Urban Orchards : Soil Lead Levels

Soil Lead Levels – Interpretations and Recommendations

Soil contamination by lead – Lead is naturally present in all soils, generally in the range of 15 to 40 parts lead per million parts of soil (ppm) or milligrams per kilogram (mg/kg). Pollution, however, can increase soil lead levels to several thousand mg/kg. The major cause of soil contamination by lead in populated areas is weathering, chipping, scraping, sanding, and sand-blasting of structures bearing lead-based paint.

In the past the uses of tetraethyl lead as an anti-knock ingredient in gasoline and lead arsenate as an insecticide in fruit orchards were important causes of soil contamination by lead. Automotive lead emissions have effectively ceased with the phasing out of leaded fuels. With the development of more effective pesticides and Integrated Pest Management (IPM), lead arsenate is no longer in use. Unfortunately lead persists in soil many hundreds of years, so the past use of these products continues to present problems in some areas.

Soil lead becomes a health risk when directly ingested or breathed as dust. Garden produce, which has accumulated lead in its tissue or has soil particles adhering to it, can also be a hazard if eaten. Lead poisoning is a particular concern for young children (under 6) because their play habits tend to maximize exposure and their bodies' rapidly developing systems are very sensitive to the effects of lead.

Soil lead levels, distribution, and sampling – The procedure used by the UMass Soil Testing lab to measure lead in soil is the same used to measure the various plant nutrients. The "extracting solution" removes a reproducible fraction of the total soil lead. This "extractable" lead is a measure of the reactive lead in the soil. By testing a large number of soils (>300) by both this routine extraction procedure and a more rigorous total soil digestion (to obtain an actual total lead result) a relationship between the two values has been established. Your test result also reports an estimated total lead level based on this relationship. This is a calculated value. Total lead levels higher than 1,000 ppm are legally hazardous. Contact your state's Department of Environmental Protection regarding removal of contaminated soil materials. Information derived from a variety of sources has resulted in classifying soil lead levels as follows:

Table 2.1 - Soil Lead Levels

Lead Level	Extracted Lead (ppm)	Estimated Total Lead (ppm)
Low	less than 43	less than 500*
Medium	43 to 126	500 to 1,000
High	126 to 480	1,000 to 3,000
Very High	greater than 480	greater than 3,000
*At present total soil levels less than 200 ppm have not been associated with elevated blood lead levels in young children. If estimaged soil lead levels are above 300 ppm however, young children and pregnant women should avoid soil contact.

Due to the nature of the contamination process, lead in soil may be very unevenly distributed. The lead in paint removed from a structure will usually be concentrated near the source, but levels may vary greatly over small distances (ex. one foot). Lead arsenate residues in old orchards closely reflect the locations of sprayed trees. Consider these facts carefully when sampling. If the purpose of testing is to establish the extent of play-area contamination, combine several, small, randomly taken samples from the surface 1-2 inches to create one sample for testing. If the concern is for lead uptake by garden vegetables, combine several vertical slices from the top 7 inches of soil (root zone) to create a sample.

Reprinted with permission from Soil and Plant Tissue Testing Laboratory, West Experiment Station, University of Massachusetts, Amherst, MA 01003.