Distribution of anthropogenic lead estimated by Pb isotopic composition in the upper layers of soil from a mixed forest at Dinghushan, southern China

The heavy metal lead (Pb) is toxic to living organisms. Forest soils are important sinks for heavy metals generated by human activities. The forest at Dinghushan of southern China has experienced long-term exposure to atmospheric pollutants from the Pearl River Delta (PRD). The objectives of this research were (a) to determine the vertical and temporal distribution of Pb in the forest soil at Dinghushan, (b) to determine whether dilute acid extraction could be used to identify anthropogenic sources of Pb in forest soil, and (c) to determine the main anthropogenic contributors to soil Pb. Lead concentrations and isotopes were measured in two sets of forest soil samples. One set consisted of archived samples from 0 to 20 cm depth collected annually from 1997 to 2010. The other set was collected throughout three profiles sampled at 5-cm intervals to the bedrock (85 cm depth) in 2011. The soil samples were air-dried, ground, and passed through a 100-mesh polyethylene sieve. Lead in the samples was digested with concentrated acid (HNO3 + HClO4, 4:1 v/v) or extracted with dilute acid (1 M HCl with a soil/solution ratio of 1:10) and was measured with an inductively coupled plasma mass spectrometer. Concentrations of Pb obtained both by total digestion and dilute acid extraction decreased with soil depth in the profile samples and increased over time in the archived ones. Soils at 0-20 cm depth had Pb concentrations of more than twice of the local soil background value. In all soil samples, the Pb-206/207 ratios was lower and the Pb-206/204, Pb-207/204, and Pb-208/204 ratios were higher with the dilute acid extraction than with the strong-acid digestion, indicating that dilute acid extraction could be used to distinguish between anthropogenic and geogenic Pb. Comparison of the Pb isotope ratios in the samples with those in the main pollutants from the PRD indicated that coal combustion and industrial emission were the main contributors to the forest soil Pb at Dinghushan. The forest soil (0-20 cm depth) at Dinghushan was contaminated by Pb. Dilute acid extraction could be used to identify anthropogenic Pb sources. From 1997 to 2010, the main contributors of anthropogenic Pb to the forest soil at Dinghushan were coal combustion and industrial emission. Measures that control Pb emission from coal combustion and industrial activity, changes in coal consumption, and re-adjustments of industry development in the PRD should reduce Pb contamination of forest soil.