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Title: Lead in urban soils - A real or perceived concern for urban agriculture

item BROWN, SALLY - University Of Washington
item Chaney, Rufus
item HETTIARACHCHI, GANGA - Kansas State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2015
Publication Date: 1/11/2016
Citation: Brown, S.L., Chaney, R.L., Hettiarachchi, G. 2016. Lead in urban soils - A real or perceived concern for urban agriculture. Journal of Environmental Quality. 45:26-36. doi:10.2134/JEQ2015.07.0376.

Interpretive Summary: Gardening and urban agriculture are of increasing importance due to many social and economic factors. At the same time, urban soils are commonly contaminated with lead and other heavy metals and xenobiotic compounds. This paper reviews information about benefits of urban gardening, and how to prevent risks from soil lead and other contaminants during gardening. Because of the deposition of automotive emission lead, of deteriorating exterior lead paint, and historic stack emissions, soils surrounding urban homes are often significantly lead contaminated. High soil lead can comprise risk to young children, both because children ingest soil and house dust by hand-to-mouth play, and because growing garden crops in contaminated soils may increase dietary metals. However, many claims about risks from lead in urban soils are incorrect because the chemical forms of lead in urban soils, especially garden soils, have low bioavailability due to reactions of lead with phosphate, organic matter, and iron oxides. For similar reasons, most plants accumulate little lead from soils. Two groups of garden crops require attention to minimize lead accumulation: low-growing leafy vegetables and herbs which are splashed by soil and small soil particles become embedded in the leaf cuticle (mulching can greatly reduce this process); and expanded hypocotyl root vegetables which accumulate lead in the xylem or core tissue which comprises the middle of these root crops but not phloem fed root crops such as potato. On the other hand, the bioavailability of lead in foods is much lower than that of lead in water. In water, 60-80% of dissolved lead is absorbed by humans, while mixing soluble lead with a small meal yields only 3-7% absorption, while growing the lead into the food lowers bioavailability even more because of the presence of calcium, phosphate, and phytate in garden foods. When garden soil lead concentrations exceed advisory levels, these leafy and root crops which may accumulate lead from contaminated soils may be grown in a raised bed garden with clean soil to prevent excessive lead accumulation in crops. The main focus of urban gardening regarding lead is to keep soil out of homes where young children might ingest the soil in house dust. The many health, nutritional, social, and environmental benefits of urban gardening are important and attention to this advice can protect families from contaminated urban soils.

Technical Abstract: Urban agriculture is growing in cities across the U.S. and it has the potential to provide multiple benefits including increased food security. Concerns about soil contamination in urban areas can be an impediment to urban agriculture. Lead is the most common contaminant in urban areas. A review of both direct (soil ingestion via outdoor and indoor exposure) and indirect (consumption of food grown in lead contaminated soils) exposure pathways are reviewed. It is highly unlikely that urban agriculture will increase incidences of elevated blood lead for children in urban areas. This is due to the high likelihood that agriculture will improve soils in urban areas resulting in reduced bioavailability of soil Pb and reduced fugitive dust. Plant uptake of Pb is also typically very low. The exception can be low growing leafy crops where soil contamination is more likely and expanded hypocotyl root vegetables (carrot, etc.). However, even with higher bioaccumulation factors, it is not clear that the Pb in root vegetables or any other crops will be absorbed after eating. Studies have shown limited absorption of Pb when ingested with food. Best management practices to assure minimal potential for exposure are also common practices in urban gardens. These include use of residuals based composts and soil amendments, and attention to keeping soil out of homes. This review suggests that benefits associated with urban agriculture far outweigh any risks posed by elevated soil Pb.