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Detecting lead hotspots in urban gardens requires different sampling strategies

gardenDETROIT — The local food movement is gaining traction in cities across America, with urban gardens contributing a healthy source of fresh produce for local citizens. Urban gardening is not without risks, however.

Many garden plots within cities were previously inhabited by residences or industrial buildings that disposed of toxic chemicals on site, creating potential health hazards from the use of lead in paint, gasoline and industrial activities.

To properly assess the risk of soil contamination in urban gardens, researchers from Wayne State University analyzed the lead content in soil across a local urban garden plot and evaluated the results of several sampling strategies, some of which failed to detect a lead “hotspot.”

The research team of Lauren Bugdalski, WSU student, Lawrence D. Lemke, Ph.D., associate professor of geology and student mentor, and Shawn P. McElmurry, Ph.D., assistant professor of civil and environmental engineering, summarized their findings in the article “Spatial Variation of Soil Lead in an Urban Community Garden: Implications for Risk-Based Sampling” in the journal Risk Analysis, published by the Society for Risk Analysis.

The research emerged from Bugdalski’s undergraduate work and was supported by a grant from Wayne State to expand undergraduate research opportunities.

Ingesting or inhaling even small amounts of lead can cause neurological disorders in children and adults. Not only is lead more easily absorbed into children’s growing bodies, they can have higher exposures because they are more likely to accidentally ingest contaminated soil when playing outside.

The current permissible contaminant level for lead in soil, set by the Environmental Protection Agency (EPA), is 400 parts per million (ppm) for residential soils where children are likely to play.

Current EPA guidance for sampling residential plots for lead recommends analyzing as few as two composite samples to ensure that the plot is safe for activities such as gardening or children’s play.

Given the varied nature of soils and the potential for multiple sources of lead contamination, the Wayne State researchers concluded that two samples might not be sufficient to detect contaminated hotspots.

Lemke said that soil sampling is important because “the highest risk of exposure generally comes from dust and soil sticking to unwashed vegetables, not from direct uptake into the vegetable plants themselves. The easiest way to reduce this risk is to thoroughly wash fresh vegetables, especially leafy plants, before eating them.”

The researchers sampled 80 sites across a 49-by-98-foot garden plot. The soil lead concentration measured at the sites was highly variable across the plot, and 2 percent of the samples exceeded the 400 ppm EPA threshold.

A prominent hotspot was detected in the northwest corner of the plot, where gardening or other activities would pose undue risks. Examination of historic city documents indicated that a paint shop had been located on the property, explaining the origin of the contaminated hotspot.

Based on the researchers’ statistical models, the collection of 20 samples in the 15-by-30-meter plot would be needed to be 95 percent confident that the hotspot would be detected.

A common practice is to average several soil samples in close proximity to create a “mixture” of the sampled site. However, the researchers cautioned that this practice could lead to failure to detect a hotspot entirely, as nearby low-concentration samples would dilute the average to permissible levels.

The researchers recommend that garden organizers interested in an untested plot check local libraries for “Sanborn Fire Insurance Maps,” which show local land uses often going back 100 years, to see if contaminants were potentially used on site.

To read the full study, visit


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