Effects of coastal reclamation history on heavy metals in different types of wetland soils in the Pearl River Delta: Levels, sources and ecological risks

The human activity of coastal reclamation for agriculture production has caused serious environmental pollution problems and threatened the sustainable development of coastal delta area. Top soils (0–10 cm) were collected to clarify the effects of reclamation years (100y, 40y, 30y and 10y) on heavy metal pollution in reclaimed wetlands and ditch wetlands in Nansha District of the Pearl River Delta. Three parallel samples were also collected from each sampling site. As, Cd, Zn, Cu and Pb were analyzed to assess their concentration levels compared with other deltas. Geoaccumulation index, enrichment factor values, new toxicity evaluation index and principal component analysis were also used in this study. Compared with the reclaimed wetland soils, the highest concentrations of the selected elements were observed in the ditch wetland soils, with the value of Cd (1.97 ± 0.38 mg/kg), As (18.21 ± 4.61 mg/kg), Pb (68.56 ± 6.28 mg/kg), Cu (103.98 ± 23.38 mg/kg) and Zn (250.89 ± 52.35 mg/kg). Meanwhile, the elevated concentrations of heavy metals under the reclamation periods of 10y and 30y were found in two types of wetland soils except for As and Cd, which indicated that the comprehensive interactions of physical and chemical properties of soil, geographical factors, river network transportation and human activities jointly regulated the changes of heavy metal accumulation with wetland types and reclamation years. The highest potential ecological risks of Cd and Cu were observed in ditch wetlands. The sum of the toxic units were lower than 4. The contributing ratios of As (18.66 ± 3.23%) and Cd (12.97 ± 1.73%) were enhanced according to the new toxicity evaluation index compared with the ratios of As (14.70 ± 2.57%) and Cd (11.39 ± 2.35%). All the enrichment factor values of Cd, Zn and Cu exceeded 1.5, implying a large anthropogenic origin. However, the sources of As and Pb in the ditch and reclaimed wetland soils of the Pearl River Delta were generally controlled by the comprehensive effect of the natural, industrial and agricultural sources. Based on the results, key strategies including reducing the amount of pesticide and fertilizer and developing environment-friendly modern agriculture and clean production process were proposed for healthy development of the wetland ecosystems in the Pearl River Delta.