Terrain-modulated deposition of atmospheric lead in the soils of alpine forest, central China.

Alpine ecosystem has a potential to intercept the transport of atmospheric metals, while the regulation mechanisms with variations in altitude and slope direction remain unclear. In this study, the soil and moss samples on the northern and southern slopes of Shennongjia Mountain were collected with altitude to quantitatively identify the sources of lead (Pb) and to decipher the regulation mechanisms of altitude and slope on the Pb distribution. The results showed that the concentrations of Pb decreased evidently with soil depth, and in the O (organic soils) and A (surface mineral soils) horizons they increased with altitude. The Pb isotopes and moss biomonitoring revealed that Pb was mainly from atmospheric deposition, and the sources included fossil fuel combustion, ore mining and smelting. Based on a binary mixing model of Pb isotopes, the percentage of atmospheric Pb in the O and A horizons and mosses averaged 58.8%, 43.7% and 71.0%, respectively. Atmospheric wet deposition strikingly controlled the distribution of soil Pb along the altitude. Canopy filtering and leaching also impacted the accumulation of Pb in the forest floor. The significant difference in the atmospheric Pb accumulation in the soils between the two slopes was not observed as expected, since atmospheric dry deposition from northwestern China contributed to the Pb accumulation on the northern slope according to the Pb isotopic ratios and air mass trajectories. The results of this study indicate that altitude determines the distribution pattern of atmospheric Pb, while slope direction screens the source region of Pb in alpine ecosystems.

Barrier effects of remote high mountain on atmospheric metal transport in the eastern Tibetan Plateau.

Anthropogenic metals adsorbed on suspended fine particles can be deposited on remote and inaccessible high mountains by long-range atmospheric transport. In this study, we investigated the cadmium (Cd) and lead (Pb) in the soils, mosses and rainfall of three transects on the Gongga Mountain, eastern Tibetan Plateau, to understand the mountain interception effects on their atmospheric transport. The concentrations of Cd and Pb in the soils and mosses displayed a pattern of eastern transect>northern transect>western transect. The distribution of Cd and Pb on the eastern transect increased from 2000 to 2900m a.s.l. (above sea level), decreased toward the timberline, and increased again with altitude; on the northern transect, it generally decreased with altitude whereas a distribution trend was not clearly observed on the western transect. The Cd and Pb concentrations in the rainfall of the eastern transect generally decreased with altitude, and they were higher inside forests than outside forests and temporally higher in the winter than the summer. The Pb isotopic ratios coupled with moss bio-monitoring distinguished anthropogenic sources of Cd and Pb on the eastern and northern transects, whereas bedrock weathering was the main source of Cd and Pb on the western transect. We proposed a conceptual model to delineate the effects of terrain, local climate and vegetation on the transport of atmospheric metals. Our results highlighted the high mountains in the eastern Tibetan Plateau as an effective natural barrier limiting atmospheric metal transport.

Altitudinal patterns and controls of trace metal distribution in soils of a remote high mountain, Southwest China.

The aim of this study is to reveal the effects of regional human activity on trace metal accumulation in remote alpine ecosystems under long-distance atmospheric transport. Trace metals (Cd, Pb, and Zn) in soils of the Mt. Luoji, Southwest China, were investigated along a large altitudinal gradient [2200-3850 m above sea level (a.s.l.)] to elaborate the key factors controlling their distribution by Pb isotopic composition and statistical models. The concentrations of Cd, Pb, and Zn in the surface soils (O and A horizons) were relatively low at the altitudes of 3500-3700 m a.s.l. The enrichment factors of trace metals in the surface soils increased with altitude. After normalization for soil organic matter, the concentrations of Cd still increased with altitude, whereas those of Pb and Zn did not show a clear altitudinal trend. The effects of vegetation and cold trapping (CTE) (pollutant enrichment by decreasing temperature with increasing altitude) mainly determined the distribution of Cd and Pb in the O horizon, whereas CTE and bedrock weathering (BW) controlled that of Zn. In the A horizon, the distribution of Cd and Pb depended on the vegetation regulation, whereas that of Zn was mainly related to BW. Human activity, including ores mining and fossil fuels combustion, increased the trace metal deposition in the surface soils. The anthropogenic percentage of Cd, Pb, and Zn quantified 92.4, 67.8, and 42.9% in the O horizon, and 74.5, 33.9, and 24.9% in the A horizon, respectively. The anthropogenic metals deposited at the high altitudes of Mt. Luoji reflected the impact of long-range atmospheric transport on this remote alpine ecosystem from southern and southwestern regions.

Spatiotemporal variation and exposure risk to human health of potential toxic elements in suburban vegetable soils of a megacity, SW China, 2012-2016.

Trace element contamination in soils of vegetable fields can threat public health. Seven potential toxic elements (As, Cd, Cr, Cu, Ni, Pb, and Zn) in suburban vegetable soils of Chengdu city, Southwest China, in 2012 and 2016, were analyzed to identify their sources with the spatiotemporal variation and assess their contamination and health risk for residents. The results showed that the concentrations of soil elements did not increase significantly in 2016 compared with that in 2012, whereas their spatial distributions altered markedly. The hot spots of soil As, Cd, and Pb as well as Cu and Zn in 2016 revealed the anthropogenic sources including agricultural activities, industrial emissions, road dust with heavy traffic, and open burning of solid waste. The apparent spatial difference of anthropogenic elements was related to the layout of land use surrounding the vegetable field. The contamination of soil elements decreased in the order of Cd > As ≈ Zn > Cu ≈ Pb > Cr ≈ Ni in 2012 and Cd > Zn > As ≈ Cu ≈ Pb > Cr ≈ Ni in 2016, and the vegetable soils were slightly to moderately contaminated by these elements through integrated contamination index. The sites affected by the trace elements did not increase in 2016 than in 2012, whereas the sites with relatively high contamination increased markedly. The non-carcinogenic risk of trace elements was generally acceptable, and children showed higher health risk than adults. The As carcinogenic risk for children varied between 5.48 × 10-5 and 1.59 × 10-4 in 2012 and between 4.40 × 10-5 and 1.82 × 10-4 in 2016, and the sites above acceptable levels (> 10-4) reached 60.6% and 48.5% in 2012 and 2016, respectively. The health risk of As in the vegetable soils should be paid more attention due to its high toxicity.