Abstract: With the rapid development of global industrialization, the pollution of hexavalent chromium (Cr(Ⅵ)) in soil and groundwater has become increasingly serious. Field investigation and laboratory tests were carried out for the soil polluted by the chromium slag of a ferroalloy plant. The adsorption, infiltration and dispersion experiments were conducted to study the adsorption characteristics and migration mechanism of Cr(Ⅵ) in silty clay. A three-dimensional kinetic mathematical model of Cr(Ⅵ) migration considering convection-dispersion-adsorption was established. The migration and distribution characteristics of Cr(Ⅵ) in groundwater with the pollution source located upstream or downstream of the contaminated site were obtained using the numerical approach. Meanwhile, the effects of dispersity (?) and distribution coefficient ( ) on the spatial and temporal distribution of Cr(Ⅵ) were revealed. The experimental results show that the Langmuir isotherm model well fits the adsorption data of silty clay. The maximum adsorption capacity of silty clay for Cr(Ⅵ) was 466.6 mg/kg. The hydraulic conductivity of silty clay under the infiltration of distilled water and 160 mg/L Cr(Ⅵ) solution was 6.5×10–7–6.7×10–7 cm/s, while it increased to 4.4×10–6 cm/s under infiltration of Cr(Ⅵ) solution with a concentration of 1 000 mg/L. The hydrodynamic dispersion coefficient (D) of silty clay was 1.4×10–4 m2/d. The value of the retardation factor ( ) was found to be 4.2–10. The results of the numerical simulation indicated that when the downstream was contaminated by Cr(Ⅵ), there was still a risk of pollution in the upstream even if molecular diffusion was not considered. The degree of pollution depended on the dispersity of the aquifer. Considering the adsorption of Cr(Ⅵ) by the aquifer, the higher the soil distribution coefficient, the smaller was the distribution range of the Cr(Ⅵ) pollution plume. Therefore, the transformation processes such as Cr(Ⅵ) adsorption should be focused on when predicting the distribution of Cr(Ⅵ) in contaminated sites.

Key words: contaminated site, hexavalent chromium Cr(Ⅵ), migration and transformation, dispersion, numerical simulation