Abstract:

Mercury enrichments or anomalies in soils in urban and town areas are a general phenomenon in China, and investigation on source, transport cycle and fate of Hg in soil is essential for evaluating environmental risks. Soil Hg may be derived from natural and/or anthropogenic sources and may be deposited by wet or dry processes. Emitted Hg may be also derived from the reemission of Hg that was previously deposited to the soil from the atmosphere. It is important to understand the relative magnitude of emission, deposition, and reemission of Hg associated with ecosystem of cities with soil Hg anomalies because for these cities being the center districts of human activity, the aggravated Hg pollution would result in serious dangers to environment and human health. The major focus of this paper is to discuss soilair Hg exchange process, occurrence forms of mercury for high concentration Hg soils in Beijing, China. This information is important for developing ecogeochemical mass balances, assessing the effectiveness of regulatory controls of Hg emission from coal combustion.  Data collected from field and laboratory are presented. An extension measurement of mercury in soil covered 1044 km2 with a density of 1 sample/km2 were carried out within the Beijing area; 19 samples of dry and wet deposition, and 20 samples of airborne particles (PM10 and PM2.5) with a relatively uniform distribution using a grid design based on 7 km×7 km were collected. 131 atmospheric mercury samples and soil gas mercury samples in mercury enrichments area and 66 samples in mercury background area were sampled. Results indicate that the average content of Hg is 0.410±0.403 mg/kg in soil, 0.194±0.058 mg/kg in dry and wet precipitation, 3.13±2.35 ng/m3 in atmosphere, 559.65±1177.36 ng/m3 in soil gas, 0.59±0.26 ng/m3 and 0.67±0.49 ng/m3 in PM10 and in PM2.5, respectively. Hg emission from coal combustion has been significantly reduced to 1836 kg in 2006 compared with the emission of 8830 kg in 1999, because the Beijing government in 2000 issued a regulation on changing from coal combustion to natural gas combustion. Consequently, average Hg concentration in atmosphere has been obviously declined from 8.324.7 ng/m3 in 1998 to 3.13 ng/m3 in 2006, and Hg content in airborne particles has been diminished from 1.18 ng/m3 in 2003 to 0.59 ng/m3 in PM10 and 0.67 ng/m3 in PM2.5 in 2006, respectively. It means that the regulatory measures adopted by the Beijing government since 2000 to change the combustion mode from coal combustion to natural gas combustion has been successful in reducing Hg emission. Quantities of cinnabar minerals, metallic and glassy spherical particles characterized by high temperature melting were found in high concentration Hg in soil, which indicated that large ash and mercury emitted from coal combustion and metal smelting industry were deposited in soils in Beijing. Coupling spatial distribution patterns of S, Hg, pH with that of cinnabar minerals showed that the contents of S, Hg and pH in soil are key controlling factors for forming cinnabar minerals. The formation of soil gas Hg is not as well understood as the formation of cinnabar minerals in soil; whether the Hg released from soil was an original component of the soil (geogenic) or the reemission of Hg previously precipitated from the atmosphere is not known. But a good linear relationship for soil gas Hg versus atmospheric Hg was confirmed (n=131，R＝0.267，p<0.01), which indicated that the sources of soil gas Hg may be the more significant contributors of mercury to the atmosphere than previously realized. The area average emission rate of mercury calculated for soil in the Beijing area is 102.42 ng/(m2·h), and the total Hg flux emitted to atmosphere from soil is 936.70 kg in 2005, which were obtained based on the linear relationship between the total Hg in soil and mercury flux of emission; while 1387 kg Hg was precipitated by the way of dry and wet precipitating processes. A prediction on mercury content in soil, mercury flux of emission from soil and precipitated flux of dry and wet precipitation in Beijing in 2050 has been made, and a scenario of soil environmental quality of mercury in Beijing in 2050 was outlined, which are based on the major objective of reducing 10% of total quantities of pollutant (including Hg) of the previous year, as indicated in Chinas 11th 5year plan.

Key words:

 Hg anomalies in soil in urban; occurrence; exchange flux; Beijing