Measurement of the Bioaccessibility of Arsenic in UK Soils

Mark R Cave, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG, Tel: +44 115 9363526 Fax: +44 115 9363261
Jo Wragg, Barbara Palumbo and Ben A Klinck, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG, Tel: +44 115 9363100, Fax: +44 115 9363261

The largest area of concern for human exposure to arsenic contaminated land is the oral/ingestion pathway followed by the dermal and respiratory exposure routes.  Whether the arsenic in these contaminated soils poses a human health risk depends on the potential of the arsenic to leave the soil and enter the bloodstream.  The recent UK Environment Agency guideline concentration for this element in domestic garden soil, 20 mg kg^-1, is significantly exceeded in many parts of England, either naturally or as the result of metalliferous mining or other industrial processes.  Whether these soils pose a human health risk depends on the potential of the arsenic to leave the soil and enter the bloodstream.  In this study, an in-vitro Physiologically Based Extraction Test (PBET), which mimics the conditions inside the human stomach, has been used to measure the bioaccessibility of arsenic in soils from the area around Gunnsilake in Devon, Cardiff in Wales and Wellingborough in Northamptonshire.  These results, in combination with, the total element compositions of the soils, a novel sequential extraction methodology called Chemometric Identification of Substrates and Metal Distributions (CISMeD), and EXAFS spectroscopy have been used to identify the speciation of the arsenic and the physico-chemical hosts of arsenic in the soil.  At all of the sampling sites, statistical analysis of the total element composition of the soils and the data from the sequential extracts has shown that iron-dominated soil components (probably iron oxides and oxyhydroxides) were the most important hosts of both bioaccessible and non-bioaccessible arsenic.  There is also evidence that the addition of phosphates or organic matter to soils can desorb non-bioaccessible arsenic bound to iron oxides and convert it into bioaccessible forms. 

Bioaccessibility: Human Physiologically Based in vitro Digestion Models Simulating Fasting and Fed Conditions

A. Oomen, C. Versantvoort, A. Sips, Centre for Substances and Integrated Risk Assessment; RIVM, National Institute of Public Health and the Environment. P.O.Box 1, NL-3720 BA Bilthoven, The Netherlands, Tel. +31 30 2742159, Fax: +31 30 2744451

It is known that the fasted or fed status can have a large impact on the oral bioavailability of compounds, as the presence of food markedly alters the conditions in the gastrointestinal tract. This food effect is often due to the dissolution (bioaccessibility) of compounds in the chyme and their transport across the intestinal epithelium. We present in vitro digestion models reflecting the conditions of the gastrointestinal tract for the fasted and fed state of man in order to study the bioaccessibility of compounds from soil. In order to mimic the intestinal absorption, we combined the digestive fluids with the Caco-2 transport model.

Our digestion model involves simulating human digestion for the fasted cq fed state in subsequently mouth, stomach and small intestine. Infant formula feedings were used as food and physiologically based conditions i.e. composition of media, pH and residence time periods typical for each compartment were applied. The effects of fasted vs fed conditions were studied on the bioaccessibility of metals (lead, cadmium, arsenic (metalloid)) and a lipophilic compound (benzo[a]pyrene).

The bioaccessibility of lead, cadmium, arsenic and benzo[a]pyrene from soil was on average 22%, 30%, 44% and 6%, respectively, for fasting conditions. The fed state had no (arsenic and cadmium) or little (lead < factor 2) effect on the bioaccessibility of the metals. In contrast, the bioaccessibility of benzo[a]pyrene was 3 to 8 fold increased under fed conditions. Combination of the digestive fluids with Caco-2 cells showed that the digestive fluids affected the transport characteristics of the Caco-2 monolayers without affecting cell integrity.

Thus, the digestion models simulating fasted and fed conditions can be used to model events occurring during digestion and solubilisation of compounds in the gastrointestinal tract. Combination with the Caco-2 monolayers may in some situations be helpful for more accurate predictions of absorption of compounds.

Arsenic Bioavailability: Consequences for Ecological and Human Health Risk Assessment in Yellowknife, Northwest Territories

Kenneth J. Reimer, Environmental Sciences Group, The Royal Military College of Canada,  Kingston, Ontario K7K 7B4, Canada, Tel: 613-541-6000 x6161, Fax: 613-541-6596, Email: Reimer-k@rmc.ca
Iris Koch, Environmental Sciences Group, The Royal Military College of Canada, Kingston, Ontario K7K 7B4, Canada, Tel: 613-541-6000 x6870, Fax: 613-541-6596
Christopher A. Ollson, Jacques Whitford Environment Limited, Suite 200 – 2781 Lancaster Road, Ottawa, Ontario, K1B 1A7, Tel: 613-738-0708  

Arsenic levels in Yellowknife, Northwest Territories, are elevated as a result of the natural geology of the area and as a consequence of historical gold-mining operations. Arsenic concentrations in background, residential and other areas not impacted by the mines are up to 150 ppm; approximately ten times the Canadian average. Soil on mine property can reach 8% arsenic by weight but typically contain 1000s of ppm arsenic. Public concern about the effects of arsenic contamination on both human and ecological health is a driving force behind the characterization of risk in Yellowknife. We have modeled bioavailability to obtain more realistic estimates of the relevant arsenic concentrations in soil.

Over one hundred soil samples, representative of arsenic contaminated areas, were studied to determine their potential risk to Yellowknife residents. Samples were subjected to an in vitro gastric fluid extraction (GFE), to simulate potential human bioavailability. These tests revealed that soil type, not total arsenic concentration, strongly influences potential arsenic exposures. Only 5.0 + 3.6 % and 2.9 + 1.7 % of the arsenic in the rock and tailings samples was extracted using GFE, while 20 + 11 % and 31 + 28 % was extracted from the mine site organic and residential organic samples. Significantly more arsenic was extracted from the organic soils than from the rock and tailings samples with respect to both percent and total arsenic extracted using GFE (ANOVA, p<0.001). The predominant form of arsenic in the extracts was arsenate.

To estimate ecological risk, a field-based study of a short terrestrial food chain, comprised of deer mice (Peromyscus maniculatus), plant and soil/tailings samples, was carried out on mine contaminated soils. The measured body burdens and robust population was consistent with exposures that incorporated the bioavailability data. These results, together with a human health risk assessment, have been used to propose realistic remediation targets for the mine properties.

Quantifying the Bioaccesibility of Cr and Cd in Soil

Melanie A. Stewart, Oak Ridge National Lab, PO Box 2008, Bldg 1505, MS 6038, Oak Ridge, TN  37831, Tel: 865-574-1902, Fax: 865-576-8646
P. Jardine, Oak Ridge National Lab, PO Box 2008, Bldg 1505, MS 6038, Oak Ridge, TN  37831 Tel: 865-574-8058, Fax: 865-576-8646
T. Mehlhorn, Oak Ridge National Lab, PO Box 2008, Bldg 1505, MS 6038, Oak Ridge, TN  37831, Tel: 865-574-7829, Fax: 865-576-8646
M. Barnett, Dept. of Civil Engineering, Auburn University, 208 Harbert Engineering Center, Auburn, AL 36849, Tel: 334-844-6291, Fax: 334-844-6290
S.E. Fendorf, Stanford University, Dept. of Geol. and Environ. Sci., Stanford,  CA  94305, Tel:  650-723-5238

Numerous Department of Defense (DoD) sites throughout the United States and elsewhere are contaminated with toxic metals such as Cr (III/VI) and Cd and are awaiting site cleanup.  Site risk assessments are based on the total metal content of the soil and generally do not take into consideration the metal sequestering properties of the soil and the soils potential ability to reduce metal bioaccessibility upon ingestion.  Soil ingestion by children is the usual risk-driver that motivates these remediation efforts.  The objective of this study was to identify the key soil physical and chemical properties that govern the extent of toxic metal bioaccessibility for Cr(III), Cr(VI), and Cd contaminated soil and to develop a statistical model to simulate the bioaccessibility of metal  contaminated soil.  Bioaccessibility was determined by use of a physiologically based extraction test (PBET) that simulates the digestion process of the stomach.  Cr  bioaccessibility varied widely as a function of soil type with most soils limiting bioaccessibility to < 30% for Cr(III) and < 50% for Cr(VI) after 100 d aging.  Statistical analysis of soil Cr(III) bioaccessibility was shown to be correlated to the clay content and the total inorganic carbon content of the soil while Cr(VI) bioaccessibility was controlled by the total organic content and the pH of the soil.  The bioaccessibility of Cr(VI)  was strongly linked to the reduction of Cr(VI) to Cr(III) by soil organic carbon.  Soils with > ~ 0.5% organic carbon, 90 – 100% of all Cr(VI) is reduced to Cr(III).  The bioaccessibilty of Cd was high in many soils and was not found to be significantly influenced by differences in soil properties.  The models for Cr(III) and Cr(VI) yielded equations based on commonly measured soil properties that could be used to predict the bioaccessibility of Cr contaminated soils with a reasonable level of confidence. 

A Study of the Geochemical Controls of Arsenic, Cadmium, Lead and Zinc Bioaccessibility from Mine Waste using a Respiratory Uptake Model

Joanna Wragg, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG, Tel: +44 115 9363328, Fax: +44 115 9363261
Mark R Cave, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG, Tel: +44 115 9363526 Fax: +44 115 9363261
Barbara Palumbo, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG , Tel: +44 115 9363100, Fax: +44 115 9363261
Ben A Klinck, British Geological Survey, Keyworth, Nottingham, UK NG12 5GG, Tel: +44 115 9363100 Fax: +44 115 9363261 

Abandoned metalliferous mines in Wales are significant sources of pollution.  Mine wastes contain high concentrations of lead ranging from 2-8% by weight often occurring as the lead carbonate mineral Cerrusite.  Airborne pollution from spoil heaps is thought to represent a significant pathway and risk to human health through inhalation by residents close to mine workings.  The mucous lining of tracheobronchial system is the main line of defense by trapping particles.  Particles can undergo solution in the bronchial tree and solutes can then pass into the vasculature and be transported to other body tissues by the blood.  A respiratory uptake test, originally developed for the nuclear industry, has been adapted to study the dissolution of potentially harmful elements (PHE) from eight samples from sites in Wales.  Samples were dried at <40°C, disaggregated, homogenised and sieved to three particle size fractions (<10mm, 10-40mm and 40-100mm).  Each particle size from each sample was subjected to a sequential extraction, using a simulated lung fluid, over 26 days at 37°C (ca. pH 7.3).  Over this period 12 extracts were taken for ICP-AES analysis.  The original samples and the residual samples after extraction were subjected to acid digestion and ICP-AES analysis to determine their total element concentrations.  Initial data interpretation shows that the PHE’s are released at different rates over the timescale of the experiment.  Further work will be presented to show how these dissolution profiles relate to the physico-chemical properties of the samples under investigation.

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