Heavy metals and nutrients (carbon, nitrogen, and phosphorus) in sediments: relationships to land uses, environmental risks, and management.

Management of sediments in polluted rivers requires an inventory of sediment quantities as well as their nutrient and metal contents and the chemical forms of pollutants. We investigated the influence of three major land uses (i.e., orange plantation-OP, OP + residential + industrial-OPRI, and residential + industrial-RI) on the quality of surface sediments (0-20 cm below water/sediment interface) at the Sanyang Wetland (China). The total contents of metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) and nutrients (C, N, and P) as well as metal speciation in sediments were examined. GIS technology was used to estimate the volume of sediments needed to calculate the amounts of stored nutrients and metals in sediments. The surface sediments in the 3.2-km2 study area contain more than 2800 Mg C, 200 Mg N, and 100 Mg P. OPRI and RI land uses contribute more total C, N, P, Zn, and Cd to sediments than OP land use. High contents of C, N, and P may provide options to recycle the sediments as fertilizer but must be used with caution due to high levels of metals. Total Cd in sediments is at an order of magnitude (up to 59 mg kg-1) higher than the Level III criterion in the Chinese Environmental Quality Standards for Soil. Sediments in all land uses have very high risks due to >50% exchangeable + carbonate-bound Zn, Ni, and Cd. It is suggested that toxicity tests be conducted to better assess the environmental risks associated with any potential use of sediments.