[Environmental Background Values of Heavy Metals and Physicochemical Properties in Different Soils in Shenzhen].

Research on the characteristics of environmental background values of soil can provide a scientific basis for setting regional standards for soils. To determine the characteristics of environmental background values and main influencing factors of heavy metals (Cr, Cu, Zn, Ni, Pb, Co, V, Cd, and Hg) in soils in Shenzhen, 500 topsoil samples (0-20 cm) were collected by decision unit multi increment sampling (DUMS) from 500 soil background sites in the whole city, including 405 latosolic red soil, 77 red soil, and 18 yellow soil samples. The results show that the concentrations of heavy metals in red soil are relatively low in general, and those of Cr, Ni, Co, and V in latosolic red soil and Cu, Zn, Hg, Pb, and Cd in yellow soil are higher. Compared with the environmental background values of soil in the Seventh Five-year Plan of China in 1980s, the concentrations of Cr, Cu, Zn, Ni, Co, and V in latosolic red soil, red soil, and yellow soil are lower, the concentrations of Cd and Hg are similar, and the concentration of Pb is higher. The spatial distribution patterns of the nine heavy metals are significantly different. Pb, Zn, and Co present an obvious zonal distribution pattern, while Cr, Cu, Ni, V, Cd, and Hg present a point-like distribution pattern. Moreover, the correlation analysis between heavy metal content and physicochemical properties of different soils showed that the correlation between heavy metal content and physicochemical properties of latosolic red soil was the most significant. Stepwise regression analysis was used to elucidate the relationship between heavy metal content in latosolic red soil, and physicochemical properties were quantified. The physicochemical properties affecting heavy metal content in latosolic red soil were found to be mechanical composition, pH, organic matter, and cation exchange capacity.

[Calculation of the Soil Risk Control Value through a Hydrus-1D Model for Groundwater Protection].

A combined Hydrus-1D and groundwater dilution model was employed to determine the risk control value and risk control volume of soil in correspondence of an electroplating workshop located in the Pearl River Delta. In particular, we considered the risk control value and risk control volume of soil in relation to characteristic chromium (Ⅵ) and nickel pollutants, which can affect the quality of groundwater. Based on the absorption of pollutants by the soil in the vadose zone, we obtained soil risk control values of 41.6 mg·kg-1 and 619.1 mg·kg-1 for chromium (Ⅵ) and nickel, respectively. These values were 10 and 45 times greater than those obtained from theoretical calculations (based on the soil/water partition equation combined with the groundwater dilution model), while the soil risk control volumes of chromium (Ⅵ) and nickel were 1804 m3 and 44590 m3, respectively. The soil risk control values calculated through the soil/water partition equation combined with the groundwater dilution model tend to be excessively conservative for the contaminated sites of the Pearl River Delta (characterized by a shallow buried depth of the underground water level and a close hydraulic connection); hence, it is necessary to carry out further hydrogeological surveys and fully consider the migration process of pollutants in the vadose zone. We conclude that the combined Hydrus-1D and groundwater dilution model for groundwater protection can be used to determine the soil risk control value, potentially saving the costs involved in later remediation and risk control.