Dynamic characteristics of heavy metal accumulation in the farmland soil over Xiaoqinling gold-mining region, Shaanxi, China

Determining dynamic characteristics and their controllers of heavy metal accumulation in soils has important consequences in the prevention and treatment of soil heavy metal pollution. In the present study, the soil samples were collected in the farmland over Xiaoqinling gold-mining region, Shaanxi, China, and analyzed their contents of heavy metals including Pb, Cd and Cu. Their contents were found to be increased on the whole during all the 3 decades studied, from 1985 to 2015. No matter what was on the studied region scale or Shaanxi province scale or China country scale, compared with their background values, these contents implied that these metals had accumulated since 1989 at least. All the accumulation rates depended heavily on the tailings from the gold-mining activities, so were kept stable as the tailings were produced continuously, until the gold mine got heavily exhausted and its mining activities were restricted since 2008, and then began to decrease in the whole region scale. The accumulation rates had their own site-specific characteristics over the studied region farmland because the rates were affected by the metal-bearing minerals, the geographical features and the local dominant wind, besides the tailings. The studied metal accumulation areas were enlarged with time and their rates were in order of Cd>Pb>Cu, which also was affected by the tailings, the metal-bearing minerals, the geographical features and the local dominant wind. The local dominant wind direction induced the metal accumulation extending toward the northwest in the studied farmland region. Therefore, weakening the influence of the wind is an effective way to prevent the heavy metal accumulation and the accumulation area enlarging in the studied farmland region.