Chinas fast-growing economy has led to an increase in the releases of many heavy metals. These releases affect both the local environment through soil and water contamination, but also the regional and global environment as many metals can be transported thousands of kilometres away from their original source through the atmosphere. The Sino-Norwegian Cooperation Project on Mercury Pollution (SINOMER) focuses on mercury releases in the People’s Republic of China, with Guizhou province as case study area. Mercury is one of the long range transboundary air pollutants and raises global concern due to its toxicity and accumulation in aquatic food webs even in remote areas. Other heavy metals are also of concern due to their toxicity, and the aim of this study is to map the distribution of some of them in the SINOMER sampling areas.
The levels of the heavy metals silver, arsenic (a metalloid), cadmium, mercury, nickel, lead, vanadium and zinc were determined in paddy soils and water in Wanshan and Qingzhen. The two sampling areas were chosen on the basis of two different mercury hotspots; mercury mining and smelting in Wanshan, and a coal fired power plant and other industries in Qingzhen. In addition to the heavy metals, selenium was determined because of its possible involvement in reducing mercury’s biological availability.
An element screening of both soil and water was performed using (High Resolution) Inductively Coupled Plasma Mass Spectrometry after digestion of the soil samples using an autoclave. This decomposition method is considered to give the more available fraction of elements in soil compared to the total amount which includes elements contained in bedrock. Additionally, mercury concentrations of the soil samples were determined by a Direct Mercury Analyzer.
Results show that mercury levels in paddy soils in Wanshan are especially high, with concentrations reaching 119 µg g-1 and a median concentration of 25 µg g-1. All except one soil sample exceed Grade C in the Chinese National Standard for Soil Environmental Quality. Only a few of the water samples exceed global average mercury concentration for freshwaters of 0.07 µg L-1. The contamination levels of cadmium, zinc and silver in Wanshan soil are moderate to high, with median concentrations of 0.9, 86 and 0.6 µg g-1, respectively. Cadmium and zinc seem to be related to mercury mining as the soil concentrations correlate positively with mercury concentrations. Concentrations of cadmium, silver and zinc in water are below global average concentrations for freshwaters; with median concentrations 0.02 µg L-1, 0.07 µg L-1 and below detection limit, respectively. Arsenic, nickel, lead, selenium and vanadium are all within background soil concentrations for the area and below global average concentrations for freshwaters (nickel, lead and selenium are under detection limits in water).
In soils in Qingzhen, Hg levels are considerably lower than in Wanshan, although still considered high compared to background values, with concentrations reaching 1.7 µg g-1 and median concentration of 0.3 µg g-1. Elevated levels of silver, arsenic, selenium and vanadium in soil are also found; median concentrations reach 0.8 µg g-1, 38 µg g-1, 1.0 µg g-1 and 113 µg g-1, respectively. Significantly high concentrations of arsenic, cadmium and vanadium are found in seepage water from a large field containing bottom ash from the Qingzhen power plant. The vanadium median concentration (91 µg L-1) is 90 times higher than the global average for freshwaters. Somewhat high levels of Hg were also found in this seepage water; above the global average for freshwater (0.07 µg L-1). These high concentrations are probably caused by high levels of particles in the water, as the Hg levels in the ash itself are not particularly elevated. Lead, zinc and cadmium levels in soil are moderate. Nickel levels in soil are within background levels, while levels in water are below detection limits.
In conclusion, the mercury contaminated Wanshan area has somewhat elevated levels of heavy metals known to be associated with mercury (cadmium and zinc), while other heavy metals concentrations are low. In the Qingzhen area heavy metals concentrations are in general elevated, showing impacts of intense industrial activities. Especially heavy metals associated with coal combustion show considerable contamination levels.