Soil lead, zinc, and copper in two urban forests as influenced by highway proximity

Heavy metals emitted by vehicles have the potential to accumulate in soil near roadways, threatening the health of soil, plants, animals, and humans. This study evaluates Pb, Zn, and Cu levels in forest O-horizons, mineral soil, and earthworms near busy roadways in the metro-Washington, DC area. The study sites comprised road-edge environments within urban parks. Six transects were sampled in each park, collecting mineral soil at 1- to 30-m distances from the road edge and dividing it into eight depth increments (0-30 cm). O-horizon plant litter and earthworm samples were also collected at these locations. All samples underwent total Pb, Zn, and Cu X-ray fluorescence analysis. Generally, Pb concentrations (in upper 0-10 cm) were 1-4.8 times higher at 3 m compared to 30 m from the road, with less consistent gradients for Zn and Cu. Concentrations peaked near the soil surface, with lower levels in the O-horizon above and deeper soil layers. Leaded vehicle fuel was phased out by the early 1980s, but legacy Pb contamination persisted in roadside forests, averaging 365 mg kg^-1 in the upper 10 cm within 3 m of the roadway (< EPA action level of 1200 mg kg^-1 for non-play areas). Zinc, often present in vehicle tires, accumulated in earthworms to 192-592 mg kg^-1, concentrations exceeding those in the soil, while Pb and Cu were less concentrated in earthworms than in either O-horizon or mineral soil. Factors such as plant uptake, erosion, wind, soil texture, and metal solubility influence how heavy metals redistribute and bioaccumulate in the O-horizon, mineral soil, and soil fauna.