Biogeochemistry of Lead in McDonalds Branch Watershed, New Jersey Pine Barrens

Abstract

AbstractLead concentrations, fluxes, and storage were measured in the vegetation, forest floor, and acid, sandy mineral soil (Quartzipsamments) of a forested watershed in the New Jersey Pine Barrens. Atmospherically deposited Pb at the McDonalds Branch Watershed was 140 g ha−1 yr−1 (8 µg L−1 in bulk precipitation) in 1980 to 1982, a substantial reduction from 350 g ha−1 yr−1 (17 µg L−1 in bulk precipitation) in 1978 to 1979. Virtually all Pb falling on the uplands was retained—75% by the organic forest floor and 25% by mineral soils, especially in B and C soil horizons. Total Pb content of the forest floor was 7.6 kg ha−1, with an accumulation rate of 100 g ha−1 yr−1 (1980–1982), or 1.3% of the forest floor pool. Estimated mean residence time of Pb in the forest floor was 220 yr. About 35 g Pb ha−1 yr−1 moved out of the forest floor in solution through the E horizon and was correlated strongly with dissolved organic matter. Approximately 30 g Pb ha−1 yr−1 accumulated in the B and C soil horizons. Less than 1 g Pb ha−1 yr−1 percolated past a depth of 2 m. Lowland muck soils and vegetation accumulated 98% of incoming Pb, with only 3.2 g Pb ha−1 yr−1 exported in the stream. Lead concentrations in the stream were correlated positively with dissolved organic matter and water level in the swamps. Lead in biota was contained mainly in the bark, fine roots, and foliage. Concentrations were: fine roots (18 mg kg−1) > bark (15 mg kg−1) > foliage (4 mg kg−1) > wood (0.5 mg kg−1). Although Pb concentrations in herbs, mosses, and lichens ranged from 10 to 60 mg kg−1, these plants accounted for very little biomass. Total Pb content of biota was 335 g ha−1, about 4% of the forest floor Pb content.