Urban gardens: lead exposure, recontamination mechanisms, and implications for remediation design.

Environmental lead contamination is prevalent in urban areas where soil represents a significant sink and pathway of exposure. This study characterizes the speciation of lead that is relevant to local recontamination and to human exposure in the backyard gardens of Roxbury and Dorchester, MA, USA. One hundred forty-one backyard gardens were tested by X-ray fluorescence, and 81% of gardens have lead levels above the US EPA action limit of 400 microg/g. Raised gardening beds are the in situ exposure reduction method used in the communities to promote urban gardening. Raised beds were tested for lead and the results showed that the lead concentration increased from an initial range of 150+/-40 microg/g to an average of 336 microg/g over 4 years. The percent distribution of lead in the fine grain soil (<100 microm) and the trace metal signature of the raised beds support the conclusion that the mechanism of recontamination is wind-transported particles. Scanning electron microscopy and sequential extraction were used to characterize the speciation of lead, and the trace metal signature of the fine grain soil in both gardens and raised gardening beds is characteristic of lead-based paint. This study demonstrates that raised beds are a limited exposure reduction method and require maintenance to achieve exposure reduction goals. An exposure model was developed based on a suite of parameters that combine relevant values from the literature with site-specific quantification of exposure pathways. This model suggests that consumption of homegrown produce accounts for only 3% of children's daily exposure of lead while ingestion of fine grained soil (<100 microm) accounts for 82% of the daily exposure. This study indicates that urban lead remediation on a yard-by-yard scale requires constant maintenance and that remediation may need to occur on a neighborhood-wide scale.