粤港澳大湾区丘陵地带某电镀场地重金属污染特征与迁移规律分析

粤港澳大湾区电镀企业的数量、生产规模均居全国前列,电镀生产排放的污染物造成该区电镀场地普遍存在土壤和地下水重金属污染现象,调查并分析区域电镀场地土壤和地下水重金属污染特征及迁移规律是粤港澳大湾区该类场地安全再利用的必要工作.在详细调查粤港澳大湾区丘陵地带某电镀场地土壤和地下水重金属污染现状的基础上,定量分析了不同深度土壤、地下水中的重金属空间分布特征,并结合场地水文地质条件探讨了粤港澳大湾区丘陵地带电镀场地土壤和地下水中的重金属迁移规律.结果表明,该电镀场地土壤和地下水已受到不同程度的重金属污染,土壤中Ni、Cr~(6+)和Cu超标率依次为20. 5%、12. 8%和2. 7%;地下水中Ni、Pb和Cr~(6+)超标率依次为41. 7%、33. 3%和33. 3%;场区内重金属污染与电镀厂生产功能分区相对应,说明重金属主要来源于电镀废物泄漏.由于该场区填土层以下为渗透性较差的粉质黏土,不利于重金属污染物向深部迁移,因而重金属污染物主要集中在表层土壤;但是在全风化花岗岩埋深较浅的电镀车间范围内,土层渗透性增大,重金属迁移深度显著增加,其中Cr~(6+)由于酸性土壤的吸附作用较弱,甚至出现了10 m深度的高浓度检出现象.虽然场区浅层含水层渗透性较弱,但是酸性土壤和地下水氧化环境有利于Cr~(6+)和Ni的迁移,因而在地下水位埋深较浅和土壤重金属迁移深度较大的电镀车间出现了Cr~(6+)和Ni超标现象;而粤港澳大湾区花岗岩中Pb含量背景值较高可能是造成场区地下水Pb超标的主要原因.因此,粤港澳大湾区丘陵地带电镀场地的Ni、Cr~(6+)和Cu主要集中在浅层土壤和地下水中,区域广泛分布的低渗透黏土层在一定程度上阻隔了重金属的扩散迁移;但是在土壤酸化和花岗岩埋深较浅的地区,以及地下水以氧化环境为主的丘陵地带,Ni和Cr~(6+)在土壤和地下水中的迁移深度将显著增加.

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

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.