RISK ASSESSMENT AND REMEDIATION OF SOIL AND AMENDMENT TRACE ELEMENTS
Title: Advances in Assessing Bioavailability of Metal(loid)s in Contaminated Soils
Submitted to: Advances in Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 31, 2009
Publication Date: April 16, 2010
Citation: Scheckel, K.G., Chaney, R.L., Basta, N.T., Ryan, J.A. 2010. Advances in Assessing Bioavailability of Metal(loid)s in Contaminated Soils. Advances in Agronomy. 17:409-439.
Interpretive Summary: Risks from elements in soil depend on the soil properties, and the nature of the consumer. Chapter considers plant uptake of elements, effects of elements on soil organisms, livestock and wildlife, and the potential effects of soil ingestion for contaminated soils, with focus on the role of bioavailability in risk assessment. Bioavailability and hence food-chain risks from soil elements are largely controlled by element chemistry in soils, and the nature of roots and plants; only a few elements in soil comprise risk thru food-chain transfer; soil Mo and Se are risk to grazing livestock in the case of alkaline soils; soil Cd is a risk to subsistence rice farmers for nearly all episodes of soil contamination. But soil ingestion can transfer soil into the stomach and intestine without the blockage of plant uptake processes. Direct ingestion of soil and dust by hand-to-mouth play and exploratory behavior of young children (or wildlife and livestock) all exposure to Pb, As, F, and some other elements which can cause adverse effects to soil consumers. The specific chemical form of the element present in the soil controls bioavailability. This knowledge can be used to conduct remediation of contaminated soils by changing the chemical species using soil amendments.
Research on risk from soil ingestion has progressed strongly in the last decade. Soil feeding studies have shown that Pb and As are only partially bioavailable to humans and appropriate test species such as pigs or monkeys. Further, soil treatments have been discovered which can strongly reduce the bioavailability of soil Pb and As. In the case of Pb, new solid forms of Pb such as chloropyromorphite are formed in phosphate amended soils; this form of Pb has very low bioavailability to primates. A field test at Joplin, MO, showed that even in highly Pb contaminated soils, bioavailable Pb was reduced by 69% using phosphate treatment. The next step is development of chemical extraction tests which correlate well with the measured bioavailability or bioaccessibility of soil elements. In the case of Pb, extraction at pH 2.5 using 0.4 M glycine-HCl yields extractable Pb highly correlated with bioavailable Pb, but extraction at pH 1.5 fails to measure the reduction in bioavailability due to phosphate treatment of soils. Soil As bioaccessibility has been measured by several extraction methods with good correlation to bioavailability to swine or monkeys. The increasing evidence that effective treatment in the field to reduce soil element bioavailability can be achieved supports moving toward adoption of such technologies. Additional evidence of element bioavailability reduction to humans or other primates remain needed to support use of chemical extractions well correlated with feeding results. Such use could greatly reduce the cost of dealing with the extensive areas of contaminated soils in the US.
The term bioavailability has many different meanings across various disciplines of toxicology and pharmacology. Often bioavailability is concerned with human health aspects such in the case of lead (Pb) ingestion by children. However, some of the most contaminated sites are found in non-public access facilities (Departments of Defense or Energy) or in remote regions as a result of mining or industrial practices in which ecoreceptors such as plant, animals and soil organisms are the primary concerns as well as the potential for food chain transfer. In all cases the endpoint requires movement of the element across a biological barrier. Still the utilized approach to base risk assessment on total metal content in soils is an outdated endeavor and has never been proven to be scientifically sound. Yet to reverse this trend, much work is required to establish base-line bioavailability measurements and to develop complementary methods that are capable of predicting bioavailability across a whole range of impacted media in a cost efficient manner. Thus, regulators have recognized site-specific human health risk assessments play a key role in decision-making processes at contaminated sites.