DEPOSITION OF PARTICULATE MATTER AS A MECHANISM FOR TRACE METAL CONTAMINATION OF URBAN GARDENS

Yard soil in the Boston communities of Roxbury and Dorchester, like other inner city settings, is often significantly contaminated by Pb, with 85% of tested soils (n=270) containing concentrations higher than the EPA limit of 400 µg/g (average [Pb] = 974 µg/g). Non-profit organization The Food Project has worked with residents interested in urban gardening to install raised beds filled with compost in order to reduce exposure. Deposition of fine particles containing high Pb concentrations, however, can result in recontamination. Previous work estimates that while consumption of well-rinsed produce accounts for only 3% of children’s daily Pb exposure, ingestion or inhalation of fine-grained soil accounts for 82%. This work investigates the deposition mechanism in order to determine if simple modifications in raised bed design can limit recontamination and exposure. By assessing trends in grain size and metal concentrations, ambient air quality, and the relative bioavailability of soil types (yard, raised bed, and compost) a more complete understanding of the risks posed by recontamination via particle deposition can be attained. While beds are spatially heterogeneous, [Pb] in an excavated yard increased from an average of 300 µg/g following remediation to 690 µg/g over the course of four years. Samples obtained by scraping splashed and wind-borne dust from the sides of raised beds demonstrate that, along a gradient from ground level to the top of the raised bed, grain size decreases while Pb concentrations increase. Between 0-10 cm height, only 36% of the total sample mass fell into the three smallest grain size fractions (<149 µm), while between 15-23 cm height, 52% of the total sample mass fell into the three smallest fractions. Bulk [Pb] towards the top of the raised bed was likewise 10-15% higher than at ground level. Finally, the high organic matter content of compost may make recently remediated beds an efficient metal sink, while the lower pH (compared to yard soil solutions) may increase the bioavailability of metals deposited in such a matrix.