Heavy Metal Pollution In Sewage-irrigated Soils Along The Liangshui River Of Beijing

Sewage irrigation is a main measure to solve water shortage for agriculture in Beijing. The entry of heavy metals into soil through sewage irrigation, and subsequent transfer from soil to plant, and/or to animal, can ultimately hazard human health. Soil samples were collected in a sewage-irrigated area along the Liangshui River of Beijing to study the pollution degree and heavy metal speciation. Total concentrations of Cu, Zn, Cr, Hg, Cd, Pb and Ni were determined for soil environmental quality assessment and correlation analysis between heavy metals and between heavy metals and soil basic physico-chemical properties. Principal component analysis was also conducted. Chemical fractions of heavy metals were partitioned with Tessier sequential extraction procedure. Results showed that the mean concentration of Cu, Zn, Cr, Hg, Cd, Pb and Ni was 66.2 mg kg-1, 86.4 mg kg-1, 66.98 mg kg-1, 0.46 mg kg-1,0.07 mg kg-1, 109.5 mg kg-1 and 28.4 mg kg-1, respectively. Soils were heavily polluted by Cu,Zn,Ni and Cd, which accorded with the calculated individual pollution indices and Nemerow synthetical pollution indices. Very significant negative correlations were found between Fe, Mn, Zn, Ni, Cd, Pb, Hg and clay content. Heavy metals Cu and Cr had very significant negative correlations with silt content, sigificant negative correlations with soil pH and electrical conductivity, positive correlations with organic matter and total nitrogen, and very significant positive correlations with available P. No significant correlations existed between all heavy metals with readily available K. The significant positive correlations between some pairs of heavy metals may indicate compound pollution of heavy metals which came from the same pollution sources with sewage irrigation. Prinicipal component analysis showed similarity between Cd, Ni and Zn, between Cu, Hg and Pb, and between Zn and Hg. Sequential extraction of heavy metals with Tessier procedure showed that Cu had the trend of residual>Fe-Mn oxides>organically bound>carbonate bound>exchangeable; Zn: residual>Fe-Mn oxides> organically bound> carbonate bound>exchangeable; Cr: residual>Fe-Mn oxides>organically bound>carbonate bound> exchangeable; Cd: residual>Fe-Mn oxides>carbonate bound>exchangeable>organically bound; Pb and Ni: residual>Fe-Mn oxides>organically bound>carbonate bound>exchangeable. Significant negative correlations were found between Zn and Pb concentrations in plants and their carbonate bound fractions in soils; There existed very significant negative correlation between Pb concentration in plants and its Fe-Mn oxides fraction in soils.The overall pollution status indicated that sewage irrigation may present long-term environmental contamination risks to soil-plant-animal system.