Lead dynamics in the forest floor and mineral soil in south-central Ontario

Recent studies have suggested that the residence time of Pb in the forest floor may not be as long as previously thought, and there is concern that the large pulse of atmospheric Pb deposited in the 1960s and early 1970s may move rapidly through mineral soils and eventually contaminate groundwater. In order to assess Pb mobility at a woodland (JMOEC) in south-central Ontario, a stable Pb isotope tracer ²⁰⁷Pb (8 mg m^–2) was added to the forest floor in white pine (Pinus strobus) and sugar maple (Acer saccharum) stands, respectively, and monitored over a 2-year period. Excess ²⁰⁷Pb was rapidly lost from the forest floor. Applying first-order rate coefficients (k) of 0.57 (maple) and 0.32 (pine) obtained from the tracer study, and estimates of Pb deposition in the region, current predicted Pb concentrations in the forest floor are 1.5–3.1 and 2.1–5.8 mg kg^–1 in the maple and pine plots, respectively. These values compare favorably with measured concentrations (corrected for mineral soil contamination) of 3.1–4.3 mg kg^–1 in the maple stand and 2.6–3.6 mg kg^–1 in the pine stand. The response time (1/^k) of Pb in the forest floor at the sugar maple and white pine plots was estimated to be 1.8 and 3.1 years, respectively. The rapid loss of Pb from the forest floor at the JMOEC is much greater than previously reported, and is probably due to the rapid rate of litter turnover that is characteristic of forests with mull-type forest floors. In a survey of 23 forested sites that border the Precambrian Shield in south-central Ontario, Pb concentrations in the forest floor increased exponentially with decreasing soil pH. Lead concentrations in the forest floor at the most acidic survey sites, which exhibited mor-type forest floors, were approximately 10 times higher (80 mg kg^–1) than at the JMOEC, and pollution Pb burdens were up to 25 times greater. Despite the rapid loss of Pb from the forest floor at the JMOEC, the highest pollution Pb concentrations were found in the upper (0–1 cm) mineral soil horizon. Lead concentrations in the upper 30 cm of mineral soil were strongly correlated with organic matter content, indicating that pollution Pb does not move as a pulse down the soil profile, but instead is linked with organic matter distribution, indicating groundwater contamination is unlikely.