The impact of tree barriers on the dispersal of lead in the soil at highway roadsides.

Tree windbreaks are common features in high-traffic roadsides that mitigate air pollution, winds, and noise. The impact of tree barriers on the dispersal of lead in the soil was studied at highway roadsides located near Holon city, Israel. A discontinuous line of trees parallel to the highway separated the highway from agricultural fields. The line includes alternating trees and openings; thus, it was divided into two groups: one consists of the areas hidden by trees (windbreaks) and the other includes the open areas exposed to the road (gaps). At each of the windbreaks and gaps, soil samples were collected from the upper 0-5 cm depth at various distances from the road: 6, 11, 27, 40, 60 m. Lead concentration and soil properties associated with lead adsorption (organic matter, texture, calcium carbonate) were determined for all soil samples. At both the windbreak and gap areas, the averages and the medians of lead content declined nonlinearly with increasing distance from the highway. At 6 m distance, the average lead content in the windbreak areas was higher than that in the gaps (176 and 157 mg kg, respectively). At the area between 17 and 40 m distance, an opposite trend was seen. High significant positive correlations were found between lead and calcium carbonate and organic matter contents at both the windbreaks and the gaps. Yet, organic matter content less than 2% did not correlate with lead.