Metal Contamination and Fractionation in Sewage-Irrigated Soils Along the Liangshui River, Beijing, China

Soil samples were collected in a sewage-irrigated area along the Liangshui River, Beijing, China, to study the pollution degree and metal speciation. Total concentrations of Cu, Zn, Cr, Cd, Pb and Ni were determined, and their chemical fractions were partitioned with a sequential extraction procedure. Results showed that soils were heavily polluted by metals in the order of Cd > Cu > Ni > Pb > Zn > Cr. The significant positive correlations between some pairs of metals, except for Ni, may indicate compound pollution of metals that came from the same pollution sources with sewage irrigation. Most metals had very significant negative correlations with soil pH, and very significant positive correlations with organic matter, total N, available P and electrical conductivity. Sequential extraction of metals showed that Cu, Zn, Pb and Ni were mostly concentrated in about 1:1 ratio in non-residual (the sum of exchangeable, acid soluble, oxidizable, and reducible fractions) and residual fractions. Cadmium was almost evenly distributed in all five fractions, and the distribution of Pb and Ni had a similar trend of residual > oxidizable > reducible > acid soluble > exchangeable fraction. The dominant form of Cr in the residual fraction reflected on its tendency to become unavailable once in the soils. The overall pollution status indicated that sewage irrigation may present long-term environmental contamination risks to soil-plant-animal system.