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龙塘镇是珠三角电子垃圾拆解区之一，采用AAS方法对该地土壤和河流底泥沉积物中5种重金属（ Cd、Cr、Cu、Zn和Pb）含量进行测定分析，结果显示周围农田土壤中Cd含量是《国家土壤质量标准》二级标准的3倍左右，是广东省土壤背景值的20倍左右；拆解区土壤Cd超标最为严重，最高达5．67倍，超过广东省土壤背景值达36．17倍；拆解区下游0—1500 m河流底泥中Cd、Cr、Cu 、Zn和Pb含量大幅增加且部分金属超标，Cd在0—40 cm深度内含量总体逐渐降低，仍远超背景值，Cu含量采样深度内递减规律最显著且在0—10 cm内含量超标，Cr、Zn和Pb除部分点位在表层积累较多外，其余与背景值相差不大且未超标．采用BCR （ the Community Bureau of Reference）法提取重金属不同形态可知，底泥中Cd主要以弱酸溶态和可还原态为主，Cu主要以可氧化态存在，Cr主要以残渣态存在，而Zn和Pb则以可还原态为主；其中Cd的可提取态比例较高（＞60％），容易释放出来形成“二次污染”．潜在生态风险结果表明，河流底泥中Cd生态危害程度属于极强（Ei＞320）；其他金属Cr、Cu、Zn 和 Pb 危害风险程度属于轻微（Ei＜40）；金属 Cd 对整个 RI 值贡献最大（＞88％），应重点加强对Cd污染的防控．

Longtang Town is one of the E?waste recycling regions in Pearl River Delta. The concentrations of five heavy metals ( Cd, Cr, Cu, Zn, and Pb) in the soils and river sediments of this region were determined with AAS method. The results showed that the concentrations of Cd in the farm land soils nearby the region were nearly 20 times higher than the background value of soil in Guangdong province, and 3 times higher than the Secondary Standard of National Soil Quality of China. The concentrations of Cd in the soils of recycling region were 5. 67 times higher than the criteria, while Cd was 36.17 times higher than that of the background value of Guangdong Province soil. The concentrations of Cd, Cr, Cu, Zn, and Pb in the 0—1500 m downstream of the river sediments had an obvious increase and some metals were above the standard. The contents of Cd decreased gradually in the depth of 0—40 cm but were above the standard. The concentrations of Cu in sediment decreased sharply with the sampling depth and were above the standard in the depth of 0—10 cm. Cr, Zn and Pb were enriched in the surface soil at some sampling sites, but the other sites the concentrations were similar to the background values. BCR ( the Community Bureau of Reference) four?step sequential extraction method was used to examine the speciation of the heavy metals in the sediments. The results show that Cd mainly exist in the acid extractable fraction and reduced fractions;Cu mainly exist in the oxidized fraction;Cr mainly exist in the residual fraction;Zn and Pb mainly exist in the reduced fraction. In particular, the content of the extraction fraction of Cd was more than 60% of the total content, which have higher possibility to produce secondary pollution. Evaluation on the potential of ecological risk of the river sediments indicate that the Ei of Cd is greater than 320 which shows considerably serious risk. The ecological risks of other heavy metals are relatively low. The contribution of Cd to the whole RI is over 88%, and is the greatest among the tested heavy metals.