Atmospheric transport and deposition of trace metals in remote reservoirs of the South China coastal region

Abstract

In the past several decades, enormous economic development has taken place in the South China coastal region with intensive industrialization and urbanization. The increasing emissions of air pollutants from various anthropogenic sources have consequently resulted in substantial enrichment of heavy metals in the atmosphere and have deteriorated the air quality. The outflows of airborne heavy metals might have significant environmental implications for adjacent regions or countries. The present study aims to investigate the atmospheric transport and deposition of some major trace metals (Cu, Ni, Pb and Zn) in some remote areas of the South China coastal region, and to further elucidate the significance of potential outflows of air pollutants to the major receptors at the Western Pacific Rim. Sediment cores were collected at four remote reservoirs (i.e. Tingxi Reservoir, Xinfengjiang Reservoir, Luokeng Reservoir and Songtao Reservoir) along the South China coastline to examine the historical inputs of heavy metals into the aqueous environment. Surface soil samples were collected in the reservoir catchment area to study the elemental concentrations of baseline soils in the drainage basin, and to provide background information of regional geochemical conditions. Moss samples were also collected simultaneously to assess the significance of current atmospheric deposition of trace metals in the study area. The analytical results showed that the concentrations of heavy metals (e.g. Cu, Pb, and Zn) were elevated significantly in upper layers of sediments at Xinfengjiang Reservoir, reflecting the increasing inputs of regional air pollutants in recent years. The lead isotopic compositions of mosses imply that the atmosphere over the study area has been strongly influenced by the regional anthropogenic sources, which might contribute substantial amounts of anthropogenic heavy metals in the surface environment by atmospheric deposition. The intensive industrial activities processing lead-containing ores and coal combustion in the PRD region are probably the major contributing sources of anthropogenic lead at the reservoir. Moreover, the long persistence of past-released Pb, possibly from vehicle exhausts of leaded gasoline, may also contribute to the anthropogenic lead in the atmosphere. The increment of Pb concentrations in top sediments at Xinfengjiang Reservoir might thus be attributed to the increasing atmospheric deposition of anthropogenic pollutants released and transported from the PRD region in the past several years. However, there is no obvious contamination sign of sediments at Tingxi Reservoir, Luokeng Reservoir and Songtao Reservoir, as the concentrations of trace elements were closely associated with the lithogenic major elements Al and Fe. The variations of trace metal concentrations along the sediment cores at these three reservoirs might be attributed to changes of the natural environment. When analyzing the moss samples, the concentrations of heavy metals accumulated in moss samples were found to be rather low at Songtao Reservoir, probably suggesting that the atmospheric deposition fluxes of heavy metals at this reservoir are less significant. In addition, the lead isotopic compositions of mosses at this reservoir were similar to the values of natural backgrounds, possibly reflecting that the air quality is well preserved in this area. The good air quality might be attributed to its specific topographic characteristics of the reservoir, which may prevent the intrusion of polluted air masses coming from the neighboring developing countries (e.g. Pakistan, Thailand, Vietnam, Indonesia) in Southeast Asia. Although the sediments are not likely to be in contaminated level at Tingxi Reservoir and Luokeng Reservoir, the low 206Pb/207Pb ratios of mosses at these two reservoirs probably indicate that there are some anthropogenic inputs of Pb by atmospheric deposition from regional emission sources. The inappropriateness of using the reservoir sediments as geochemical archives of the environment changes might be attributed to the low transmission efficiency through the reservoir catchments due to their small catchment areas, which may subsequently affect the deposition and accumulation of airborne heavy metals in the sediments of the two reservoirs. Further studies need to be conducted to provide additional evidence.