Heavy Metal Contamination and Migration in Correspondence of an Electroplating Site on the Hilly Lands of the Guangdong-Hong Kong-Macau Greater Bay Area, China

Abstract

The Guangdong-Hong Kong-Macau Greater Bay Area presents the highest number of electroplating corporations in China; some of them of very large scale. Electroplating emissions are the cause of widespread heavy metal contamination of both soil and groundwater in the Guangdong-Hong Kong-Macau Greater Bay Area. Hence, the reuse of electroplating sites in this area should be preceded by an analysis of heavy metal characteristics and migration in the soil and groundwater. We performed such analyses in correspondence of a relocated electroplating site on the hilly lands of the Guangdong-Hong Kong-Macau Greater Bay Area, and quantitatively determined the spatial distribution of heavy metals. Moreover, we discussed the migration of heavy metals under the specific hydrogeological conditions of the area. The results showed that the soil and groundwater in correspondence of the electroplating factory were polluted by heavy metals in different degrees. The over-standard rates of Ni, Cr6+, and Cu in the soil were 20.5%, 12.8%, and 2.7%, respectively; meanwhile, those of Ni, Pb, and Cr6+ in the groundwater were 41.7%, 33.3%, and 33.3%, respectively. The pattern of heavy metal pollution reflected the functional division of the electroplating factory, the contaminants should have mainly derived from the leakage of electroplating wastes. A low-permeable silt clay layer located below the fill soil layer limited the downward transportation of heavy metals, which were hence mainly concentrated in the surface soils. However, in another area of the site characterized by shallow-buried and completely decomposed granite (having high permeability), heavy metals could be transported much deeper. The adsorption of Cr6+ by the soil tends to be weak in an acid-acidic environment, explaining the relatively high concentrations of Cr6+ recorded in the upper 10 m of soil. Although the conductivity of the shallow aquifers was low, the occurrence of acid soil and of an oxidizing water environment should have favored the transport of Cr6+ and Ni in the groundwater, causing high concentrations of Cr6+ and Ni in correspondence of the electroplating workshops (characterized by a relatively low water table and deep heavy metal transport depth). The excess of Pb in the groundwater probably resulted from the high Pb content of granite in the Guangdong-Hong Kong-Macau Greater Bay Area. Overall, we observed high concentrations of Ni, Cr6+, and Cu in the shallow soil and groundwater located in correspondence of the electroplating site on the hilly lands of the Guangdong-Hong Kong-Macau Greater Bay Area. The presence of low permeable clay restricted the downward diffusion of heavy metals. However, in the presence of acid soil and shallow buried granite, or of oxidized groundwater, the migration depth of Ni and Cr6+ was significantly higher.