Heavy metals (Cr and Ni) distribution and fractionation in cropland soils from reclaimed tidal wetlands in Pearl River estuary, South China

摘要

Heavy metal contamination was compared for soils with different land use (cropland and wetland) in the reclaimed region of the Pearl River estuary. A sequential extraction technique was used to produce five chemical fractions (exchangeable (F1), carbonate (F2), Fe-Mn hydroxide (reducible) (F3), organic (F4) and residual (F5)) to study the effects of reclamation and tillage on the distribution and mobility of the heavy metals Cr and Ni. The results revealed that generally the contamination levels increased with a longer history of reclamation (100> 40> 30> 10 years) whilst an undisturbed reference wetland was least affected. The most substantial quantities of both Cr and Ni were observed in the residual fraction, which may be a consequence of the mixing effects of soil tillage and planting. With the exception of the 20-30cm layers from the wetland and 10 year old cropland samples, the total concentration of Cr and Ni at all sites and all soil layers exceeded the moderately polluted level according to the SQGs of the US EPA.