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AMENDING SOILS WITH PHOSPHATE AS MEANS TO MITIGATE SOIL LEAD HAZARD: A CRITICAL REVIEW OF THE STATE OF THE SCIENCE
Citation:
Scheckel, K., G. Diamond, M. Burgess, J. Klotzbach, M. Maddaloni, B. Miller, C. Partridge, AND S. Serda. AMENDING SOILS WITH PHOSPHATE AS MEANS TO MITIGATE SOIL LEAD HAZARD: A CRITICAL REVIEW OF THE STATE OF THE SCIENCE. Sam Kacew (ed.), Submitted to: JOURNAL OF TOXICOLOGY AND ENVIRONMENTAL HEALTH - PART B: CRITICAL REVIEWS. Taylor & Francis, Inc., Philadelphia, PA, 16(6):337-380, (2013).
Impact/Purpose:
Lead (Pb) is a ubiquitous environmental contaminant and significant environmental health hazard for children (CDC 2012). CDC (2012) recently concluded that 2.5% of the U.S. children, approximately 450,000 of children between the ages of 1–5 years, have elevated blood Pb levels (>5 µg/dL) that may pose a risk for cognitive development and other adverse effects. CDC (2012) targeted the U.S. population 97.5 percentile blood Pb level as a trigger for Pb education, environmental investigations, and additional medical monitoring. Lead has been found in at least 1,272 of the 1,684 current or former National Priorities List hazardous waste sites (ATSDR 2007). Many of these sites possess elevated soil Pb levels resulting from specific industrial processes such as mining, milling, and/or smelting. Other sources of soil Pb contamination, prevalent in most urban environments, include historic use of leaded gas, Pb based paints, incineration of contaminated materials, and various industries that utilize Pb such as secondary smelters and battery cracking operations (Caravanos et al. 2006; USEPA 2006, 2012b). Amending agents that reduce soil Pb bioavailability alone or in combination with other actions (e.g., capping) that treat soil in situ, are attractive alternatives to soil excavation for mitigating health risks from exposure to soil contaminated with Pb. Application of amending agents has the potential to be less expensive than excavation and replacement with clean soil (USEPA 2007, 2012a). Sourcing of clean soil from another site may have negative impacts on that site’s ecology and sustainability. In situ remedies can also be less disruptive in terms of noise, emissions of dust and equipment exhausts, and disruption of the landscape (e.g., loss of bushes, shrubs, trees). Lower costs and less disruption can promote greater public acceptance of remediation activities in urban areas. The remediation strategy implemented in the South Prescott community of West Oakland, CA is an example of community acceptance of in situ remediation. In this case, soil was amended with ground fish bone (a phosphate substrate) to convert soil Pb to pyromorphite (a highly insoluble Pb-phosphate mineral) and then covered with a green cap such as sod, clean soil with mulch, raised garden beds, or gravel (South Prescott Community Forum 2012). Of the aforementioned agents, phosphate agents have been studied most extensively for their effects on oral bioavailability of Pb and are the focus of this review. Demonstration of efficacy is an absolute requirement for utilizing phosphate amendments as a means for lowering soil Pb bioavailability. Therefore, in addition to summarizing relevant literature, this review attempts to evaluate evidence for efficacy of phosphate amendments for decreasing soil Pb bioavailability in humans. In making this evaluation one needs to consider the physical and chemical interactions of Pb and phosphate that would be expected to influence bioavailability, effects of phosphate amendments on soil Pb bioaccessibility (i.e., predicted solubility of Pb in the GIT), and results of bioavailability bioassays of amended soils conducted in humans and animal models. Practical implementation issues such as what would constitute adequate evaluation of efficacy at a site, available methodology for evaluating efficacy, and potential effects of phosphate on mobility and bioavailability of co-contaminants in soil are also discussed. The final section summarizes the conclusions, identifies major uncertainties and data gaps, and offers suggestions for further research. Although this review is focused on phosphate amendments, it also serves as a general template for evaluation of efficacy and implementation issues for other amending agents that might be considered for reducing bioavailability of soil contaminants.
Description:
Ingested soil and surface dust may be important contributors to elevated blood lead (Pb) levels in children exposed to Pb contaminated environments. Mitigation strategies have typically focused on excavation and removal of the contaminated soil. However, this is not always feasible for addressing widely disseminated contamination in populated areas often encountered in urban environments. The rationale for amending soils with phosphate is that phosphate will promote formation of highly insoluble Pb species (e.g., pyromorphite minerals) in soil, which will remain insoluble after ingestion and, therefore, inaccessible to absorption mechanisms in the gastrointestinal tract (GIT). Amending soil with phosphate might potentially be used in combination with other methods that reduce contact with or migration of contaminated soils, such as covering the soil with a green cap such as sod, clean soil with mulch, raised garden beds, or gravel. These remediation strategies may be less expensive and far less disruptive than excavation and removal of soil. This review evaluates evidence for efficacy of phosphate amendments for decreasing soil Pb bioavailability. Evidence is reviewed for (1) physical and chemical interactions of Pb and phosphate that would be expected to influence bioavailability, (2) effects of phosphate amendments on soil Pb bioaccessibility (i.e., predicted solubility of Pb in the GIT), and (3) results of bioavailability bioassays of amended soils conducted in humans and animal models. Practical implementation issues, such as criteria and methods for evaluating efficacy, and potential effects of phosphate on mobility and bioavailability of co-contaminants in soil are also discussed.
