systems which may not be in true equilibrium. We conclude that appropriate application of spectroscopic methods provides more conclusive and accurate results in environmental systems (i.e., Pb, Zn, Cd, etc.) examining P-induced immobilization.

Blood lead changes during pregnancy and postpartum with calcium supplementation

Pregnancy and lactation are times of physiologic stress during which bone turnover is accelerated. Previous studies have demonstrated that there is increased mobilization of lead from the maternal skeleton at this time and that calcium supplementation may have a protective effect. Ten immigrants to Australia were provided with either calcium carbonate or a complex calcium supplement (approximately 1 g/day) during pregnancy and for 6 months postpartum. Two immigrant subjects who did not conceive acted as controls. Sampling involved monthly venous blood samples throughout pregnancy and every 2 months postpartum, and quarterly environmental samples and 6-day duplicate diets. The geometric mean blood lead at the time of first sampling was 2.4 microg/dL (range, 1.4-6.5). Increases in blood lead during the third trimester, corrected for hematocrit, compared with the minimum value observed, varied from 10 to 50%, with a geometric mean of 25%. The increases generally occurred at 6-8 months gestation, in contrast with that found for a previous cohort, characterized by very low calcium intakes, where the increases occurred at 3-6 months. Large increases in blood lead concentration were found during the postpartum period compared with those during pregnancy; blood lead concentrations increased by between 30 and 95% (geometric mean 65%; n = 8) from the minimum value observed during late pregnancy. From late pregnancy through postpartum, there were significant increases in the lead isotopic ratios from the minimum value observed during late pregnancy for 3 of 8 subjects (p < 0.01). The observed changes are considered to reflect increases in mobilization of lead from the skeleton despite calcium supplementation. The identical isotopic ratios in maternal and cord blood provide further confirmation of placental transfer of lead. The extra flux released from bone during late pregnancy and postpartum varies from 50 to 380 microg lead (geometric mean, 145 microg lead) compared with 330 microg lead in the previous cohort. For subjects replete in calcium, the delay in increase in blood lead and halving of the extra flux released from bone during late pregnancy and postpartum may provide less lead exposure to the developing fetus and newly born infant. Nevertheless, as shown in several other studies on calcium relationships with bone turnover, calcium supplementation appears to provide limited benefit for lead toxicity during lactation.

Methodologies to examine the importance of host factors in bioavailability of metals

Bioavailability provides a link between intrinsic toxicity and the ability to produce that toxic effect in an organism. Biomonitoring tools are essential to assess the health of ecosystems and their component parts, including humans. While field and laboratory data are available, two critical issues to our understanding of bioavailability are often missing: 1) knowing the relationship between dose and tissue concentrations, and 2) species extrapolations. Understanding of high to low dose extrapolation is also critical. Methods to understand the importance of host factors in bioavailability of metals must assess gender, age, nutritional status, individual variability, temporal changes, and critical habitat effects. Methods to examine these variables include correlational, observational, experimental, epidemiological, and modeling studies, or a combination of these. Data gaps include developing more representative studies of human and animal populations, better analytical tools for rapid determination of metal content in the field, improved analytical characterization of metal bioavailability, and concurrent studies of different metals.